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COLE, M.R.I.A., F.G.S., \t PROFESSOR OF GEOLOGY AND MINKRALOOT IN THE ROYAL COLLEGE OF SCIENCE FOR JREIAND. AND DIRECTOR OF THB GEOLOGICAL SURVEY OF IRELAND. TUflitb IFiumerous 31lustration0, SIXTH EDITION, REVISED. LONDON: CHARLES GRIFFIN AND COMPANY, LIMITED; EXETER STREET, STRAND. 1909. \Att Rights Reserved.] V V TO PROFESSOR J. W. JUDD, C.B., LL.D., F.R.S., tbte 3Boofc is inscribed, IN GRATEFUL RECOLLECTION OF THE AUTHOR'S FIRST LESSONS IN PRACTICAL GEOLOGY, AND OF THE FRIENDLY HELP ACCORDED TO HIM THROUGHOUT THIRTY YEARS. 2T O O f\ O O PREFACE TO THE SIXTH EDITION. WHILE the size of the book has not been increased in this edition, alterations have been made in more than one hundred places, whereby, it is hoped, the fairly wide subject-matter has been brought up to date or more adequately set before the reader. The book has always aimed at aiding those who enquire into the materials and history of the earth's crust, for purposes of research or for any special enterprise. At the same time, the student is not likely to forget that practical geology is fundamentally to be followed in the open country and the open air, and that these "Aids" are intended to help towards the determination of what has been personally gathered from the mountain-side or in the plain. For those who take pleasure in learning from the earth around them, a smaller volume has been written, "Open- Air Studies in Geology," in which special appeal is made to the rock-masses as we meet them in the field. But all the wide developments of geology must rest on patient determinative work ; and the invention of a new piece of apparatus for the laboratory, or the correct appreciation of a fossil, may open up the clue to some long -sought secret of the earth. While certain modern restrictions in nomenclature have been introduced, which undoubtedly tend towards exactitude, the limits of the names of rocks and fossil genera have been kept as wide as possible. It has seemed equally unnecessary to relegate Terebratula to an obscure position, because of its imperfect definition a century or more ago, as to set aside viii PREFACE. "granite" and "basalt," and a score of familiar petrograpbic terms. In stratigraphy, I make no excuse for continuing to use De Lapparent's name " Gotlandian" for the beds called " Upper Silurian" by the older Geological Survey. The restriction of the name "Silurian" to these strata has raised frequent diffi- culties, especially in consulting classical maps and memoirs ; and, in areas where the Ordovician and Gotlandian beds are not well marked out from one another, there is a great con- venience in retaining " Silurian " to cover both the systems. I have again and again found myself thanking Mr. A. Morley Davies in my teaching work for pointing out to me this mode of escape from a very troublesome situation. The third edition of this book was greatly aided by the advice of Mr. L. Fletcher, F.R.S., as regards the description of the optical properties of minerals. Dr. F. A. Bather helped me at the same time in the choice of genera of fossil crinoids. Most of the kind suggestions made from time to time by Prof. Bonney, F.KS., Dr. J. J. H. Teall, F.R.S., and Dr. A. H. Foord, are now embodied in this edition. GRENVILLE A. J. COLE. DUBLIN, September, 1909. PREFACE TO THE FIRST EDITION. THIS little work is intended as a companion to any ordinary text-book of geology ; and it is hoped that it may be of special service to those students who have made excursions into the field, and who wish to determine their specimens for themselves. Mr. Joshua Trimmer, in 1841, issued hia Practical Geology and Mineralogy, with the object of en- couraging readers who were beyond the reach of oral in- struction. The book necessarily contained some theoretical matter ; but at the present day the abundance of excellent text-books has enabled these Aids in Practical Geology, while originating in the same idea, to be kept within still stricter limits. The section on blowpipe- work has been inserted as an aid to travellers ; while the description of the hard parts of fossil invertebrates will probably assist those readers who find it impossible to distinguish genera by means of mere names and figures. In arranging the genera thus discussed, those forms have been first dealt with which exhibit most com- pletely the characters of their class or sub-division. Hence highly developed types are often treated of before those which -may have preceded them in time, or which may have degenerated from them. By kind permission, I have been able to utilise many of the figures of fossils illustrating Phillips's Manual of Geology, and have supplemented these by a few sketches and diagrams explanatory of special features. A large section of the book has been devoted to rocks and to the ordinary minerals of the earth's crust, since these will X PREFACE. always present themselves to the observer during any ex- pedition or in any country. As for the names used for igneous rocks, I have endeavoured to retain the comprehen- sive terms of pioneers, such as d'Aubuisson, Brongniart, and Haiiy. The more exact determinative knowledge of the present day has introduced us to many new rock- varieties ; but these can be distinguished by the addition of a mere mineral prefix. In 1878 Prof. J. W. Judd, F.R.S., organised the instruction in Practical Geology at the Royal School of Mines in London ; and it is difficult to express briefly how much this book owes, in respect of any merit it may possess, to the courses then instituted and continuously developed from year to year. My great indebtedness, also, to Prof. Judd's published papers, and to the works of Brush, Lacroix, Levy, Rosen- busch, Teall, Zirkel, and Zittel, will again and again be apparent in the text. Numerous friends have, in addition, assisted me from time to time. At the risk of passing over some of the most generous, I must express my sincere thanks to Messrs. J. E. Duerden, L. W. Fulcher, J. W. Gregory, and T. H. Holland. And let me add, with Isaak Walton, that " I have found a high content in the search and conference of what is here offered to the reader's view and censure ; I wish him as much in the perusal of it." GRENVILLE A. J. COLE. DUBLIN, December. 1890. TABLE OF CONTENTS. PART I. THE SAMPLING OF THE EARTH'S CRUST. CHAP. >GB I. On certain observations in the field, ..... 1 II. On the collection and packing of specimens, ... 10 PART II. THE EXAMINATION OF MINERALS. III. On the occurrence and some physical characters of minerals, 14 IV. Simple tests with wet reagents, ..... 32 V. Examination of minerals with the blowpipe, ... 37 VI. Simple and characteristic reactions of the constituents of common minerals, ........ 60 VII. Blowpipe-tests useful in the determination of common minerals, ......... 66 VIII. Quantitative flame -reactions of the felspars and their allies, 78 IX. Examination of the optical properties of minerals, . . 85 PART III. THE EXAMINATION OF ROCKS. X. Introductory, .... . . . 92 XI. Rock-structures easily distinguished, . . . . 94 XII. Some physical characters of rocks, . . . . .103 XIII. The chemical examination of rocks, 105 Xll CONTENTS. CHAP. PACK XIV. The isolation of the constituents of rocks, . . . 110 XV. The petrological microscope and microscopic preparations, 123 XVI. The more prominent characters to be observed in minerals in rock sections and as isolated crystals under the microscope, . . . . . . . . .134 XVII. The characters of the chief rock-forming minerals in the rock -mass and under the microscope, . . . .154 XVIII. Sedimentary rocks, 186 XIX. Igneous rocks, 215 XX. Metamorphic rocks, 271 PART IV. THE EXAMINATION OF FOSSILS. XXL Introductory, 287 XXII. Fossil generic types. Rhizopoda (p. 294); Spongiae (p. 298) ; Hydrozoa (p. 302) ; Actinozoa (p. 304), . . 294 XXIII. Fossil generic types. Polyzoa (p. 311); Brachiopo* 2 ^r^OB^RV^TIONJS .IN THE FIELD. him the Earth becomes a great reality, for he surveys it through the extent of his collections and his studies ; but the ordinary student, gathering together a few relics from the curiosity- cabinets of relatives and friends, finds that they appeal to him but little; they have no associations, they have long been separated from their kindred, they are " fossil " in the worst of senses. But let him, having a knowledge of first principles and of museum-types, go out to see things for himself. Furnished with the maps and books of experienced workers, let him re-examine the evidence on which they have relied. A week's holiday thus spent amid varied surroundings, as on the Welsh border, or in Antrim, or around Edinburgh or Bristol, will provide material for long and careful study. Once in the field, the complexity of the subject will dawn upon him ; but at the same time he becomes assured that, wherever he may wander, he will find congenial work. The first visit to a district commonly raises numberless questions, when the specimens gathered are examined at his leisure ; and the suggestions of the laboratory or the microscope must be tested in a second or third excursion by re-examination of the relations of the rock-masses in the field. In the field itself broad names are assigned to objects, detailed determination being left for comparative and instrumental work ; but in these after-hours of study every scene comes back Tividly before us, and even the lichens that may yet cling in hollows and betray the collection of an imperfect and weathered specimen, serve their turn with the naturalist and remind him of the wide, open-air, and eminently natural character of his work. The art of observing in the field, and of balancing the evidence oi various exposures, must be to a great extent learnt by oral tradition and personal guidance ; and the study of any geological map, with its outliers, its sinuous outcrops, its inliers, its repetitions by faults or foldings, should be carried on, wherever possible, in the actual district that has been mapped. The practical construction of maps, and of sections from them, is discussed in Sir A. Geikie's Outlines of Field Geology (Macmillan & Co.), and Penning's Field Geology (Bailliere, Tindall p-jr = T-TV The graduation adopted gives this result at once, for we have only to read the figure coincident with the point C. Dr. J. Holms Pollok has independently devised a very small and portable balance on Coates's principle, which is made by Messrs. Baird & Tatlock, London, for 16s. In Mr. K Parish's balance, two pans are hung one above the other from a fixed point on one arm of the beam, the lower pan * Walker's balance is made by Mr. G. Lowdon, Reform Street. Dundee. Price 31s. 6d. \-Philosophical Magazine, vol. Iviii. (1821), p. 108- From Journ. of Acad. of Nat. Sciences, Philadelphia, vol. i., Part 2. + lbid., p. 109. From same source. African Journ. of Science, ser. iii., vol. x. (1875), p. 352. 28 BOMB PHYSICAL CHARACTERS OF MINERALS. being immersed in water. The beam is then equipoised by a small sliding weight, clamped by a screw, working on the arm that bears the pans. The specimen is laid in the upper pan, and balanced by the addition of a light pan, into which sufficient sand is thrown, suspended from the point corresponding to A in Coates's instrument The specimen is now transferred to the lower pan, and the balancing-pan is slid inwards, care being taken not to disturb the sand. The reading now made gives the specific gravity without calculation, the graduation being on the plan employed by Coates. One merit of this instrument is that fragmentary materials can be determined, as no suspending thread is required ; but in practice it is probable that the results obtained by it are not superior to those given by Walker's balance, while it is more complicated in construction. Prof. Jolly's spring-balance or Federwage is, however, simple and yields excellent results. A long brass spiral spring, which may be exchanged for one of greater delicacy if the specimen is exceptionally small, is hung from a sliding rod, set in a pedestal some 3 feet high. One end of the spring may thus be brought 5 feet above the table. The base of the instrument is pierced by three levelling-screws, and a long slip of looking-glass, with even graduations marked on it, is let into the face of the pedestal. Two light pans are hung, one below the other, from a wire hooked to the lower end of the spring, and on the wire is fixed a little bead, acting as an index. The lower pan is sunk well in a tumbler of water, the support of which can be slid up and down the pedestal ; and the sliding- rod is carried so high that the pans come to rest somewhere opposite the upper divisions on the graduated mirror. Looking along the top of the index-bead until it appears to coincide with its image in the mirror, the position of rest, o, of the spring is noted, in terms of the fine graduations used. It will be seen that this reading corresponds to the determination of the sink- ing-weight of the araeometer, only in this case the figure will vary according to the adjustment of the spring at starting. Place the specimen in the upper pan, having previously drawn the tumbler to a lower position to avoid the wetting of both pans. Readjust the tumbler until the pans swing freely and as much of the lower suspending-wire is immersed as before. Take a second reading, 6 ; then b - a = w, the value in air. Transfer the specimen to the lower pan, and readjust. The reading, e. will be less than 6, and G = - . o c SOME PHYSICAL CHARACTERS OF MINERALS. 29 Though not so suitable for travellers, this makes an admirable laboratory-instrument,* and, the readings being merely propor- tional, the utility of the spring as a weight-measurer is not affected by expansion due to change of climate. We must conclude the present section with an account of the use of dense liquids in determining the specific gravity of mineral particles. If a solution of known density is to hand, and a specimen, though it has been completely freed from bubbles, floats upon the surface, while others sink with more or less rapidity, some idea of their relative specific gravities may be obtained. Further, if the liquid is diluted until a particular specimen swims about in it and remains sluggishly wherever it is placed, the liquid and the mineral will be of the same specific gravity. That of the liquid may be determined by throwing in a series of specimens already determined, until one is found that will neither float nor sink to the bottom ; or by suspending a weight from a chemical or Jolly's balance, and comparing the readings given when it is immersed in water and in the liquid respectively. Prof. Sollas ("Gran- ites of Leinster," Transactions of the Royal Irish Academy, vol. xxix., 1891, p. 430) has even employed a minute hydro- meter. This method of determining specific gravities, which can be used even in the case of very small specimens, was brought into prominence by Mr. E. Sonstadtt as recently as 1874, and has since been largely utilised. Sonstadt's solution consists of a saturated solution of potassium iodide in water, in which is stirred up as much mercuric iodide as it will dissolve. " It will then dissolve more iodide of potas- sium, then more mercuric iodide, and so forth. The iodides dis- solve very slowly at the last, and as it is best not to accelerate the solution by the application of heat, considerable time must be allowed when a liquid of maximum strength is required. The ' solution, after filtering, is fit for use. ... It may be diluted to any extent, and then concentrated by heat, without injury." The maximum density obtainable falls just short of 3-2, and is about 3 -17 in hot climates, these figures being higher than those first given by Sonstadt. In addition to its use in determining specific gravities, Son- stadt pointed out that his solution would serve to separate * Supplied by Krantz, Rheinisches Mineralien-Contor, Bonn, at 37s. t "New Method of taking Specific Gravities," Chemical News^ vol. xxix., p. 128. 30 SOMK PHYSICAL CHARACTERS OF MINERALS. mineral particles of one kind from others with which they might be mixed, as in the case of diamonds occurring in quartz sand. This application has been so far extended by Thoulet in France, and Goldschmidt in Germany, that the solution has often been named after these workers instead of after its original dis- coverer. Rohrbach's * solution of iodide of mercury and iodide of barium has a density as high as 3-588, but decomposes on addition of water, and must be reduced to the density required by a specially prepared dilute solution. Neither of the foregoing liquids are satisfactory for the traveller, or even for laboratory use, on account of their dangerously corrosive and poisonous character. They have been largely superseded by the solution of borotung- state of cadmium, first prepared by D. Klein,f and now very widely used. This is also a pale yellow liquid, with a density of 3-28 ; it can be diluted with water and again concentrated by heating over a water-bath until a hornblende crystal just floats upon the surface. Any overheating will cause the salt to crystallise out on cooling down, when a fresh dilution will be necessary. Though poisonous, the borotungstate is not irritant like the mercury solutions; it can be carried about in a stoppered bottle in the solid state, and dissolved in distilled water when required. A few ready-made solutions of known density, kept carefully stoppered, will be very useful in the discrimination of gems. The only objections to this liquid are that it decomposes carbonates, so that specimens before use should be treated with a mild acid ; and that it tends to crystallise readily upon the stoppers of bottles or the glass rods used in stirring. The rods and vessels used should always be washed with distilled water, the resulting very dilute solutions being kept together in a bottle, to be concentrated by evaporation when time allows. Another liquid that is of great utility has been brought for- ward by B. Brauus.j He uses methylene iodide, which must be -diluted with benzene and not with either water or alcohol, and which, to preserve its pale straw-colour and transparency, must be kept as much as possible from the light. When it has become darkened, as must eventually happen, the colour can be restored by putting mercury or copper filings into the bottle and * Ntiues Jahrbuch fur Mineralogie, IPE-REACTiONS OF MINERALS. 75 Add. Soluble in H 01 ; chlorine evolved, known by its smell. Barium sulphate is usually precipitated on addition of H 2 S O 4 . Manganite (H 2 Mn 2 4 ) is otten crystallised, and yields no barium. 66. Pyrargyrite. Ag 3 Sb S 3 ( - 3 Ag 2 S . Sb ? S 8 ). Like proustite, with antimony in place of arsenic. Ch. Silver oxide encrusta- tion on antimony oxide. Add. Streak purplish-red (Miers). 66a. Pyrite (see Iron Pyrites). 67. Pyrolusite. Mn 2 . Fus. Infusible. Bor. and Micr. Manganese reactions. Cl. tube Commonly a little water. Evolves oxygen, a glowing splinter of wood inserted in tube being re-kindled as each puff of gas arises. Add. Soluble in H 01 with evolution of chlorine, which is know\i by its smell. 68. Pyromorphite. Pb 4 (PbCl)(PO 4 ) 3 . Flame With sul- phuric acid, phosphorus reaction, the green flame surrounding an inner blue one due to lead. Fus. Yery easy, Micr. With copper oxide, chlorine reaction. Ch. White lead chloride en- crustation ; nearer assay, lead ditto. In R. F., metallic lead. 69. Pyrrhotine. Fe 7 S 8 ( = 6 Fe S . Fe S 2 ). Fus. About 2. Bor. and Micr. Iron reactions. Cl. tube Scarcely any sulphur. Add. Soluble in HOI; evolves H 2 S on boiling. Magnetic before reduction, and attracts its own powder. See Iron Pyrites. 70. Quartz. SiO 2 . Fus. Infusible. Micr. TJndissolved. Ch. Fuses readily with soda; cobalt nitrate added to the pro- duct produces a deep blue glass, as in ordinary fusible silicates. Add. Hardness = 7; specific gravity only 2-65. 71. Redruthite (Copper Glance). Cu 2 S. Flame With HOI copper flames. Fus. About 1-5. Bor. and Micr. Copper re- actions. Cl. tube No sulphur. Ch. With soda, or when roasted in O. F., metallic copper. With soda, sulphur reaction. Add. Sectile. 72. Rhodonite. MnSiO s . Fus. About 2-5. Bor. and Micr. Manganese reactions. In latter, silica. Add. With H Cl commonly effervesces, through presence of some carbonate, but is only slightly decomposed. 73. Rock-salt. NaCl. Flame Intense sodium. Fus. About 1. Micr. With copper, strong chlorine reaction (p. 61). Add. Soluble in water. Taste characteristic, but similar to sylvine. 74. Rutile. Ti0 2 . Fus. Infusible. Bor. and Micr. Good titanium reactions. Barely soluble in latter. Ch. The soda- residue, boiled with tin in H 01, gives a strong titanium reaction upon standing. 76 fcLOWflPE-REACTlOtfS OF MINERALS. 75. Sal-ammoniac. N H 4 01. Fus. Swells up and volatilises without fusion. Micr. With copper oxide, chlorine reaction. Cl. tube Volatilises, forming dense white sublimate and fumes. Add. Evolves ammonia, known by its smell, when pounded up or fused with sodium carbonate. Soluble in water. 75a. Siderite (see Chalybite). 76. Smaltine. (Oo, Fe, Ni) As 2 . Graduates into chloanthitej richer in cobalt (see Chloanthite). 76a. Smithsonite (see Calamine). 77. Soda-Nitre. NaN0 3 . Flame Strong sodium. Fus. Very easy. CL tube Fused with bisulphate of potash, gives off brown fumes, well seen on looking down tube. Ch. Flares up like nitre. Add. Soluble in water. Saline taste. 78. Sphene. Ca Si Ti O 5 . Fus. Fairly easy. EOT. and Micr. Titanium reaction. Silica in latter. Ch. The soda-residue, boiled with tin in H Cl, gives a clear titanium reaction. 79. Spinel. (Mg, Fe) (Al 2 ,Fe 2 ) O 4 . Fus. Infusible, thus differ- ing from similarly coloured garnets. Add. Specific gravity less than zircon (about 4-0 and 4*5 respectively). See notes on Zircon. 79a. Stibnite (see Antimonite). 796. Stream Tin (see Cassiterite). 80. Strontianite. SrCO 3 . Flame Strong strontium. Add. Effervesces in cold H 01. Even very dilute solutions give, on standing, a precipitate with sulphuric acid. Compare with aragonite or calcite, and see p. 36. 81. Sulphur. S. Flame Burns with a blue flame. Cl. tube Volatilises, giving sulphur sublimate. 82. Sylvine. K 01. Flame Strong potassium ; otherwise like rock-salt. 83. Talc. H 2 Mg 3 (Si O 3 ) 4 . Fus. Infusible. The lamellae bend away from one another during heating. Micr. Silica. Cl. tube A little water. Ch. With cobalt nitrate, a fair magnesia reaction. Add. Hardness clearly less than that of micas. 83a. Tinstone (see Cassiterite). 84. Titanic Iron Ore (ilmenite and titaniferous magnetite). Ilmenite = m Ti Fe O 3 + n Fe 2 O 3 . Fus. Practically infusible. Bor. Iron reactions. Micr. Iron and titanium. Ch. In B. F. magnetic residue. The soda-residue, boiled with tin in H 01 gives a satisfactory titanium reaction. Some specimens are magnetic before reduction. 85. Topaz. (F, OH)o Al 2 SiO 4 . (See Groth, Tab. Uebersicht, BLOWPIPE-REACTIONS OF MINERALS. 77 1898, p. 116.) Fus. Infusible. Micr. Silica. Ch. With cobalt nitrate, alumina reaction. Add. Fused on glass slip as described on p. 62, dulls and etches the surface. Distinguished from quartz by hardness = 8, specific gravity = 3 '5, and presence of good cleavage (basal). 86. Tourmaline. For composition, see p. 183. Flame Some specimens give boron flame when fused with fluor-spar and bisuphate of potash. Fus. Various, but often easy. Micr. Silica. Add. Distinguished from hornblende by hardness = 7, and very common occurrence of trigonal prisms. 87. Vivianite. Fe 3 (P(X) 2 + 8H 2 0. Flame With sulphuric acid, phosphorus. Fus. Easy. Bor. and Micr. Iron reactions. Cl. tube Becomes white; gives off water. Ch. Magnetic residue. Add. Soluble in H 01. Reduces the ammonium molyb- date solution, the blue colour mingling with the yellow precipitate due to phosphoric acid. Blue colour characteristic, but alters to brown, becoming then red by transmitted light. Blue crystals strongly pleochroic. 88. Websterite. (A1O) 2 SO 4 + 9H 2 O. Fus. Infusible. Cl. tube Much water. Ch. With cobalt nitrate, fine alumina reaction, With soda, sulphur reaction. Add. Soluble in H Cl. Specific gravity = 1-66. Alunite (K 2 A1 6 S 4 O 22 + 6H 2 O) has higher hardness and specific gravity, and is insoluble in H 01. 89. Witherite. BaCO 3 . Flame Barium. Fus. 2. Add. Effervesces in H Cl. (See also p. 36.) 90. Wolfram. (Fe, Mn)WO 4 . Fus. Decrepitates, but /uses about 3. Bor. Iron and sometimes manganese. Micr. Iron and tungsten. Ch. The soda-residue, boiled with tin in H Cl, gives a fine tungsten reaction. Add. Carbonate of soda bead gives manganese reaction. Lustre and cleavage characteristic. 91. Wollastonite. Ca Si 3 . Flame Fine calcium with H 01. Fus. About 4; glows strongly. Micr. Silica. Add. Gives a silica-jelly with H 01. Some carbonate often present. 92. Zinc-Blende (Blende). Zn S. Fus. About 6. Cl. tube Thin sulphur. Ch. Zinc encrustation, at times excellent with cobalt nitrate, poor in other examples ; best produced when specimen is in R. F. Some varieties give cadmium encrustation. Often magnetic residue. With soda, sulphur reaction. Add. Soluble with effervescence in hot H Cl, sulphuretted hydrogen being evolved. 93. Zircon. ZrSiO 4 . Fus. Infusible. Ch. The soda- residue, after thorough fusion, treated in a dish with hot water, gives abundant minute hexagonal platy crystals (zirconia) and FLAME-REACTIONS OP THE FELSPARS. rhombohedra ( ? sodic zirconate). Examine on glass slip under microscopic power magnifying about 400 diameters. For dis- cussion of this reaction, see Levy & Lacroix, Mineraux des Roches (1888), p. 117. If the soda and the zircon are not finely pul- verised together and completely fused, a residue of zircon frag- ments alone appears. See notes on Garnet and Spinel. INDEX TO METALLIC COMPOUNDS. Aluminium, 28, 29, Iron, 21, 23, 35, 38, Sodium, 29, 73, 77. 46, 85, 88. 41, 44, 45, 50, 52, 54, 55, 69, 79, 84, 87, 90. Strontium, 19, 80. Antimony, 4, 66. T'ln 1 ft Lead, 1, 20, 36, 68. 111, 19. Arsenic, 22, 26, 48, 55, 64, 76. Magnesium, 15, 31, 33, Titanium, 74, 78, 84. Barium, 12, 89. 51, 79. Tungsten, 90. Bismuth, 13. Manganese, 65, 67, 72, 90. Uranium, 63. Zinc, 16, 35, 42, 92. Calcium, 2, 5, 7, 17, Mercury, 25. 31, 34, 40, 78. Zirconium, 93. Molybdenum, 56. Cobalt, 22, 26, 76. Nickel, 22, 48, 76. SILICATES. Copper, 11, 14, 24, 27, 30, 53, 71. Potassium, 59, 82. 3, 6, 10, 24, 32, 37, 42, 43, 46, 49, 57, 58, 60, Chromium, 23. Silver, 8, 47, 64, 66. 61,62,78,83,85,86,91. CHAPTER VIII. QUANTITATIVE FLAME-REACTIONS OF THE FELSPARS AND THEIR ALLIES. PROF. SzABd of Budapest, by making more precise certain flame- reactions indicated by Bunsen, developed in 1876 a new method for the determination of the felspars and allied silicates found in common rocks.* Practice has again and again shown that the * "Ueber eine neue Methode die Feldspathe auch in Gesteinen zur bestimmen." Franklin- Verein, Budapest. An abstract occurs in Proc. American Assoc. for Advancement of Science, vol. xxxi., 1882, without illustrations. FLAME-REACTIONS OF THE FELSPARS. 79 observations made in this manner in the Bunsen-flame are as reliable as they are simple and expeditious. Gas is required, but a careful observer working on typical minerals with a blast- lamp might no doubt profitably construct a table of reactions with which to compare the results given in the same flame by undetermined specimens. The Bunsen-burner used by Szab6 has a tube of 1 cm. diameter. A three-rayed support, screwed on over the upper end until it rests 3 cm. below the orifice, carries a removable iron cone 6 cm. high, 5'3 cm. in lower diameter, and 3 cm. in diameter above. The flame is 13 to 14 cm. high when the cone is not employed. The position of highest temperature, the fusion-place, is about one-fourth of the total height of the flame above its base. The particle of felspar or other mineral to be tested must be of a fixed bulk and about this size when fused to a globule ; it must be carefully selected from a roughly powdered sample of the rock of which it forms a part, and must not be touched by the finger nor immersed in water that is not distilled. The lens should, as usual, be used in the selection of such fragments, and the character of their cleavage can often be noted as a pre- liminary. Should the mineral fly to pieces in the flame, Szab6 recommends that a sample of the mineral should be allowed to decrepitate by heating in a closed tube, the fragment finally used being selected from the material thus already broken up. The particle is supported on a platinum wire of about this thickness , of which 1 decimetre should weigh only 32 milligrammes. The usual small loop is made at the end of the wire. To secure the particle on the wire, a matter which some workers have found troublesome, but which need cause little annoyance considering the ease with which the observations are finally made, Szab6's directions should be carefully followed. Dip the wire loop in distilled water and touch the granule with it, quickly raising it, so that only the upper surface becomes wetted ; turning the wire, the dry surface of the particle comes upwards. Bring it in this position gradually near to the base of the Bunsen- flame, the water thus drying off slowly ; a card should be held beneath the specimen to catch it if it becomes shaken off. Finally let it enter the flame and remain there for two seconds, the surface in most cases becoming fused to the platinum wire. The wire can be supported in various ways in the flame, the points selected being in the outer envelope at (a) the base of the flame, (b) at 5 millimetres above the base, and (c) the fusion- 80 FLAME REACTIONS OP THE FELSPARS. place, about 5 mm. above the top of the iron cone. The present writer has described the following support as one that practice has shown to be useful.* (Fig. 9.) Fig. 9- A small gallipot, such as is used for Liebig's extract, forms a case that is clean, strong, and adequately heavy. A brass wire, about 3 mm. in diameter, passes through the cork of this, and rises 15 centimetres above it, carrying a stout cork A, which can be slid up and down to any level. A steel wire or knitting- needle, some 25 cm. long, is pushed horizontally through A, the last 7 cm. on either side being then bent forward at right angles. Two small corks, B and B', are carried by the parallel arms thus formed, and support, by means of a knife-slit in the top of each, the fine platinum wires employed. B can be slipped off the steel wire, the mineral fragment can be attached, with Prof. Szab6's precautions, to the platinum loop, and the carrier replaced without fear of loss by jarring. B' can be used for a type- specimen to be compared with that under examination, the wires on both corks being adjusted to exactly the same level, and one or other being brought at will into the flame. The cork A being set approximately at the proper height, the rotation of the steel wire within it moves B and B' equally in vertical planes, and gives a delicate means of fine adjustment. To secure uniformity of position in successive experiments, the platinum loop carrying the specimen is brought to the exact level of the top of the Bunsen-burner, or to the level of the top of the iron cone. A small plate of wood, C, of the thickness of 5 mm., * "On a simple apparatus for flame-reactions." Geological Magazine* 1888, p. 3X4. FLAME-REACTIONS OP THE FELSPARS. 81 is then slid under the gallipot, the specimen being thus raised tc the positions adopted by Prof. Szabo, without any of the diffi- culties that often arise from the jarring or stiffness of motion in more elaborate supports. The dimensions above given are those adapted to a Bunsen- burner of ordinary height and ordinary diameter of base. For packing, the erection can be taken down, and the 5-millimetre plate and the smaller corks can be kept inside the gallipot till required. Observation of Fusibility. This forms the least reliable part of Szab6's reactions, as a temperature slightly under that of his standard flame makes the species seem equally difficult to fuse. The scale is unfortunately numbered in the reverse order fco that of von Kobell. It is here sim- plified ; the numbers refer to the result obtained, whatever part of the flame is used ; thus, a mineral may have a fusibility of 1 in the point b and of 3 in the point C. The product must be examined with a lens. 0. Infusible. 1. Edges and corners alone rounded. 2. General form unaltered, but edges, corners, and faces fused. 3. Form altered, but not to a globule. 4. Fuses to a globule. The time of heating is in each observation one minute, the specimen being tried first in the position a (base of flame), then moved to b (5 mm. above the base), then to C (5 mm. above the iron cone), notes being made of its appearance on withdrawal from each portion of the flame. Determination of Sodium and Potassium Held in position b (first row, fig. 10), the assay imparts a certain degree of coloura- tion to the flame. Five degrees are recognised, No. 5 being the most intense. The observer, with a drawing of these degrees before him, notes down the figure corresponding to the flame given by his assay, and picks up an indigo prism or cobalt glass 5 mm. thick, through which he views the flame with the object of detecting potassium. Three degrees of the characteristic violet-red flame may be distinguished by a good eye (lowest row, fig. 10). All this can be done in the one minute assigned to the observation; at its expiry the wire is withdrawn and the degree of fusion also noted. The cone is put on and the same assay brought to C, the fusion-place. In one minute similar observations of sodium I FLAME-REACTIONS OP THE FELSPARS J. 2 3 I. S. 3. Fig. 10. FLAMK-REACTIONS OP THE FELSPARS. 83 should be slightly stronger than in b, the degree 3, for instance, now representing an intenser flame than 3 in the previous observation. The assay is now dipped in distilled water and then into powdered gypsum, which thus adheres to and surrounds it. On reheating in position C, the gypsum assists decomposition, the sodium and potassium being converted into sulphates. The observation should be made when the assay has been two minutes in the flame. It is unnecessary to observe the sodium reaction except as a check ; but the potassium flame is intensified and four degrees are distinguishable (fig. 11), No. 1 representing a quantity too minute for previous detection. See, however, p. 85. Determination of Calcium. The lime in felspars and allied minerals can generally be inferred from the diminution of the soda and potash ; but its flame may be seen as follows : Put fragments of the mineral in a glass tube with cold con- centrated hydrochloric acid sufficient to completely cover them. Close with wax and leave for 24 hours. Then open and plunge in a fairly thick platinum wire, coiled at the end. The drop thus brought out is held in position b, and the first colour observable is due almost entirely to the calcium. A direct- vision spectroscope shows the red, orange, and green lines dis- tinctly. The flames due to sodium, potassium, and lithium follow in order, and the degrees observed during this operation often serve to distinguish minerals in which the actual percent- age of soda, &c., is the same, a higher degree being shown by those more easily decomposed by acid. 84 FLAME-REACTIONS OP THE FELSPARS. TYPICAL REACTIONS. MINERALS. In position b. Time 1 minute. In position c. Time 1 minute. In position e with Gypsum. Time 2 minutes. Drop of the HCl solution, after digestion of particles for 24 hours. Position b. Na. K. Degree of fusion. Na. E. Degree of fusion. K. Ca. Na. E. i. Orthoelase, . 1-3 2-3 1-3 1-3 2-3 S-4 3-4 1-2 2. Soda-Ortho- \ clase, J 3-4 1 S-4 3-4 1-2 4 2-3 1-2 3. Albite, . 5 S-4 5 4 1-2 4. Oligoelase, . 3-5 S-4 4-5 4 1-2 2 5. Andesine, . 3-4 2-3 34 3-4 1-2 0-1 2-3 0-2 6. Labradorite, 2-3 0-2 2-3 2-3 1-2 1-2 2-3 0-1 7. Bytownite, . 2-3 0-1 2-3 1-2 0-1 2-3 1-3 0-1 8. Anorthite, . 1-2 0-1 1-2 0-1 0-1 2-3 1-3 0-1 9. Leueite, 2-3 3 1 2-3 3 2 4 2-3 4 10. Nepheline, . 5 1-3 2-3 5 2-3 S-4 3-4 5 3-4 11. Nosean, 5 1 1 5 1-2 2 2-3 5 3 12. Haiiyne, 6 1-2 1 5 1-3 2-3 2-3 0-3 5 1-3 13. SodaUte, . 5 0-3 1-3 5 0-3 8-4 0-3 0-1 5 1-3 A table based on those prepared by Szab6 is given above. The degree of fusion is stated according to the modified scale adopted on p. 81. The observer draws up a blank form on the same lines, notes into it each observation numerically as soon as made, and compares the whole result with the series given in the table. A good plan is to take each result separately and write down all the felspars or allied minerals to which it might possibly correspond. A comparison of the brief lists thus formed enables one to pick out the name that occurs most frequently, or the two OPTICAL PROPERTIES OF MINERALS. 85 between which the mineral must lie. These reactions, performed upon minute grains, and occupying altogether about ten minutes, have a great value to the geologist, however simple they may at first appear to the chemist, mindful of his refined but lengthy methods of analysis. The determination of potassium in the flame is rendered easier by the following process,* which allows of a complete decomposi- tion of the silicate, and which is unaffected by the presence of a bright sodium flame. On a loop of platinum wire, 2 mm. in diameter, form a bead of sodium carbonate, and view the flame given by it through 5 mm. of blue glass to see that no potash is present. Moisten this with water, and pick up about 2 cubic mm. of the mineral i.e., about twice as much as is used in Szab6's process. Place the wire on the support in the position C, 5 mm. above the cone. The specimen does not tend to fall off, but is soon attacked and dissolved. Leave for two minutes by the watch. Then observe the flame through 5 mm. of blue glass. The bright sodium flame is cut off, except for a marginal column, which comes through blue. If potassium is present, it is revealed by a pink-violet inner fringe to this blue column. Judging from its extent and also its intensity near the assay, three grades can be established. Grade 1 = about 4 per cent, of potash. C\ ___ O > o = 12 ,, ,, The ease of manipulation in this method, and the completeness of decomposition, seem to recommend it. It is also of service in examining the glassy or fine-grained groundmasses of igneous rocks. All true orthoclases fall in grade 3 j but each observer should establish the grades for his own eye upon specimens already known. CHAPTER IX. EXAMINATION OP THE OPTICAL PROPERTIES OP MINERALS. IN a book written for the geologist rather than for the mineral- ogist, a detailed account of the optical properties of minerals, as observed in slices cut from them in known directions, would be out of place. The appearance of the common rock-forming minerals in *G. Cole, "Potassium in Silicates," Geol. Mag., 1898, p. 103. 86 OPTICAL PROPERTIES OF MINERALS. ordinary microscopic sections is stated in Chapters xvi. and xvii. It will be well, however, to indicate at once some of the terms that may be employed in describing the phenomena then ob- served.* It has long been customary to introduce into these terms the hypothesis of an incompressible ether, of elasticity variable with the direction of its displacement; but, as Mr. Fletcher has pointed out, changes of view as to the nature oi the ether have rendered such phraseology undesirable. If we take a plate cut from a doubly refracting crystal, but not perpendicular to an optic axis, and cause a ray of ordinary light to strike it in a direction perpendicular to its surface, this ray, on entering the crystal-plate, is split into two, which respectively vibrate in planes that are perpendicular to one another. The traces of these two planes on the surface of the plate may be called the vibration-traces for that particular section. They have also been styled the " vibration-directions," and "directions of greatest and least elasticity," for that particular section. The ray corresponding to one of these traces is propagated with greater velocity than that corresponding to the other trace. We may call one trace, then, the fast-ray vibration-trace, and the other the slow-ray vibration-tracef, or "fast-ray trace" and "slow- ray trace " where shortness is desirable. These correspond to Prof. Groth's "vibration-direction of greater light-velocit}^" and "vibration-direction of less ligh t- velocity ;" and to the ri p and ri g of MM. Michel Levy and LacroixJ, who distinguish the directions as those of rays with smaller or greater refractive index respectively. It is of interest to observe that the vibration-traces represent the directions of the axes of the ellipse which is formed by the intersection of the plane of our crystal plate with the optical iiidicatrix, or surface of reference for rays propagated in any direc- tion in the crystal. The vibration-trace of the slow ray corre- sponds to the long axis of this ellipse (^) 5 and vice versd, and the lengths of the two axes are proportional to the refractive indices for rays of the same colour vibrating parallel to them respectively, and travelling normally to the surfaces of the plate. * For a discussion of the principles underlying the optical properties of minerals, the reader should specially consult Fletcher's Optical Indicatrix (Min. Mag., vol. ix., 1881, p. 278; also issued as a separate work, revised, published by H. Frowde, Ib92); also Miers, Mineralogy, 1902, and Groth, Physikalische Krystallographie, 3rd or later edition. tl am indebted to Mr. L. Fletcher, F.R.S., for generous help in the discussion and selection of the above terms. Les MinAraux des Roches, pp. 3 and 4. Or, as Mr. Fletcher mnemonically puts it to me, the longer trace is the longer-time vibration -trace. OPTICAL PROPERTIES OF MINERALS. 87 The indicatrix of a biaxial crystal is an ellipsoid, the three axes of which, perpendicular to one another, have different lengths; it has three planes of symmetry, each of which contains two of the axes, and is perpendicular to the third axis. If a ray of light is propagated along a line, other than an optic axis, lying in one of these planes of symmetry, it travels with one or other of two different velocities. In one case the ray vibrates per- pendicularly to the plane of symmetry, and therefore parallel to the axis which is perpendicular to that plane ; the vibration- direction in the other case is parallel to the plane of symmetry. The velocity of the former ray is constant, in any particular crystal, whatever the direction of propagation of the ray may be within the plane of symmetry; its refractive index is also constant, and it is, in this respect, an " ordinary " ray. There are, however, three different indices of refraction for the three ordinary rays that correspond to the three planes of symmetry respectively. Seeing that rays of the various colours composing white light suffer refraction in different degrees, these indices are determined with monochromatic light, the sodium flame being generally used. They are styled the principal indices of refraction of the crystal. The smallest is usually designated a, or n p of French authors ; the mean is /5, or n m ; and the greatest is y, or n g . Since the value of /3 is not the arithmetic mean between a and y, the result of ^ is employed to express o the average refractive index of the mineral. This may be briefly termed the " refractive index of the mineral." A very high refractive index for rock-forming minerals is 2*712, that of rutile; while 1*433, that of fluor-spar, is a low refractive index. By definition, the axes of the optical indicatrix of a biaxial crystal are proportional to the principal indices of refraction ; the length of the shortest axis is thus expressed by a or n p , that of the mean axis by (3 or n m) and that of the longest by y or n r In the case of a doubly refracting biaxial crystal, there must be one section in which the difference in the velocities of the slow ray and the fast ray, travelling perpendicularly to the faces of the plate, is as great as possible for that particular mineral ; such a section contains the longest and shortest axes of the optical indicatrix. Consequently, the fast ray for this section will be the ray of least refractive index in the crystal as a whole; and its direction of vibration will be that of the shortest axis of the indicatrix, the German a, or the "axis of greatest elasticity" of the older elastic theory. The slow ray for such a section will similarly be the slowest possible ray, or ray of greatest refractive OPTICAL PROPERTIES Of MINERALS. index, and its vibration-direction will agree with the longest axis of the indicatrix, i.e., with the German , or the "axis of least elasticity " of the older elastic theory. In uniaxial crystals i.e., crystals of the HEXAGONAL and TETRAGONAL systems any section parallel to the principal axis (optic axis) of the crystal contains these two directions, one of which corresponds to the principal axis, and the other to any line perpendicular to it ; and such a section is required in describing the optical properties of a mineral of either of these systems. If the principal axis is the vibration-direction for the slowest ray, the crystal is called positive ; if for the fastest ray, it is called negative. In biaxial crystals, the section containing the longest and shortest axes of the indicatrix is always the plane of the optic axes. Two of the axes of the indicatrix bisect the acute and obtuse angles between the optic axes, and are therefore styled the acute and obtuse bisectrices respectively ; they are necessarily perpendicular to one another. If the vibration-direction for the slowest ray bisects the acute angle between the optic axes, the crystal is called positive; if this angle is bisected by the vibration- direction of the fastest ray, the crystal is called negative. In crystals of the RHOMBIC system, the axes of the indicatrix are coincident in direction with the crystallographic axes, and the optic axial plane is parallel to one of the three planes of symmetry of the crystal. The vibration-directions for the slowest and fastest rays correspond to those of the two crystallographic axes that are contained in a section parallel to the optic axial plane. The third axis, perpendicular to the plane of this section, is the French n m , the German Jj, and the " axis of mean elas- ticity " of the older elastic theory. For the section containing J) and a, fr will coincide with the slow-ray vibration-trace ; but for that containing fr and t, Jj will coincide with the fast-ray vibra- tion-trace. In crystals of the MONOCLINIC system, the optic axial plane is either parallel to the plane of symmetry i.e., to the clinopinacoid or is some plane perpendicular to this. In the former case, the bisectrices make angles with the vertical crystallographic axis, which vary for different mineral species. In the latter case, one of the bisectrices, corresponding either to a or t, according to the species, coincides in direction with the orthodiagonal ; the other coincides with the trace of the plane of symmetry. In the TRICLINIC system, there is no relation between the axes of the optical indicatrix and those of crystallographic form. PLEOCHROISM. 89 The terms, as applied to crystal-sections, may now be summed up as follows : Fastest-ray vibration-trace = direction of vibration for rays of least refractive index and greatest velocity, the direction of pro- pagation being perpendicular to the section ; = " axis of greatest elasticity." Symbol a, , or n p . Slowest-ray vibration-trace = direction of vibration for rays of greatest refractive index and least velocity, the direction of pro- pagation being perpendicular to the section ; = " axis of least elasticity." Symbol , 7, or n r It should be remembered that the two latter symbols in each case represent actual numbers, which are the refractive indices for rays vibrating parallel to the directions for which they respectively stand. While the applications of the optical properties of minerals will be dealt with in connection with the microscope (pp. 141 to 153), yet one important property may be treated here, since it can be observed in ordinary specimens, without the use of sections. This is the phenomenon of pleochroism. Pleochroism. Several minerals which are coloured and yet fairly transparent exhibit their pleochroism in ordinary crystals. When held up between the eye and the light, either in the fingers or cemented to a little stick, and turned about in various directions, a change of tint may be perceptible according to the direction in which the light traverses the crystal. Yivianite and transparent andalusite can easily be examined in this way. The extreme colours thus observed are the " face-colours " (Fldchenfarben) of Haidinger ; but the " axis-colours " (Axen- farben} prove with more certainty that a mineral is pleochroic and enable one to correctly describe its characters. The colour of any face is, in fact;, compounded of the colours of two groups of rays into which the light entering the crystal has been divided. Brewster* in 1819 passed polarised light through a number of specimens, noting the extreme differences of tint pro- duced by bringing different directions in the crystal parallel to " the plane of primitive polarisation." This is the method now made use of in observing the axis-colours of minerals in micro- scopic sections (see p, 143). Haidinger, f however, by his Dichroscope (dichroskopische Loupe), made both the axis-colours * Phil. Trans. Roy. Soc., 1819, p. 11. \-Abhandl. bdhm. Gesell. der Wissenschaften, v. Folge, Band 3 (1844); reprinted in Poggendorff'sAnnalen, Bd. Ixi., p. 302. See also Pogg. Ann, t Ixv. (1845), p. 4. 90 PLEOCHROISM. visible at once. This instrument can be easily obtained, is very portable, and proves in many cases valuable in the examination of transparent minerals, such as gems. A brass tube, some 6 cm. long, encloses a cleavage-prism of calcite, the longer edges parallel to the axis of the tube. On each end of the calcite is cemented a small glass prism of about 18, which makes the terminal surfaces vertical and prevents the rays from being so strongly refracted as to necessitate a stouter instrument. Prof. Church states that as an alternative method the ends of the calcite may be cut off perpendicular to the longer edges. At one end of the tube is a magnifying lens, at the other a small square aperture, the image of which is sharply seen through the lens. But this image is doubled by the calcite; and, if a pleochroic mineral is held against the aperture, the two squares seen will be of different colours. The mineral must be viewed in some direction other than that of an optic axis, and must be turned about until the maximum difference of colour is observable in the two images of the square, in which case the vibration-traces of the two groups of rays emerging from the crystal-face are parallel to those of the calcite rhombohedron. Consequently the rays of one group are already vibrating parallel to one of the planes into which the doubly-refracting calcite would tend to bring them ; these rays therefore are not doubly refracted by the calcite, but come through to the eye entirely in one of the two images formed. Similarly the other set of rays from the crystal, being at right angles to the first, conies through entirely in the other image formed by the dichroscope. Thus the colours due to each set are completely separated for examination, and this will again occur on rotation of the dichroscope through 90. It will be remembered that uniaxial crystals are dichroic and biaxial are trichroic. Thus, if a prism of beryl or tourmaline is rotated about its longer axis, no change of face-colour will be perceptible to the eye, and, a far surer test, the colours of the two squares seen with the dichroscope remain respectively the same and at their maximum difference. But if a biaxial crystal, such as topaz, is thus examined, changes will take place in the axis-colours as different faces are viewed. Such observations should, however, be conducted on cylinders or on sections of equal thickness, and the complete determination of the character of the pleochroism of a particular mineral is beyond our present aim. But the mere fact that a mineral is pleochroic is often of considerable value. Thus garnet and red spinel may be dia- PLEOCHROISM. 91 tinguished from ruby, minerals of the cubic system being optically similar in all directions, and consequently exhibiting no pleochroism, while ruby gives pink-red and yellow-pink axis-colours. Glass imitations of emerald, ruby, or sapphire may be similarly detected, and this too in cases where the nature of the specimen renders other tests undesirable. Dr. Lang has fitted a cap to the end of the dichroscope that bears the aperture, capable of rotation in various directions. The mineral is fixed by wax to the aperture in this cap, and can thus be oriented in regard to the instrument (Groth, Krystallo- graphie, ed. 3, p. 154). We may note in conclusion that Haidinger's first experiments were conducted with a plain cleavage -prism of calcite, at one end of which a stop with a square opening was placed; and such a prism, costing a few pence, and a card with a hole cut to suit, so that the two images fall side by side, serves all purposes of ordinary observation or demonstration. The dichroscope can now be obtained from London mineral-dealers or opticians, at prices varying from about 15 to 20 shillings. PART III. THE EXAMINATION OF ROCKS. '* As for the earth, underneath it is turned up as it were by tire. The stones thereof are the place of sapphires, and it hath dust of gold." The Book of Job. " No arrangement can pretend to define and separate those objects which the hand of nature has neither denned nor separated. " JOHN MACCULLOCH, A Geological Classification of Rocks. 1821. CHAPTER X. INTRODUCTORY. WHILE a mineral may be to a large extent discussed and determined in the laboratory, a rock, considered as a part of the earth's crust, and not as a mere aggregate of chemical compounds, requires a very full knowledge of its mode of occurrence before it can be properly treated of and described. In fact, after a study of a number of type-specimens, the student is recommended to go out to some well-described district, and to endeavour to recognise the varieties of igneous and sedimentary rocks by careful observation in the field. In this way alone can he appreciate the various modes of weathering, the massive or minuter structures due to jointing, the smooth or rugged outlines, that characterise the masses of which his hand-specimens form a part. He will meet with many difficulties of determina- tion, and will procure a store of well-selected material on which to work during less propitious days. Questions will arise, even during microscopic examination, that will send him back to gain further information in the field ; and in the end his investigations will have far more geological value to him than any knowledge gathered in type-collections or museums. The notes that follow presuppose that the specimens have been collected in the field; that at any rate something is known about- THE STUDY OF ROCKS. 93 their mode of occurrence and their relation to other parts of the same mass. Weathered specimens should be avoided as a rule, but often reveal structures hidden in the unaltered portions. Collections made from stream-beds or taluses are often useful for showing the general character of a district ; but rocks so gathered are seldom of value for detailed study. Nothing short of striking the rock-mass in situ with the hammer, and taking in with the eye its position and surroundings, even to the broader features of the landscape, should content the geologist who would follow worthily the founders and masters of the science. The points of interest presented by various types of rock will be dealt with later. Broadly speaking, in the case of sedimentary rocks specimens should be collected showing weathered surfaces and also freshly exposed bedding-planes, since minute structures, fossils, f service to remind the reader of the successive operations performed during a simple rock analysis, such as would suffice for broad determinative purposes. Naturally the list of substances that might be looked for and separately estimated in an elabor- ate analysis of material from the earth's crust is as long as that of the known chemical elements ', but the proportions in which the below-mentioned oxides occur are often of fundamental geological importance. Unless, however, such substances as manganese, titanium, barium, (fee., are separately determined, the analysis must be regarded as only approximate, and as serving for classificatory purposes rather than for refined discussion. This is clear from the detailed papers by Messrs. Clarke and Hillebrand, which should be in the hands of all who would analyse silicates. (Bull. U.S. Geol.l Survey, No. 176, and the. complete revision by Hillebrand, Bulletin, No. 305, issued in 1907. See also Washington, Manual of the Chemical Analysis of Rocks, 1904, Wiley & Sons, New Yoik.) CHEMICAL EXAMINATION OF ROCKS. 107 SUMMARY OF DETERMINATIVE CHEMICAL ANALYSIS OF A ROCK. 1. Loss on Heating. Weigh about 1 gramme of the air-dried powdered rock in a platinum crucible, dry in an air-bath at 100 to 110 0., cool in a desiccator, and weigh again. Determine thus the loss of moisture, which may be due in part to combined water, as in zeolites. Then ignite strongly over a gas blowpipe, and weigh again. Ignite a second time and weigh, repeating this until the weight is constant. The difference thus found is due to loss on ignition, which generally represents water. Where much iron is present, its oxidation may affect this result. Where it is necessary to determine carbon dioxide, a sample of the powder must be decomposed by acid in an apparatus in which either the gas evolved is all-owed to escape and is deter- mined by loss, or in which it is collected in an absorption- tube by soda-lime and weighed. (See Hillebrand, op. cit., p. 150.) 2. Silica. Add to the ignited powder in the crucible, or to a fresh sample if the heating has caused it to fuse or frit together, about four times its weight of sodium carbonate, mixing carefully and very thoroughly with a rod or platinum spatula. Fuse at first over a Bunsen-burner, the lid of the crucible being kept on, and avoiding too great heat at the outset. Then apply the blowpipe until the whole mass runs freely together and ebullition ceases. The flame should be directed obliquely, and should not envelope the whole crucible. Remove and stand the crucible on a cool surface, such as an iron plate, so that the fused mass may crack away from the wall of the crucible. Place in a porcelain or platinum dish with hydrochloric acid and water, covering quickly with a clock-glass to avoid loss by effervescence of the carbonates. Warm, and allow to stand until decomposition is complete. Evaporate to approximate dryness in a water-bath (Hillebrand, op. cit., p. 76). Moisten again with strong hydrochloric acid, ado! water, and warm. The silica should now float about lightly in the liquid when Stirred, while all the bases are in solution. Filter off the silica ; evaporate the filtrate, treat as before, and add the small quantity of silica thus obtained to that already in the filter. Ignite for about twenty minutes, and weigh. If gritty matter occurs amid the silica, the fusion has not been satisfactory, and the process must be begun again. 3. Alumina and Ferric Oxide. Add to the filtrate a few drops of nitric acid, in order to ensure the conversion of ferrous to ferric salts. Then add ammonium chloride, and ammonia in very slight excess, and boil. Filter off the precipitate of alumina 108 CHEMICAL EXAMINATION OF ROCKS. and ferric oxide, obtaining the nitrate a. When thoroughly washed, re-dissolve the precipitate into another vessel, and divide the subsidiary nitrate thus obtained into two measured quantities. Thus it may be made up to half a litre by dilution in a marked flask, and 250 cc. may be drawn off with a pipette. In this portion precipitate alumina and ferric oxide as before ; filter, ignite, and weigh. Draw off 100 cc. from the portion remaining in the flask, and determine the iron in this, reduced to the ferrous state, volumetrically by means of permanganate of potash . Make a check-determination by drawing off another 50 or 100 cc. Divide the weight of iron found by "7, which will give the weight of ferric oxide. Deduct this from the joint oxides, the alumina being thus found by difference. Ferrous and ferric oxides must be separately determined in all exact analyses. (See especially Hillebrand, op. cit., p. 131.) 4. Lime. Heat the original nitrate a nearly to boiling, add ammonia in excess, and then excess of ammonium oxalate. Allow to stand for 12 hours. Filter, and ignite strongly ; weigh, and repeat till the weight is constant. The precipitate is thus converted into lime. 5. Magnesia. Ammonia being in excess, add hydric disodic phosphate to the nitrate, stirring very carefully with a rod, since the precipitate clings to any parts of the beaker that may have been in the least degree abraded by touching. Stand for 12 hours and filter cold. Wash the precipitate with a mixture of 1 part ammonia (sp. gr. *88) and 3 water, and ignite, the filter being burnt separately in the lid of the crucible. Where a large quantity of magnesia is expected, a porcelain crucible should be used, to avoid injury to the platinum. The ignited precipitate is the pyrophosphate (Mg 2 P 2 O 7 ). To estimate as magnesia, multiply by -36036. 6. Potash and Soda. These alkalies are best determined by the Lawrence-Smith method. Mix intimately 1 part of the powdered rock (about half a gramme) with one part of ammonium chloride and 8 parts of pure calcium carbonate. Heat for about an hour in a deep platinum crucible, which is best supported almost horizontally over a flat-sided Bunsen-flame, and under a conical iron shield. The flame must be applied very gradually at first to avoid rapid volatilisation of the ammonium chloride, and the temperature should at no time rise above dull redness. The decomposition is effected without complete fusion. Dissolve out the fritted mass in water in a dish and filter. The filtrate contains the metals of the alkalies in the form of chlorides, with some portion of the materials used in decomposition. CHEMICAL EXAMINATION OK ROCKS. 109 Precipitate the lime from the filtrate by ammonium carbonate j filter and evaporate down, testing the filtrate as it becomes more concentrated with a drop or two of ammonium carbonate solution. If lime is still present, precipitate it and filter again. Evaporate to dryness in a small dish, and gently drive off by further heating the ammonium chloride and ammonium carbonate. A dark stain may appear, which is due to impurities in the ammonium carbonate, and may be neglected. Excessive heat must be avoided, lest a portion of the chlorides of the alkali- metals should be lost. Weigh the joint chlorides in the dish while the latter is still slightly warm. Dissolve up in water, add platinic chloride, and evaporate almost to dryness on a water-bath. Add alcohol, and allow to stand for some hours, the precipitate of potassium platinio chloride being insoluble in alcohol. Filter on to a weighed filter, -wash with alcohol, and dry at 100. Weigh with the filter without ignition. To calculate this precipitate as potash, multiply by *19272. Divide this result by '63173, which gives the weight of the potassium chloride in the joint chlorides. Deduct this from the joint weight and multiply the remainder by -53022. This gives the weight of soda. 110 ISOLATION OP THE CONSTITUENTS OP ROCKS. CHAPTER XIV. THK ISOLATION OP THE CONSTITUENTS OP ROCKS. IN the case of a coarse-grained rock, clearly composed of hetero- geneous materials, it is not difficult to break out with the hammer or the pliers fragments or crystals of individual con- stituents, which can then be submitted to special tests. It is, however, highly desirable that a microscopic section should have been previously prepared, in order that the purity of the crystals which are to be examined, and their freedom from enclosures, may be satisfactorily ascertained. This precaution is especially neces- sary when chemical or microchemical tests are about to be applied. Where the selected grains are small and translucent, examination of them when mounted in water under the microscope will often assure the observer of their purity or the reverse. Many sedimentary rocks, such as sandstones, can be broken up with the pliers or even with the fingers, and the grains spread out on paper for identification. Other rocks, such as clays, may be broken up after prolonged treatment in water, the materials of varying fineness being successively washed off into separate vessels, and an often valuable residue of larger grains, small fossils, the bisectrix which emerges in the field, and the thinner arm of the black cross that can be formed by them lies along the trace of the optic axial plane. This occurrence of two dark curves sweeping across the field and uniting at every 90 of rotation to form a cross is one of the best features by which biaxial crystals can be determined. In default of so good a figure, the curvature of the single bars that come into view must be noted, in opposition to the permanent straightness of those of uniaxial crystals. The typical figure given by biaxial minerals may be well studied in muscovite, and, commencing with a thick piece, the specimen should be thinned down until the hyperbolae are accompanied by the merest trace of coloured rings. In certain special cases, finally, where it is known that the section is perpendicular to the acute bisectrix, the optical sign of the crystal can be simply determined. The trace of the optic axial plane is set at 45 to the diagonals of the nicols ; since it is one of the vibration-traces of the crystal-section, determine with the quartz wedge in plane polarised light whether it corresponds to the ray of greatest or least velocity (sea p. 147). If greatest, that is to say, if compensation occurs when the wedge is thrust along this direction, then the vibration-trace perpendicular to the optic axial plane is that of the slow ray in the section. Since this latter direction is always the vibration- direction of the ray of mean velocity in the crystal as a whole, the acute bisectrix, the normal to the section, must be the vibration-direction for rays of least velocity in the crystal, which is, therefore, positive. If the experiment gives a reverse result, the crystal is negative. 154 ROCK-FORMI.NG MINERALS. CHAPTER XVII. THE CHARACTERS OP THE CHIEF ROCK-FORMING MINERALS IN THE ROCK-MASS AND UNDER THE MICROSCOPE. UNDER this heading are given the characters presented by common minerals as they occur in rocks, the order followed being alphabetical. The minerals of first importance are printed in thick type. Each description is divided into two parts : I. The most striking characters of the mineral as it appears embedded in the rock-specimen, with one or two additional notes. II. The characters it exhibits in microscopic sections. Many of these characters can of course be observed in isolated grains (p. 132). The abbreviations used are as follows : Comp. Chemical composition. Syst. Crystallographic system. Form. Ordinary form, or outlines in sections. Cleav. Cleavage. End. Enclosures. Zon. Zone - structure. Refr. Index. Average index of refraction (determined with yellow light). The figures are mostly those given by Le"vy and Lacroix, Tableaux des Mineraux des Roches. Colour. Colour as seen in ordinary light. Variations according to the face viewed (face- colours). Appearance of alteration-products. Pl eo . Pleochroism as observed by means of the single nicol (axis- colours). D. Refr. Double refraction. This is "strong" when the difference between the greatest and least index of refraction in the crystal is large (say -040), and "weak" when this difference is small (say '005). In the former case we have "high " colours, in the latter " low." Extinct. Positions of extinction. Opt. sign. Optical sign ; including character of the acute bisectrix, and its position. Twins. Characteristic twinning. ROCK-FORMING MINERALS. 155 Actinolite. A non-aluminous amphibole. Special points: T. Long prisms, distinctly green; often in radial bunches. II. Colour Pale green to colourless. Pleo. Faint tints of green. Note. See Hornblende and Tremolite. . See Soda-Pyroxenes. Albite. See Plagioclases. A MBLYSTEGITE. See Ebombic Pyroxenes. AMETHYST. Like quartz. Special points : I. Yiolet colour; occurs in cavities of rocks. II. Pleochroic when the section is thick enough for the colour to appear. Amphiboles. See Actinolite, Anthophyllite, Hornblende, Soda-Amphiboles, Tremolite. ANALCIME. Comp. Some varieties = Na Al Si 2 6 + H 2 O. Syst. Probably cubic. I. In cavities of rocks ; colourless glassy-looking or opaque white icositetrahedra, commonly in groups. II. Refr. Index. Near leucite (1-487). D. Refr. Sometimes anomalous, in grey and greyish- white tints; sometimes forms an isotropic ground-mass between other minerals. Note. Fuses easily, and gelatinises in HC1. Compare leucite, which it resembles externally. If the substance is transparent in the mass, it is very probably analcime rather than leucite. Analcime may occur as a decomposition-product of nepheline. ANATASE. Comp. Ti O 2 . Syst. Tetragonal. I. Occurs as brilliant blue-black to black modified bipyramids, which, though commonly about 3 mm. long, catch the eye by their lustre on the surfaces of rocks. Is found sometimes on dis- solving limestones or extracting the heavy minerals from sands. II. Refr. Index Very high (2-52). ANDALUSITE. Comp. A1 2 Si O 5 . Syst. Rhombic. I. Sometimes seen as well-marked grey or pink prisms in schistose rocks, nearly square in section, or merely rod-like and poorly bounded (specific gravity = 3-18). II, Form Small granular, as in Cornish granites, to rod-like, as in schists. Refr. Index 1'638. Colour Colourless, or face- colours faintly pink or green. Pleo. Remarkable, from palest 156 BOCK-FORMING MINERALS. green to a brownish or pinkish red. Obscured in the imperfect specimens in schists. D. Refr. -OIL Note. See Chiastolite. Anorthite. See Plagioclases. ANORTHOCLASE. See Microcline. ANTHOPHYLLITE. Comp. (Mg, Fe) Si 3 with A1 2 O 3 at times (GEDRITE). Syst. Rhombic. I. Not known as a common constituent, but may assume importance when closely looked for. Occurs as groups of long prisms in some metamorphic rocks, and among the zones of secondary minerals in some altered gabbros. II. Form and Cleav. Practically the same as hornblende, but with rhombic symmetry. Colour Colourless in examples at present known. Extinct. Rhombic, i.e., straight, extinctions. Soda-hornblendes and brown ferriferous hornblendes have, how- ever, only a small angle of extinction. Apatite. Comp. Ca 4 (Ca JF, 01]) (P O 4 ) 3 . Syst. Hexagonal I. Sometimes visible as yellowish-white streaks in metamorphic rocks, scratchable with the knife; but, despite its abundance, commonly too small for detection with the eye. II. Form Long prism, giving hexagons and acicular forms. Occurs as minute crystals included in the other minerals of the rock ; rarely in larger prisms. Cleav. Not seen. End. Absent. Refr. Index Higher than felspars (1*637). Colour Colourless; no decomposition-products. D. Refr. Somewhat weaker than felspars. Opt. Sign Negative. (Fig. 27.) Note. Nepheline crystallises in shorter prisms, commonly contains enclosures, and readily decomposes, showing yellow -brown sections. Primary quartz does not occur in prisms in igneous rocks; quartz is positive. Aragonite. Comp. Ca C 3 . Syst. Rhombic. I. Common in the deposits of warm waters (pisolitic grains, &c.) and as a constituent of the shells of many genera. Forms also radial groups in the cavities of altered rocks. Specific gravity = 2*93; calcite = 2-72. Slightly harder than calcite, which it consequently scratches. Michrochemical test, see p. 36. II. Form Prismatic, the compactness of grouping often veiling this in sedimentary rocks. Refr. Index Higher than calcite ( 1 -632). Colour Colourless. D. Refr. Colours like cal- cite. Optic axial plane, the macropinacoid. Extinct. Rhombic extinctions. Opt. sign The vibration-direction for the fastest ray coincides with vertical axis; it is the acute bisectrix, and ROCK-FORMING MINERALS. 157 the mineral is thus negative. In calcite also the vertical axis is the vibration-direction for the fastest ray. ARFVEDSONITE. See Soda-Amphiboles. Augite. Comp. m(Ca,Mg,Fe)SiO 3 + n(Ca,Mg,Fe)(Al 2 ,Fe 2 )SiO 6 (for constitution, see Groth, Tab. Uebersicht, 1898, p. 146). Kicher than hornblende in Ga. Syst. Monoclinic. I. Black and often short prisms, or granular groups between felspars. Sometimes dark or pale green. Cleavage-surfaces often visible. Slightly scratched with knife. Forms sometimes ophitic masses, and appears as if uniformly infilling the spaces between the felspars, or around granular olivines. II. Form Prismatic, with eight-sided cross-sections, both pinacoids being developed as well as the prism. Angle of the latter 87 (figs. 38 and 40). Vertical sections show the trace of one or both the characteristic half-pyramid planes. Often granular or ophitic. Gleav. Prismatic, showing thus in cross- sections a series of cracks intersecting nearly at right angles. End. Glass and crystalline enclosures fairly common. The schil- lerised forms pass over into diallage and "pseudo-hypersthene." Zon. Common in large examples. Refr. Index 1*72. Colour Typically yellow -brown to purplish - brown. Occasionally pale green. The more strongly green varieties are described under soda-pyroxene. Pleo. Very slight, except in the soda- pyroxenes. D. Refr. Fairly strong, the colours commonly being the pinks, yellows, and greens of the second and third orders. Optic axial plane is the clinopinacoid. Extinct. On clinopinacoid 40 to 50 away from principal axis. Hence typically a large angle as opposed to hornblende. This is reduced in soda- pyroxenes. Opt. sign Positive. The vibration-direction for the fastest ray points towards the obtuse angle that is usually seen in vertical sections i.e., that formed by the traces of a pyramid plane and the orthopinacoid. Twins Fairly common, a number of repetitions arising towards the centre of the crystal, and an untwinned portion occurring on either side. Composi- tion-plane the orthopinacoid. Note. Hornblende very commonly arises as a partial or complete replacement of augite, being developed from it by paramorphic change, the resulting pseudomorph being UBALITB. The outline of the augite is, however, commonly not preserved, actinolitic needles spreading through the mass and projecting from it, or larger hornblendic forms appearing round about it and in most intimate connexion (fig. 27). For allied pyroxenes, see coccolite, diallage, diopside, and soda-pyroxenes. Also rhombic pyroxenes. Bastite. A name at one time applied to the serpentinous and gchillerised pseudomorphs after rhombic pyroxene that often 158 ROCK-FORMING MINERALS. occur in serpentine-rock and sometimes side by side with but slightly altered augite. I. Generally resembles diallage. II. Generally resembles diallage. Colour Pale brown or green, in the latter case often with separated granules of magnetite. Pleo. Fair in the green examples. D. Eefr. Usually shows effects due to the presence of residual unaltered pyroxene. Extinct. Rhombic; parallel thus to the cleavage and schiller-planes in most sections. Biotite. Comp. (H, K) 2 (Mg, Fe) 2 (A1 2 , Fe ? ) (SiO_ 4 ) 3 after Grotti). A typical and common ferro-magnesian mica. For general characters see micas. Special points : I. Commonly dark green or bronze-black. A very abundant constituent of igneous rocks, particularly in the syenite and diorite groups. II. Colour Brown or green ; but sometimes colourless. The striking pleochroism gives dark basal sections (i.e., those showing no cleavage), and far lighter vertical sections. The latter are very often straw-yellow, the former reddish-brown. Decomposes to green chloritic products. Pleo. Intense in vertical sections, often yellow-brown to grey-brown; darkest tint when shorter diagonal of nicol is parallel to basal cleavage. Not perceptible in basal sections, the typical mineral being practically uniaxial. D. Refr. Strong (-058). Colours seen only in very thin sections. Basal sections practically isotropic. Convergent light gives a nearly uniaxial figure. (Figs. 24 and 27.) Even when the section is in a position of extinction, light spots usually appear in it, owing to local bending of the mineral during grinding (H. S. Jevons, Geol Mag., 1893, p. 82). Note. Distinguished in rock from hornblende by lustre, platy character, and hardness ; in section by single cleavage, ragged fibrous edges, the light spots above mentioned, and the fact that the basal sections are the darkest and show no cleavage. Compare notes on phlogopite. Biotite, like horn- blende, is often altered, by the action of a hot enclosing magma, into mere black skeletal forms. Bronzite. See Rhombic Pyroxenes. Calcite. Comp. Ca C O 3 . Syst. Hexagonal (Trigonal). I. Recognised by its cleavage-surfaces and hardness ( = 3). II. Form. Oval or irregular granules, fitting against one another, in veins or cavities, or forming the mass of a crystalline limestone. Cleav. Rhombohedral, often bent by pressure, giving two or even three series of obliquely intersecting lines, which are very clear, and along which reflections often give rise to inter- ROCK-FORMING MINERALS. 159 ference colours in ordinary light. Refr. Index 1-486 for rays vibrating parallel to optic axis, and 1'658 for those vibrating perpendicular to it. Hence the rapid rotation of the polariser produces a sort of twinkling effect, owing to the difference of "relief" (p. 141) shown by rays vibrating in different directions ; and calcite can often thus be picked out in the preliminary examination of a slide. Colour Colourless. D. Refr. Very strong, so that colours of the fourth and higher orders are alone visible in ordinary slides, the tints being often practically white. Opt. sign Negative. Twins Yery common, repeated, parallel to the negative rhombohedron, x (0112), the traces of their composition-planes running in many sections parallel to the cleavages. (PI. II., fig. 1.) Note. See Dolomite and Aragonite, and fig. 23. For useful micro- chemical reactions, see p. 36. CASSITERITE. Comp. Sn 2 . Syst. Tetragonal. I. In some granites, in orange-brown to black-brown lustrous grains. Can be easily isolated by washing the powdered rock ; test with the blowpipe. II. Form Prisms, squares, and granules. Cleav. Distinct. Zon. Sometimes zones of deeper colour. Refr. Index Higher even than garnet (2 '02 9). Colour Yellow to red-brown, varying in patches in the same grain. Pleo. Conspicuous in the browner parts. D. Refr. Exceptionally strong (*097), but not so strong as in rutile. Pink and green colours of high orders. Opt. sign Positive. Twins Common, geniculated. Chalcedony. Comp. Si O 2 . I. Blue-grey to browner and more flinty aggregates in the hollows of lavas, in limestones, and associated with flint and chert, which are in fact more massive varieties. Forms alternate layers with quartz or opal in agates. Not scratched by the knife. II. Form, Radial aggregates or minute granules. Refr. Index Yery slightly lower than quartz (1-537). Colour Colourless to brownish. D. Refr. Like quartz ; colours brilliant, of about first order. The aggregates consist of delicate fibres, in which the fastest ray vibrates parallel to the long axis. These are not, therefore, prisms of ordinary quartz. CHIASTOLITE. A white variety of andalusite with enclosures of dark amorphous matter regularly arranged. In sections parallel to the vertical axis, bands of this dusky matter may be seen running down the length of the prism ; in transverse sections, a 160 ROCK-FORMING MINERALS. diagonal cross appears, commonly with a dark square at the centre and at each angle of the section. Or the accumulation of foreign matter at the angles may leave an interstitial white cross of the purer mineral towards the centre. The mineral commonly arises in slaty rocks as a product of contact-meta- morphism. Chlorite. The name of a group of minerals composed of silicates of magnesia, ferrous and ferric oxides, and alumina, in various proportions, with much water. Syst. Probably all monoclinic, though many approach the hexagonal system. Similar crystallographic characters occur in the mica group. I. While resembling dark green micas, the chlorites are softer, being very easily scratched with the thumb-nail. Their lamellae are less elastic than those of mica, and show more markedly the effects of injury and pressure. II. Form Hexagonal plates and fibrous-looking, fan-like, or spherulitic aggregates. Often develops from green amorphous masses in the cavities of rocks, or as a pseudomorphic product of ferro-magnesian minerals. Cleav. Basal, distinct. Often dis- torted by pressure. Refr. Index About 1 -6. Colour Yellow- green to blue-green. Pleo. Noticeable in sections showing cleavage; yellowish when the shorter diagonal of the nicol is per- pendicular to the b;isal cleavage, and green when it is parallel to it. D. Refr. Weak (001 to 'OH); colours mostly first order; a deep blue is characteristic. Extinct. In many examples parallel to the cleavage; in some as much as 15 from the vertical axis (clinopinacoidal sections of clinochlore). Note. Compare green biotite and serpentine. CHLORITOID (see Ottrelite). OHROMITE. Comp. (Fe, Or) (Fe 2 , Cr 2 )0 4 (often with MgO and A1 2 O 8 ). Syst. Cubic. I. Black grains and crystals, resembling magnetite, commonly in oli vine-rocks. II. Form Granules, or squares and hexagons, derived from octahedra. Colour Black and opaque unless especially thin, when it becomes a rich claret-brown. D. Refr. None. Isotropic. Note. Compare magnetite. See also spinelloids. OOCCOLITE. A granular ferriferous diopside. Occurs in some crystalline limestones. Colour various shades of green, almost or completely colourless in thin sections. See Augite. CORDIERITE. Comp. H 2 (Mg, Fe) 4 A1 8 Si 10 O 37 (Farringtori). Syst. Rhombic. I. Typical colour a delicate blue, inclining to grey; forms glassy-looking patches in some granitoid and gneissic rocks, and ROCK-FORMING MINERALS. 161 may occur as prisms, though rarely, in lavas. Not scratched by the knife, but easily decomposed and becoming thus much softer. II. Form Mostly irregular grains. End. Fibrous enclosures of sillimanite common. Refr. Index 1*536; hence shows no pitted appearance. Colour Colourless to faint blue. Decomposi- tion products greyish or yellowish-green. Pleo. Though marked in thick specimens, feeble in ordinary sections. Bluish when the shorter diagonal of the nicol is parallel to base ; pale yellow when it is parallel to vertical axis. D. Refr. Colours commonly first order only. Extinct. Rhombic. Note. See Finite. Distinguished from quartz by biaxial character. Diallage. A common form of monoclinic pyroxene. I. Conspicuous by its sub-metallic lustre when the rock is turned about in the hand. The lustrous separation-planes give it a " foliated " character, amounting, indeed, to a new cleavage parallel to the orthopinacoid. It cannot well be distinguished from bronzite, hypersthene, and bastite in the rock-mass. II. Like the pale brown augites, but has, in all sections but those approaching the orthopinacoid, a series of strong striae, which are the traces of planes of schillerisation. End. Numerous brownish secondary enclosures on the separation-planes (fig. 30). Note. Diallage should now be closely linked with augite, crystals of the latter being at times diallagic on the edges. It passes very commonly into hornblende. DIOPSIDE. Comp. MgCa(Si0 3 ) 2 . A pale green scarcely ferriferous monoclinic pyroxene, otben colourless in sections. See Augite ; also Olivine. DIPYRE (see Scapolites). Dolomite. Comp. CaMg(CO 3 ) 2 . Syst. Hexagonal (Tri- gonal) ; almost isomorphous with calcite. I. Occurs in cavities of rocks rich in calcic and magnesic silicates and may easily be mistaken for calcite. Curved faces of the rhombohedron frequent. Forms, in minute or coarser granules, whole masses of "limestone," which may be dis- tinguished from ordinary limestones by higher specific gravity (2-85 about) and action with acids (see pp. 36 and 70). II. Like calcite, but twinning is by no means common in rock- building forms. Well outlined sections of rhombohedral crystals are characteristic in dolomite, but are rare in calcite (PI. II., fig. 2). EL.SEOLITE (see Nepheline). Enstatite (see Rhombic Pyroxenes). Epidote, variety Pistacite. Comp. H Oa 2 (Al, Fe) 3 Si 3 13 - Ca 2 (Al, Fe) 2 (Al O H) (Si O 4 ) 8 . Syst. Monoclinic. I. When in fair-sized crystals or grains, shows the char- 11 162 ROCK-FORMING MINERALS. acteristic yellow-green colour. Occurs often as a decomposition- product in fibrous groups. II. (see fig. 29). Form Prismatic, but the plane of symmetry is perpendicular to the long axis of the crystals. Hence what appear to be cross-sections of the prism, with an angle of 115 27', must be read as bounded by the base and the orthopinacoid. Occurs very often as irregular granules, spreading through the rock where decomposition of lime-silicates has gone on. At other times colourless or coloured little prisms are seen projecting into cavities which have since been filled with pale green chlorite. Cleav. Basal perfect, orthopinacoidal often good. Hence on the rhomboidal sections, which at first suggest a pale hornblende, there are at times two cleavages parallel to the outline; and a slight obliquity in the cutting will make the characteristic angle of epidote agree with that of the hornblende prism. Refr. Index 1*75, or almost as high as that of common garnet. Colour At times colourless, but typically pale yellow or a faint yellow-green in which yellow largely predominates. Pleo. Faint. D. Refr. Stronger than common pyroxenes and amphiboles. Optic axial plane parallel to clinopinacoid and therefore perpendicular to the longer direction of the crystals. Extinct Parallel and perpendicular to the edge formed by the base and orthopinacoid in sections parallel to these planes, and thus "straight" also in almost all the sections that look prismatic. In clinopinacoidal (rhomboid) sections extinction is practically parallel to the trace of the orthopinacoid ; in the rhomboid sec- tions of an amphibole it would occur parallel to the diagonals. Opt. Sign Vibration-direction for the fastest ray is nearly parallel to principal axis and is the acute bisectrix. Twins Occasionally seen ; composition-plane parallel to orthopinacoid. Note. Compare Zoisite. Felspars (see Orthoclase, Microcline, and Plagioclases). FLUOR-SPAR. Comp. Ca F a . Syst. Cubic. I. Occurs in altered rocks, sometimes with tourmaline ; common colour violet, appearing in patches between the other minerals. Hardness = 4. II. Form Sometimes shows defined edges ; generally irregular. Cleav. Octahedral, perfect, the intersections at times suggesting calcite. Zon. Coloured zones occasionally, somewhat imperfect and sporadically developed. Refr. Index 1-433; lower than that of the balsam. Colour Colourless, but often with violet patches irregularly developed. This colour is characteristic in small grains that might otherwise remain undetected. D. Refr. None. Isotropic. ROCK-FORMING MINERALS. 163 Garnet. The name of a group of minerals, the composition of which may be stated as (Ca, Fe, Mg, Mn) 3 (A1 2 , Fe 2 , O 2 ) (Si O 4 ) 3 . Syst. Cubic. I. Commonly-red (Almandine and Pyrope) ; or pale brown (Grossularite). Not scratched with the knife. Forms domin- ated by the rhombic dodecahedron, often rounded. The other minerals of the rock are often bent round the hard resisting garnets, which produce an eye-structure and stand out like knots. The easy fusibility of common garnets is a guide in cases of doubt. II. Form In some lavas sharply outlined, but almost always in ovoid or spheroidal grains, small or large. These, in some " flaser-gabbros," form a zone around decomposing minerals. Cleav. Not always seen ; parallel to rhombic dodecahedron. End. Common, of all kinds, sometimes regularly arranged. Zon. Occasionally seen, as in the coloured zones of melanite (lime-iron-titanium garnet). Refr. Index Exceptionally high, about 1-770. The outlines become thus very strongly marked where they come against most of the other minerals in a slide. Colour Colourless to pink or brown (melanite). Commonlv a pale but unmistakable pink. D. Refr. Isotropic, but with fairly fre- quent anomalous double-refraction, showing grey tints. (Fig. 43.) GEDRITE (see Anthophyllite). GLAUCONITE (see p. 191). Comp. Si 2 , K 2 O, Fe 2 O 3 , H 2 O, with A1 2 O 3 , ifce. GLAUCOPHANE (see Soda-Amphiboles). Graphite. Comp. C. I. Occasionally forms considerable beds and masses. Best recognised by characters stated on p. 71. II. Opaque ; steel-grey by reflected light, resembling granular magnetite. Gypsum. Comp.Ca, S O 4 + 2 H 2 O. Syst. Monoclinic. I. Found in washings of clays ; also as crystalline masses (gyp- sums of the Alps;; alabasters). Scratchable with the thumb-nail. II. Form Commonly seen as little rhomboidal cleavage- flakes. ' The angle between the pyramidal and orthopinacoidal cleavages, which bound these forms, is 113 51', and is often useful for measurement. Cleav. Clinopinacoidal perfect; the two cleavages above mentioned are also developed. Refr. Index 1-524. Colour Colourless. D. Refr. Weak ; beautiful clear colours. Optic axial plane is the clinopinacoid. Twins Fairly common, on orthopinacoid, producing the "arrow-head twin." HAEMATITE. Comp. Fe 2 O 3 . Syst. Hexagonal (Trigonal). In sections shows clear orange-red plates, or granular patches associated with magnetite or decomposing ferriferous minerals. By reflected light characteristic red colour. See Limonite. 164 ROCK-FORMING MINERALS. HAUYNE. Comp. Na 2 Ca(NaS0 4 Al)Al a (SiO 4 ) q . SeeNosean. Syst. Cubic. I. Sometimes recognisable as blue or dark-grey crystals or granules on broken surfaces of lavas. Yitreous lustre when blue and fairly fresh. II. Form Hexagonal or square, resulting mainly from sections of rhombic dodecahedra. Often minute. End. Abundant enclosures grouped in straight lines, often rod-like and at right angles to one another. Under a ^-inch objective these appear like a black cross-hatching, which is particularly marked towards the centre of the crystal. Zon. Often a darker or lighter zone at edge. Where the crystal is corroded, the dark zone often follows the outline and is thus seen to be a phenomenon of alter- ation. Refr. Index About 1*5, or lower than the balsam, but not so low as fluor-spar. Colour Colourless, with grey-blue dusky centre; or blue throughout ; or clear blue towards margin and colourless within. D. Refr. Isotropic; but occasional anomalies, as in leucite, the colours being very low greys. Note. See Nosean. LAPIS-LAZULI appears to be an allied species. Hornblende. Comp. w(Mg, Ca, Fe)SiO 3 + w(Mg, Ca, Fe) (A1. 2 , Fe 2 ) Si 6 (for constitution, see Groth, Tab. Uebersicht, 1898, p. 151). Closely comparable to the monoclinic pyroxenes, but poorer in Ca. Syst. Monoclinic. I. Like augite, but prisms often longer, and of more fibrous aspect. Tends to form in radial bunches in fissures and on joint- surfaces. Common in minutely fibrous and actinolitic forms as a product of paramorphism from pyroxene. SMARAGDITE, which is thus produced, is a bright green, as seen in some gabbros. On many rock-surfaces the form of the cross- sections of the prism (see below) can be clearly seen. II. Form Prismatic, commonly with six-sided cross-sections, the bounding planes being the prism and the clinopinacoids. Prism-angle about 124, the rhombus thus formed being cut off at its acute angles by the traces of the clinopinacoids. Often minute fibrous groups and veins occur. Surrounds altering pyroxenes, and often occurs as patches in them. Cleav. Prismatic; in cross -sections the cleavages commonly show very clearly, the obliquity o^ their angle contrasting with the rectangular cleavages of the pyroxenes (figs. 26, 35). End. Being itself so often a product of alteration, does not pass into schillerised types such as are prevalent among pyroxenes. Zon. Somewhat rare ; a dark zone of alteration sometimes appears at edge, where the crystal ROCK-FORMING MINERALS. 165 has been acted on by a hot matrix, separation of magnetite having taken place. Refr. Index About 1 -65 ; thus shows pitted sur- faces. Colour Pale yellowish to strong brown (varieties with much ferric oxide) ; or pale green to dark bluish-green. SMARAG- DITE is often almost or completely colourless in section; in thicker sections, a clear grass-green. The face-pleochroism of hornblende causes sections parallel to the principal axis to be of a distinctly darker colour than the cross-sections. Pleo. Very marked in the coloured varieties, the axis-colours giving yellow-brown to deep brown or almost black, and pale yellow-green to strong dark green. In vertical sections the maximum coloration occurs when the shorter diagonal of the nicol is parallel to the principal axis. Compare tourmaline and biotite. D. Refr. Fairly strong ; colours like augite. Optic axial plane is the clinopinacoid. Extinct. On clinopinacoid sometimes almost " straight ; " angle with principal axis commonly about 10 or 12, rising to 22. Hence in random prismatic sections typically a small angle, as opposed to that of augite. The longer direction in prismatic sections is practically parallel to the slowest-ray vibration-trace. Twins Fairly common, mostly simple; composition-face parallel to the orthopinacoid. The pleochroism makes their detection easy by means of the single nicol only, the two halves appearing differently tinted. Note. The cleavages in cross-sections form a safe means of distinguishing the amphiboles from the pyroxenes ; the pleochroism is also commonly an excellent guide, but it must be remembered that pale hornblendes cannot be strongly pleochroic, while soda-pyroxenes are so in a fair degree. Note also the common form of the cross-sections. The destructive action of the magma, referred to above under "Zoning," often leaves only black and opaque skeletons of the hornblende crystals (compare Biotite). For allied amphi- boles see actinolite, soda-amphibole, and tremolite. Also anthophyllite. Hypersthene (see Rhombic Pyroxenes). Ilmenite (see Titanic Iron Ore). IRON. Native iron is of very rare occurrence except in meteo- rites. It may be micro-chemically treated by placing a drop of acid solution of cupric sulphate on a grain or section ; if native iron is present, copper will be at once deposited. It also decom- poses the solution of ammonium molybdate used for detection of phosphates, a fine blue precipitate being formed, which does not occur when iron oxides are examined. Metallic iron is whiter and more lustrous than magnetite when viewed with re- flected light under the microscope. Opaque in transmitted light. Iron Pyrites. Comp.Fe S 2 . Syst. Pyrite is cubic (the easily decomposing MARC A SITE is rhombic). I. The cubic form is commonly recognisable with the eye or 166 ROCK-FORMING MINERALS, lens; the hardness ( = 6-5) distinguishes it from pyrrhotine (4) and copper pyrites (3 -5). The colour is also whiter and more brassy. II. Form Mostly squares. Colour Opaque by transmitted light; shows a brass-yellow colour, almost silvery -yellow, by reflected light. Easily thus distinguished from magnetite. Sometimes decomposed to opaque brown or brownish-white pseudomorphs. (PL II., fig. 3.) Kaolin. Comp. H 4 Al 2 Si 2 O 9 Syst. Monoclinic or triclinic. I. The soft white decomposition-product of felspars, often found as a powder between the crystals of granitoid rocks or in the matrix of el vans, &c. II. Form Occasionally shows well-defined hexagonal plates. Colour Colourless. The powdery products occurring in sections of felspars appear opaque white by reflected light. D. Refr. Being extremely thin, the little plates may give low colours, though the double refraction is in reality strong. Extinct. Basal plates are not isotropic ; they have been said to extinguish parallel to lines which are not perpendicular to any of the bounding edges. KYANITE. Comp. A1 2 Si O 5 . Syst. Triclinic. I. Known by its beautiful blue colour and easy macropinacoidal cleavage, the mineral becoming truly lath-shaped on fracture. Sometimes in little blue granules, the colour being more delicate than that of haiiyne. Found in some rocks of metamorphic origin. II. Form Often granular. Cleav. Distinct. Refr. Index High ; slightly above olivine. Colour Colourless, but blue in thick sections. D. Refr. Between felspars and augite. Labradorite (see Plagioclases). Leucite. Comp. (R, Na) Al(SiO 3 ) 2 . Syst. Probably cubic I. Small or large opaque white spheroidal crystals (icositetra- hedra more or less rounded by external action). The small ones appear dull white throughout, but the larger show on fracture a translucent interior with almost a gummy lustre. Hardness very little below that of felspars. At present known almost exclusively in lavas. II. Iform Octagonal or almost circular sections ; several often grouped together. Cleav. Not visible. End. Foreign bodies, such as glass-enclosures, are often grouped regularly in zones or radially, forming sometimes a considerable part of the bulk of the crystal. The smaller leucites in the groundmass of lavas seem particularly to affect this habit. Zon. As above stated, marked by enclosures. Refr. Index 1'508, and thus below that of the balsam. Colour Colourless, but becoming earthy brown where decomposed. D. Refr. The smaller crystals are commonly ROCK-FORMING MINERALS. 167 isotropic; but the larger show a complex system of lamellae crossing one another and light or dark grey in colour. In very thick sections (say 2 or 3 mm.) these lamellae appear brilliantly coloured, like thick glass under stress. (Figs. 37, 40.) Note. Infusible, gives strong potassium flame-reaction (p. 85), and does not gelatinise in H Cl. Compare Analcime. LEUCOXENE (see Titanic Iron Ore). Limonite. Comp. H 6 Fe 4 O 9 . A common earthy brown pro- duct of the alteration of ferriferous minerals. Orange-brown stains around sections of such minerals in altered rocks may be attributed to limonite. Seen commonly as a cementing material in sandstones. Yellowish-brown by reflected light. Magnetite. Comp. Fe 3 4 . Syst. - Cubic. I. Iron-black grains or octahedra, with lustrous fracture-surfaces. Not scratched by knife. Well developed in some chlorite-schists. II. Form Sections of octahedra (squares and hexagons), or mere grains and patches. In the glassy groundmass of rocks forms skeleton-crystals of cross-like patterns, or rods and strings of united crystallites. Occurs also as a product of the decom- position of minerals, when iron is left behind after the removal of other bases. Thus the cracks of olivine are often marked out by magnetite (fig. 31), and the hornblendes of some lavas become dissolved away, leaving a skeletal pseudomorph of magnetite granules. Colour Opaque even in thinnest sections ; steel grey by reflected light. Note. Compare Chromite, Iron pyrites, and Titanic iron ore. MELILITE (see p. 262). Micas. The members of this important group that are most frequently met with have so many characters in common that these are treated together here. Comp. Two broad chemical groups may be formed, the alkali micas and the magnesium-iron micas ; writing the bases in descending order of importance, the micas, of the latter group are silicates of magnesia, alumina, iron and alkalies, while those of the former are silicates of alumina, alkalies, iron, and magnesia. Syst. Probably all are monoclinic! I. The micas appear as lustrous little plates, silvery, bronze- coloured, green, or black, with a most exceptionally good basal cleavage. The knife scratches them easily, producing a very characteristic grating sound, audible even when minute flakes are operated on. The thumb-nail scratches them with difliculty, if at all (compare chlorite). Viewed from the side, the cleavage gives them a lamellar appearance and the characteristic lustre is lost. II. Form Hexagonal basal sections, often mere platy areas JOS ROCK-FORMING MINERALS. with ragged edges. The lamellar character can commonly be seen by examining the margin of such sections with a ^- or J-inch power. Vertical sections are rectangular; but commonly the traces of the prism planes are lost, the edges of the cleavage-planes giving a ragged fibrous boundary, and the traces of the basal planes being on the contrary very sharp. Often bent and de- formed among the other more resisting minerals. Cleav. Basal, exceedingly well marked. End. Dark patches often appear ; some may be true enclosures, while others are developed around colourless enclosures of zircon, &c. Zon. Coloured zones sometimes (but rarely) visible in basal sections. Refr. Index Higher than quartz ( = about 1 -58). Colour Colourless to brown and green. Darkest in basal sections. Biotite decomposes to green chloritic products. Pleo. Very strong in coloured varieties. Darkest tints occur when the cleavage-lines are parallel to the shorter diagonal of the nicol. D. Refr. Exceptionally strong (about '04), being higher than common epidote. The optic axial plane is parallel to the clinopinacoid or the orthopinacoid ; the basal sections (or, better still, cleavage-flakes taken from the rock-specimen direct) show admirable figures with convergent light, the optic axial angle being to about 70. Extinct. Vertical sections extinguish perpendicular and parallel to the basal cleavage, the minute deviations from this rule not being recognisable in rock-sections. Opt. Sign Negative ; the principal axis is practically the vibration-direction for the fastest ray, the deviation of the acute bisectrix from it being inappreciable. Note. See Biotite, Muscovite, and Phlogopite. In cases of doubt it is best to speak merely of "dark mica" or "light mica" until better tests can be applied. MICROCLINE and SODA-MICROCLINE (ANORTHOCLASE). Comp. Like orthoclase and soda-orthoclase. Syst. Triclinic; pseudo- monoclinic. I. The common felspar of graphic granite. On its surfaces the lens generally reveals a structure of opaque little whitish rods crossing at right angles and alternating with somewhat more translucent areas. Flesh-red, yellowish, or green (Amazon- stone). G = 2-57, to 2-60 in soda-microcline. II. Resembles orthoclase, but shows with crossed nicols a more or less defined system of repeated twinning, the minute lamellar components crossing one another nearly at right angles and producing a coloured mesh-work.* As Rosenbusch shows, * Sabersky's ingenious explanation of this effect (Neues Jahrb.fur Min. t Beilage Bd. vii., 1891, p. 360), as due to the crossing of the lamellee of only one set of twin-components, those of the albite type, is difficult to verify, but must clearly be taken into consideration. EOCK-FORMING MINERALS. 169 if one group be set upright in the field, and rotated until ex- tinction occurs in one of the sets of lamellae composing it, one set from the group lying perpendicular to the first is, in most sections, simultaneously extinguished. Black bands crossing one another at right angles are thus conspicuous in the field. In the zone parallel to the microdiagonal, the maximum angle of extinction for the albite-type of lamellae is 18. In basal cleavage-plates, extinction occurs at 15J from the trace of the pinacoidal cleavage (at in orthoclase). Refr. Index 1-526. (See Plagioclases, and fig. 25.) Muscovite. Comp. H 2 KA1 8 ( Si (X) 3 (after Grotti). Atypical and common alumina-alkali mica. For general characters, see Micas. Special points : I. Commonly light-coloured and silvery, but approaching black as the crystals become thicker. Very common in mica-schists (often with SERICITB and other varieties) ; also in many granites. II. Colour Colourless or palest yellow-brown. Pleo. Visible if the mica shows even a trace of colour. D. Refr. Strong (041). Colours commonly pinks and greens of third and fourth orders, or high white. Sections parallel to the basal cleavage are not, as in typical biotite, practically isotropic. This point may be conveniently observed in cleavage-flakes. Optic axial angle wide (50 to 70), the figure with convergent light being a striking one and easily obtained with fairly thick cleavage-plates. Optic axial plane perpendicular to clinopinacoid. The trace of the optic axial plane, which is easily found by the axial figure, is the vibration- direction of the slow ray in the cleavage-flake. (Figs. 24 and 43.) Note. Often occurs in minute forms in altered potash-felspar. SBBICITE is covered by the above description. See Micas, Biotite, and Phlogopite. NATROLITE. Comp. Na 2 Al 2 Si 3 O 10 + 2 H 2 0. Syst. Rhombic. I. A common product of the decomposition of such silicates as nepheline, soda-felspars, 5-10 3-51 99-99 B. 69-64 17-35 1-04 1-97 trace 1-40 0-21 4-08 3-51 0-72 99-92 c. 69-95 13-32 4-90 f-79 0-66 3-47 3-31 1-27 095 99-62 Fi II. While the general appearance of the constituents under the microscope is irregular and allotriomorphic, the felspars often preserve their prismatic forms, and the mica and horn- blende show still better traces of bounding planes. The quartz is, however, granular, and commonly abounds in liquid enclosures. The ground may be composed of micropeg- matitic quartz and felspar. Two or three sections may be required to determine the relative proportions of potash- felspar and soda-lime varieties. A large series of rocks known familiarly as granites must be passed over to the quartz- diorites, having, indeed, but little potash in their composition, together with a deficiency in silica. Where muscovite is present, orthoclase may naturally be expected to be predominant. Magnetite is not conspicuous. Zircon and apatite are common, as minerals of early development in the rock; the minute prisms of zircon are best seen where included in the micas. Sphene and garnet are frequent accessories. By alteration, tourmaline and topaz come in, the former replacing various silicates, and the latter representing in particular the felspars. Fluor-spar and secondary quartz, often with good outlines, accompany these changes. Varieties of Granite. GRANITE WITH MUCH PLAGIOCLASE, and witk biotite or hornblende. This is the "Granitite" of G. Rose; Rosenbusch, however, uses "granitite" for any biotite-granite. See analysis C. nv 24. Granite. Near Dublin, x 12. 6, Dark Mica, with deep brown patches included. m t Muscovite. Near the top of the field a hexagonal (basal) sec- tion occurs, o, Orthoclase. g, Quartz. HOLOCRYSTALLINE IGNEOUS ROCKS. 219 TOURMALINE-GRANITE. The smaller constituents become en- tirely replaced by tourmaline, blue or brown in section, and by secondary quartz, the porphyritic felspars often remaining only slightly attacked. The Cornish " Luxwlyanite " is one variety of such altered granites, and quartz-schorl-rock is a final stage. GREISEN is a somewhat uncommon ally of granite, in which felspar is absent. Topaz, however, abounds in the typical rock from Zinnwald, Saxony, and probably represents altered alum- inous silicates. The other constituents are quartz and a pale mica, often a lithia-bearing species. APLITE (Retz ; more correctly HAPLITE ; ALASKITE of Spurr, 20th Ann. Hep. U.S. Survey, p. 189). Practically composed of quartz and orthoclase only. Commonly pale in colour and fine- grained ; often micropegmatitic. Flakes of muscovite glimmer here and there. Occurs often as veins in granite, and even forms considerable rock-masses. The silica rises to at least 76 per cent. GRAPHIC GRANITE (PEGMATITE of Haiiy*). The later and er- roneous extension of Haiiy's term to any coarse nmscovite-granite occurring in veins necessitates a return to the descriptive name "graphic granite." I. The rock is commonly a coarse aplite, mica occurring here and there in nests and bun- ches, being excluded from the parts that exhibit the typical structure. To the eye the continuous cleav- ages of the felspar are easily apparent, the quartz being apparently in detached fragments resembling east- ern characters, embedded in the more opaque fel- spar(-7r^y / aara). Sometimes hand-specimens of the rock cleave as if composed mere- ly of coarsely developed fel- spar. The lens often shows a microcline structure. II. The felspar is usually microcline, showing the cross-twinning (fig. 25, a); the section should be thinly ground. The quartz a Fig. 25. a, Graphic Granite. Vein in Stanner Rock, Herefordshire, x 8. Nicols crossed. Clear quartz; micro- cline with characteristic twinning. 6, Micropegmatitic intergrowth of quartz and felspar, in Eurite. Stanner Rock, Herefordshire, x 160. Nicols crossed. The felspar is in a position of extinction. Other micropegma- titic areas and felspar crystals lie around. * Traite de Mineralogie, 2nde edit., tome iv., p. 436. 220 HOLOCRYSTALLINE IGNEOUS ROCKS. between crossed nicols appears optically continuous over con- siderable areas, being in fact intergrown in large crystals with the felspar. The hook-like forms of the quartz come out distinctly if the surrounding felspar is placed in a position of extinction. ORBICULAR GRANITE. A rare form, in which the minerals are grouped into bold crystalline spheroidal aggregates, usually coated outwardly with dark mica.* The relations of the spher- oidal part to the mass of the granite, and any facts bearing on the origin of the structure, should be carefully noted in the field. (See p. 102.) Eurite. This name, given by d'Aubuisson f in 1819, seems to cover admirably by its original definition the fine-grained and compact forms of granite, known commonly in England as " Quartz-Felsite " and on the Continent as " Mierogranulite," "Quartz-Porphyry," &c. The old "petrosilex" and most of the "compact felspars" and "hornstones" must come under eurite; also the Cornish "elvans" or "elvanites." "Felsite" is so loosely defined by its originator, and is so differently used by different writers, that its reputation as a rock-name is lost. It must be remembered that the " quartz-porphyries " of the Continent are in large part altered rhyolites; those divisions described by Rosenbusch as "microgranites" and " granophyres " correspond, however, to eurite. Structure. Essentially compact in appearance, and micro- granitic with the lens. Commonly with porphyritic orthoclase and quartz. Constituents Like granite. I. Colour pale as a rule, but occasionally deep grey or red- brown. Commonly yellowish or pinkish-brown, or pale grey. The knife scratches fresh specimens with difficulty, and the most compact varieties resemble flint. The joints are clean, and the surfaces often have hard white decomposition-crusts. Without the microscope it is impossible to separate hand-specimens of true eurites from acid lavas that have become holocrystalline by secondary devitrification. The porphyritic quartz that is so often seen is a good guide as to the highly siliceous character of the rock. It sometimes occurs in well-bounded double pyramids with a short prism, as in the rock of Auersberg in the Harz. Tourmaline appears sometimes in radial nests as a secondary product. * For an account of one of these rocks see F. H. Hatch, " On the spheroid- bearing granite of Mullaghderg," Quart. Journ. Geol. Soc., vol. xliv. (1888), p. 548. t TraiM de Geognosie, tome ii., p. 117. HOLOCRYSTALLINE IGNEOUS ROCKS. 221 Specific Gravity. About 2 -65. Typical Analyses. Theoretically, every granite should have its corresponding eurite. The proportion of soda often links these rocks with the quartz-apbanites. A. Fine-grained "Elvan," Mellanear, Cornwall. J. A. Phillips, Quart. Journ. Geol. Soc., vol. xxxi. (1875), p. 335. Mica. B. Quartz-porphyry, Triberg, Baden. G. H. Williams, Neues Ja' rb. fur Min. die., Beilage Bd. ii. (1883), p. 609. C. Soda-Eurite. Llyn-y-Gader, Cader Idris. Holland, Quart. Journ. Geol. &oc., 1889, p. 435. SiO 2 A1 2 3 MnO CaO MgO K 2 Na^O H 2 71-46 15-38 0-30 2-27 trace 0.47 0-22 5-51 2-79 1-70 100-10 77-68 12-95 0-96 0-37 6*30 0-21 4-37 3-18 0-71 100-73 C. 72-79 13-77 3-32 trace 1-94 0-62 2-99 4-12 1-08 100-63 II. The groundmass is microgranitic and often micropeg- matitic. The quartz and felspar are not unfrequently grouped in micropegmatitic intergrowths of spherulitic form around the porphyritic crystals ("granophyre " of Rosenbusch) fig. 25, 6. All stages appear to exist between these aggregations arid spherulites with rays composed of crystalline fibres. Some types of altered spherulites in devitrified lavas are, again, in- distinguishable from these holocrystalline aggregates in the compacter eurites ; but, as a rule, the structures are distinct. Careful observation will show that where such an aggregate- growth occurs round a quartz crystal, the quartz of the micropegmatite is in optical continuity with that of the crystal. The porphyritic crystals, even of quartz, are often well bounded, though sometimes cracked and broken by movement, or corroded by the molten ground. A fluidal structure is occasionally set up. The ferro-magnesian constituent is commonly biotite, often giving wisp-like yellowish sections. Muscovite occurs in many Cornish "elvans," and forms at times little fan-like bunches; 222 HOLOCRYSTALLINE IGNEOUS ROCKS. but neither mineral is prominent, and the rock has a marked aplitic tendency. Varieties of Eurite. EURITE RICH IN SODA (SoDA-EuRiTE). This includes very many of the micropegmatitic types above referred to ; many of the porphyritic felspars are albite or oligo- clase, and the rock is linked thus with the quartz-aphanites. Rosenbusch's "Quartz-Keratophyres" come here. See analysis C. II. SYENITE AND COMPACT SYENITE GROUP. The complete or almost complete absence of free quartz, and the predominance of orthoclase, are the distinctive characters of this group. Syenite (Werner). Structure Granitic. Constituents 1, Oithoclase, or other potash felspar; 2, Amphibole, Pyroxene (usually segirine), or Mica. Commonly some quartz ; also albite or oligoclase. I. This rock is rare as compared with granite and quartz- diorite, and it must often be a matter of opinion as to how much quartz is permissible in a true syenite. With the older writers the term was synonymous with hornblende-granite. The Mica-Syenites (or " Minettes ") contain biotite, often in abundance, and the dark lustrous plates conceal the felspar in the fine-grained varieties, so that the flame-test or the microscope must be brought to bear to distinguish the rock from mica-diorite. Sphene may often be recognised, occurring as small hard yellow crystals. Specific Gravity. About 275 or somewhat higher. Typical Analyses. A. Hornblende-Syenite. Plauenscher Grund, Dres- den. Washington, Am. J. Sci., vol. xxii. (1906), p. 132. Average of analyses. Typical rock of Werner. B. Hornblende - Syenite. Follmersdorf , Silesia. H. Traube, Neues Jahrb., 1890, Bd. i., p. 212. Specific gravity, 2-86. C. Elgeolite-Syenite. Litchfield, Maine. Bull. U.S. Geol. Surv., No. 148, p. 65. With mica and sodalite. A. 60-60 1679 2-77 2-17 Si0 2 A1 2 3 MnO CaO MgO KoO Na 2 Loss on Other cc ignit >nstit on uents 4-47 2-14 4-57 4-40 0-86 1-33 100-10 B. 65-63 13-85 2-02 2-80 trace 3-43 2-79 6-25 1-84 1-17 9978 C. 60-39 22-57 0-42 226 0-08 0-32 0-13 4-77 8-44 0-57 99-95 HOLOCRYSTALLINE IGNEOUS ROCKS. 223 II. Similar to granite, but the quartz must be insignificant or absent. In some cases traces of pyroxene remain, the hornblende having arisen by par am orphic change ; these residual crystals appear as pale and usually greenish areas surrounded by irregu- lar zones of horn- , Sng entTrel S y C of augite and orthoclase, though certainly rare, must be classed as Augite-Syenite, and such masses probably underlie many tra- chytic volcanic areas, being altered into ordinary hornblendic types by the time that they are exposed by earth -movement and denudation. Zircon and sphene are particularly com- mon in syenites. Varieties of Syenite. NEPHELINE-SYENITE (EL^EOLITE-SYENITE). I. The nepheline, in the coarse elseolite form, resembles brownish or greenish quartz, but may be distinguished by the knife. The varieties with hornblende have been called " Foyaite" from Foya in Algarve, and those with mica " Miascite " from Miask in the Urals; but the well-known examples from the Bamle area are rich in soda-pyroxene. Zircon is common, and forms large yellow crystals in the coarse biotite-nepheline-syenite of Miask. Blue sodalite occurs in a Transylvanian variety ("Ditroite"), and in a similar rock in the Ice River Valley, Canadian Rocky Mountains. A type allied to these rocks, but of distinctly basic character, has been described by Prof. Lawson as "Malignite" (Bull. Geol., Univ. of California, vol. i., p. 337); it consists of nepheline, soda-pyroxene, and apatite, with orthoclase enclosing them ophitically. Lawson regards this rock as the plutonic repre- Fig. 26. Syenite. Plauenscher Grund, Dres- den, x 8. h, Green hornblende, o, Orthoclase, fairly prismatic in habit, g, Accessory inter- stitial quartz, sp, Sphene, marked out by its high refractive index and lozenge - shaped sections. 224 HOLOCRYSTALLINB IGNEOUS ROCKS. sentative of the leucitites of Vesuvius; in both the silica is about 48 per cent. II. Some practice, and the observation of the uniaxial figure with convergent light, is needed to detect the irregu- lar grains of nepheline (elseolite) in these granitic rocks. The felspars often show a microcline structure, and are sodic types. Soda -lime felspars are common. The ferro- magnesian silicates are interesting, being, in addition to biotite, forms of soda-amphibole and soda-pyroxene, the former being rich dark-brown or green and highly pleochroic, ap- proaching arfvedsonite, while the latter, distinguished by its cleavages, approaches segirine, and is green and less pleo- chroic. Compact Syenite. We may use this name for the fine-grained types corresponding to eurite, in the absence of any well-defined term. The " Orthophyre " of Coquand* comes partly here, partly with eurite ; also many " felsites." " Orthoclase-porphyry " has been used for porphyritic forms. Structure. Microgranitic or microcrystalline. Sometimes with porphyritic orthoclase. Constituents^ Like syenite. I. These rocks are difficult to distinguish from eurite with the eye, though more yielding to the knife. Colour commonly reddish or pinkish in the varieties rich in felspar. Many " Mica- traps " come here, which are dark with lustrous mica ; these are the compacter " Minettes," and they pass into a group too poor in silica to be included under Syenite. This outlying group, with many of Rosenbusch's "Vogesites" (see below), comes under the " Lamprophyres " of that author. Specific Gravity. 2-7, but higher in the varieties rich in biotite, and approaching 2*8. II. Quartz must be carefully sought for and found practically wanting. The alteration of the felspars in many examples, such as the compact mica-syenites, makes even microscopic deter- mination difficult ; but the flame test will give an idea of the amount of potash present. The absence of free silica prevents the development of micro- pegmatitic and the so-called "granophyric" structures, such as are common in the eurites. The porphyritic orthoclase crystals, which are characteristic, often preserve their outlines well. Varieties of Compact Syenite. Rosenbusch terms the varieties rich in soda "Keratophyre." Those with hornblende or pyroxene * Traitt, des Roches, 1857, p. 65. HOLOCRYSTALLINE IGNEOUS ROCKS. 225 form his " Vogesite." Varieties with a felspathoid, intermedi- ate between felspathoid-syenite and phonolite, have been styled "Tinguaite." The nepheline in a red rock from the Val Fiemme near Predazzo is porphyritic, and, though altered, shows its characteristic outlines. III. QUARTZ-DlORITE AND QUARTZ-APHANITE GROUP. In this group of very common rocks, usually styled simply "diorites," there is free silica in the form of quartz; but the fact that the felspar is oligoclase or even labradorite keeps the total silica-percentage below that of the granites. They come thus at the head of Prof. Judd's Intermediate Igneous Rocks. Quartz-Diorite. Structure Granitic. Constituents 1, Quartz; 2, Plagioclase ; 3, Amphibole, Pyroxene, or Mica. I. An immense number of the " granites " of commerce come under this head. The striation of the plagioclase and the absence of the twinning of orthoclase are noticeable with the lens. Otherwise these rocks resemble granite. The colour is generally grey, but red felspars may occur. The remarks made on the mica-syenites apply equally to the fine-grained Mica- Diorites, which mostly contain quartz. Dark-coloured quartz- mica-diorites from the neighbourhood of Brest have been named " Kersanton," after a village so-called, and Delesse employed " Kersantite " for varieties with amphibole or pyroxene in addition to mica, the types occurring in the Yosges. "Tonalite" (vom Rath, after Monte Tonale in Western Tyrol) is a quartz-biotite-diorite in which the minerals are well developed, the white felspar contrasting boldly with the dark bronze-coloured mica. There is no doubt that masses of quartz-diorite arise as products of admixture where granite intrudes into more basic masses. Any pyroxene in the latter is then liable to recrystal- lise in the new joint rock as hornblende. Specific Gravity. Approaching 2-85 or even 2-9. Typical Analyses. The silica-percentage has been commented on above, these rocks falling short of the typical " acid " group. A. "Tonalite," Adamello Range, Tyrol. Vom Rath, Zeitsch. d. deutsch. Geol. Gesell, 1864, p. 257. Much Quartz. Both Hornblende and Biotite. B. Quartz- Pyroxene -Diorite, Vildarthal, Tyrol. Teller & von John, Jahrb. d. Geol. Reichsanstatt, 1882, p. 589. Enstatite and Augite. 15 226 HOLOCRYSTALLINE IGNEOUS ROCKS. A. B. SiO* 66-91 59-97 Al 2 O a 15-20 16-93 Fe 2 8 2-41 FeO 6-45 4-83 CaO 3-73 5-10 MeO 2-35 3'61 K^O 0-86 1'32 Na 2 O 3-33 3'87 Loss on ignition . . 0*16 1'60 98-99 99-64 II. The microscopic features of granite recur here, with plagioclase (commonly oligoclase) in place of orthoclase. The greater number of so-called "hornblende-granites" must be placed as quartz-hornblende-diorite when viewed in section. Where the hornblende can be shown to have arisen from pyroxene, the rocks are sometimes classed as " Epidiorite," and in these cases the quartz is very likely of secondary origin. The fibrous irregular nature of the secondary amphibole will often distinguish epidiorite from true quartz-diorite. The typical " epidiorites " show a schistose structure in section ; the felspar is granular ; the hornblende is sometimes fibrous and actinolitic, sometimes also granular. Residual pyroxene of paler colour may occur. Though pale augite may be expected in quartz-diorite, in marked contrast to the richly coloured amphiboles and micas, yet rhombic pyroxene is rare. Sphene and apatite are common; and magnetite and titanic iron assume importance as the proportion of silica diminishes. Quartz -Aphanite. This series includes almost all the compact hornblende-diorites or Aphanites of Haiiy. See Aphanite. Structure. Microgranitic or micro crystalline, the felspars being occasionally rod-shaped and the structure approaching that of dolerite. Constituents Like quartz-diorite. I. The quartz may be barely visible, though widely dissemin- ated. Dark green fibrous hornblende, or abundant flakes of mica, may render the rock almost black, and in the hand it may with fairness be mistaken for dolerite. Many quartz-aphanites are, indeed, altered dolerites, and would be styled by various authors " fine-grained epidiorites "or " quartz-hornblende-dia- bases." The micaceous varieties include many fine-grained " kersantites." Specific Gravity. About 2*85. II. Plagioclase and quartz, the important distinguishing HOLOCRYSTALLINE IGNEOUS ROOKS. 227 minerals, must be looked for. They may be found in micro- pegmatitic intergrowths, sometimes globular, as in the eurites. Some of the rocks styled " granophyre " must come over to this division. In the more basic types pyroxene is common. It is impossible to make any microscopic distinction between the quartz-aphanites and many "fine-grained quartz-diabases." The quartz in the latter, however, is sometimes clearly secondary, occurring in strings and veinules. IY. DIORITE AND APHANITE GROUP. The supposed importance of distinguishing rocks containing amphibole from those containing pyroxene led to a double nomenclature in this group ; but the corresponding lavas, the andesites, were for the most part investigated at a later period, and were arranged under one common name. The pyroxenic "gabbros" and "dolerites" pass, again and again, into amphibolic "diorites" and "aphanites" by paramorphic changes, and these types cannot be legitimately divided. The limits of the group, however, must, as in other cases, be chemical rather than purely mineralogical, and many " hornblende-gabbros " without olivine may have only 45 per cent, of silica, and are more basic than some members of the " olivine-gabbro " group (see Brogger, Gesteine der Grorudit-Serie, 1894, p. 93). Diorite (#a%,* quoted by d'Aubuisson in 1819, from dtoptgu, "I distinguish," indicating the distinctness of the typical minerals, hornblende and felspar), Gabbro in part. Structure Granitic to ophitic. Constituents 1. Plagioclase (commonly Oligoclase or Labradorite) ; 2, Amphibole, Pyroxene, or Mica. I. Quartz must be practically absent. Hornblende and biotite will commonly be found side by side ; some quartzless " ker- santites " come here. The Gabbros (Pyroxene-Diorites) contain augite or diallage, and sometimes enstatite (" Norites "), these minerals often enclosing the prismatic felspars ophitically. The rock -called " Gabbro " (von Budi) or " Euphotide " (Hauy} con- sists typically of diallage and plagioclase, and may be regarded as falling in this group when it contains 50 per cent, of silica or more. The olivine which usually marks the basic varieties is often difficult to recognise in the field. The lime-soda felspars of the pyroxene-diorites (gabbros) easily become opaque and dull, passing into the saussuritic condition. Amphibole developes in the diallage, which often becomes green, as in the ornamental stone, " Verde di Corsica," and in a * Traitt de Min., 2. ed. (1822), tome iv., p. 540. The rock was distin- guished from syenite by Haiiy only by its smaller proportion of "felspar." HOLOCRYSTALLINK IGNEOUS ROCKS. similar gabbro near Saas. Besides passing into diallage, the augite sometimes developes the three series of schiller-planes that produce the dark lustrous " pseudo-hypersthene " variety. This was naturally often described as hypersthene by the older writers, so that the rocks called " Hypersthenite " must now be accepted with the utmost caution and submitted to microscopic tests. A pyroxene-diorite passing into the " epidiorite " state commonly shows patches of grey-green silky matter, due to the actinolitic amphibole. Specific Gravity. From about 2-85 to 3-0. Typical Analyses. A. Hornblende-Biotite-Diorite. Unalaska Island, Alaska. Hillebrand, Bull. U.S. Geol. Surv., No. 168 (1900), p. 226. B. Hornblende- Augite-Diorite. Near Inchnadampf, Sutherland. Teall, British Petrography, p. 265. C. Gabbro. White Face Mountain, New York. Steiger, Bull. U.S. Geol. Surv., No. 168 (1900), p. 36. Labradorite, augite, hornblende. Si0 CaO MgO Na 2 H 2 Ti0 2 Other substances A. B. C. 58-63 52-47 53-18 16-23 12-15 23-25 1-91 3-47 1-53 4-20 5-23 1-82 6-59 9-71 11-18 4-28 9-94 2-60 2-09 2-26 0-86 3-51 2-81 3-97 1-32 1-62 1-13 0-74 CO 2 0-54 Ti0 2 0-45 0-43 ... 0-54 99-93 100-20 100-51 II. Many hornblende- and mica-diorites, when submitted to the microscope, have to be handed over to the quartz-diorite group. The conditions that produce hornblende and quartz are to some extent similar, since neither mineral results experimentally from mere dry fusion. On the other hand, the pyroxene-diorites are found free from quartz, and give rise to true diorites by paramorphic change. In the Pyroxene-Diorites (Gabbros) the plagioclase is oligo- clase or labradorite, frequently the latter. The saussuritic products within the felspars occasionally make the sections dull and nearly opaque. The passage from augite to diallage may be noted, and amphibole appears on the edges of the altering pyroxenes, or sporadically within them. A good deal of chlorite occurs between the constituents ; this arises from the alteration of the ferro-magnesian minerals. BOLOCRYSTALLINE IGNEOUS ROCKS. Magnetite and titanic iron ore are prominently seen. Epidote is a common alteration-product in the diorites, owing to the large proportion of lime present (compare fig. 29). Among the pyroxene-diorites the ophitic structure is common. The felspar is well bounded and lies at random in the field, and the pyroxene has settled down round it, filling up the interstices, and forming crystals of considerable size. Thus the pyroxene areas will be found between crossed nicols to be optically con- tinuous over a large portion of the section, and the consistent direction of their cleavages will point to the same conclusion. Though often called "ophitic plates," it must be remembered that such developments of pyroxene occur in three dimensions and are not limited by the thickness of the slide. Varieties o/Diorite. NEPHELINE-DIORITE (EL^OLITE-DIORITE; " Theralite " of Rosenbusch). A rare rock corresponding to the Fig. 28. Granular Pyroxene- Diorite. Near Huntly, Aber- deen, x 35. &, Biotite. g, Garnet, noticeable by its high refractive index, p, Plagio- clase in irregular grains of approximately equal size, r.p, Rhombic pyroxene (hyper- sthene). Fig. 27. Altered Pyroxene-Mica-Dior- ite. Stanner Rock, Herefordshire, x 12. ap, Hexagonal and other sections of apatite included in the other minerals. 6, Biotite. h, Green fibrous hornblende, occasion- ally in well marked crystals, de- veloping at the expense of augite. ma, Magnetite, p, Plagioclase much altered. In the centre of the field is a pale crystal of original augite, with rectangular cleavage - cracks. Hornblende is developing in this by paramorphic change. nepheline-syenites ; the deep-seated representative of the nephe- line-andesites or " tephrites." GRANULAR DIORITE. A number of "epidiorites* are granular, HOLOORYSTALLINE IGNEOUS ROCKS. as described under quartz-diorite ; the felspar in these forms a mosaic, and is often a product of recrystallisation. In addition, there are some remarkable pyroxene-diorites in which the min- erals are of granular form and unusually clear and fresh in sections. The minerals are, perhaps, all of secondary origin, when these rocks are associated, as they often are, with schists. The minerals are commonly plagioclase, green monoclinic py- roxene, hypersthene or amblystegite, magnetite, and often garnet. The last-named must be distinguished by its isotropism from the hypersthene, the pink colour being the same in many sections. These rocks are often aphanitic, and have very probably a com- posite origin. They are further discussed on p. 286. Aphanite (Hauy,* 1822, from &pavigopai, "I disappear," indi- cating the indistinctness of the constituents in opposition to those of the coarse-grained diorites). Dolerite (Hauy, f 1822, from doXepog, " deceitful ") in part. This group includes many " Porphyrites," and the quartzless plagioclastic members of the " Lamprophyres " of Rosenbusch. Structure Microgranitic and microcrystalline. At times ophitic (manydolerites). Constituents Like diorite. I. Hornblende fibres may be seen occasionally with the lens, as may the glancing surfaces of ophitic augite in the pyroxene- aphanites or dolerites. These pyroxenic rocks have rod-shaped felspars, and are typically dark -coloured and almost black. Hornblende-aphanites are often grey-green, with a slightly silky lustre. When altered, as they frequently are, the aphanites are easily scratched with the knife, and are quite distinct from the corre- sponding types in the more acid groups. "Diabase" is a good field-term for altered greenish rocks allied to diorite, gabbro, aphanite, or dolerite. Hausmann J in 1842 denned it as a rock of any grain containing "hypersthene" (i.e., lustrous augite), labradorite, and chlorite. The term has since been unduly limited. The ophitic types show a small nodular structure on weather- ing, due to the thick crystals of pyroxene coming into prominence and preserving the felspars included by them, while the inter- stitial material is more easily destroyed In these altered types, calcite can often be detected with the eye, and fragments of the rock commonly effervesce in acid. * TraiU de Mm., 2nde. 4dit., t. iv., p. 543. Quoted, with "Diorite," by d'Aubuisson in 1819 (Gdognosie, p. 148). t/**d.,p. 573. J " Ueber die Bildung des Harzgebirges." Gottingen. HOLOCRYSTALLlfcE iGNEOtS ROCKS. Specific Gravity. About 2-85 to 2-95. Typical Analyses. A. Dolerite without Olivine. " Whin-Sill," Durham. Teall, Quart. Journ. Qtol. Soc., vol. xl. (1884), p. 654. B. Fine-grained "Diabase" (altered and chloritic Dolerite). Near Wieda, Harz. Schilling, Die Grunstein-genanntc Oesteine des Sudhaizes, 1869, p. 26. C. Hornblende-Mica-Aphanite (altered). Gill Bank, near Staveley. Houghton, Quart. Journ. Geol. Soc., 1879, p. 170. Si0 2 Ti0 2 A1 2 3 MnO CaO MgO K 2 Na 2 H 2 C0 2 99-67 B. 46-60 21-60 2-86 6-40 9-25 6-48 0-94 3-20 3-10 0-45 100-88 c. 46-17 16-95 5-46 0-83 0-10 10-23 7-13 3-96 2-42 2-87 4-84 100-96 II. The plagioclases, usually labradorite or bytownite, are rod- shaped, and the hornblendes are commonly also in prismatic forms. The pyroxenes, however, form typically (as in the olivine- dolerites) areas of almost gran- ular crystals occupying the interstices of the felspar mesh, or ophitic crystals enclosing the felspars (compare fig. 39). Magnetite is prominent. The porphyritic crystals are more commonly plagioclase than a ferro - magnesian constituent. Chloritic decomposition - pro - ducts, epidote, and calcite are common in altered varieties. Varieties of Aphanite. NE- PHELINE-APHANITES and Nephe- line-Dolerites occur. The rock of the Lobauer Berg in Saxony, with nepheline, plagioclase, augite, abundant apatite, and magnetite, is a good example. Fig. 29. Altered Dolerite (Dia- base). Mynydd-y-Gader, Cader Idris, N. Wales. x 24. a, Characteristic pale-brown aug- ite. e, Almost colourless epi- dote, associated with pale chlo- ritic areas, in which it crys- tallises out, giving elongated sections, p, Prismatic plagio- clase. , Titanic iron ore, 232 HOLOCRYSTALLINE IGNEOUS ROCKS. GRANULAR APHANITE. Many granular diorites are of suffi- ciently fine grain to be classed as aphanites. SUPPLEMENT. Rocks occur, allied to Diorite and Aphanite, but with a felspathoid in place of the felspar. To these the somewhat loosely employed terms Nephelinite, Leucitite, and Noseanite have become restricted. One of the best known types is the Nephelinite of Katzenbuckel in the Odenwald, composed of nepheline, a good deal of smaller nosean, augite, some biotite, much apatite, and magnetite. Some " Tinguaites " fall here (p. 225). Rosenbusch, again, has placed in his " Lamprophyre " group an interesting series of . fine-grained rocks, named by him Camptonite, and characterised by some 40 per tent, of silica and 5 per cent, of alkalies. In the field, most of these would be collected as aphanites ; yet they are clearly an outlying and far more basic group. For a well investigated British series, see Flett, "Trap-dykes of the Orkneys," Trans. Roy. Soc. Edin., vol. xxxix. (1900), p. 874. Many fine-grained " kersantites " and " mica-traps" must be referred to the same outlying group of ultrabasic rocks without olivine. V. OLIVINE-GABBRO AND OLIVINE-DOLERITE GROUP. These are the typical basic holocrystalline rocks. Olivine -Gabbro. The gabbros without olivine are treated under diorite ; but in chemical composition some diorites over- lap into this basic group (see p. 227). Structure Granitic ; often ophitic. Constituents 1, Plagio- clase (commonly labradorite; sometimes anorthite); 2, Pyroxene, rarely Amphibole or Mica; 3, Olivine. Magnetite or Titanic iron ore is always present. I. The difference between gabbro and olivine-gabbro is not always clear in hand-specimens, since the olivine decomposes readily to dark patches, in which magnetite is largely developed. The typical pyroxene is brown-black augite, or the schillerised form, diallage. Rhombic pyroxenes are determined micro- scopically. The felspar is usually grey to blue-grey, and is often saussuritised, losing its vitreous lustre altogether. Mica is rarely seen; but hornblende may replace by paramorphism much of the original pyroxene. The olivine, when fresh, ap- pears in hard yellow-green glassy grains, contrasted with the darker and less transparent pyroxene. If the latter is diopside, it may be difficult to distinguish it from the olivine; its more marked cleavage-surfaces should be noted. HOLOCKVSTALLINE IGNEOUS ROCKS. Fig. 30. Olivine-Gabbro. Near Huntly, Aberdeen, x 7. d, Diallage, with numerous in- clusions developed by schiller- isatipn. On its margins it is passing by further change into brown strongly pleochroic horn- blende, as indicated by the darker bands, ol, Olivine with fibrous marginal zone at contact with the felspars (development of actinolitic and other amphi- boles; "dynamo-metamorphic" zone of Rosenbusch). p, Large crystals of plagioclase. ol Fig. 31. Gabbro rich in Olivine (Troctolite). Coverack, Corn- wall, x 12. a, Irregular and very subordinate augite. ol, Olivine, altered, with develop- ment of serpentine and mag- netite along the cracks. The surrounding felspars have be- come full of rifts which radiate from the decomposing olivine. p, Plagioclase (anorthite). py, Thin zones of pale brown py- roxene occasionally occurring on the margin of the olivine. Ophitic structure on a coarse scale is probably as common as the granitic. Weathering gives a brown rough surface, on which the pyroxene stands out. Specific Gravity. About 2-9 to 3-0. As low as 2-8 when much altered. Typical Analyses. A. Olivine-Gabbro. Cuillin Hills, Skye. Pollard, in Barker, "Igneous Rocks of Skye,"Geol. Surv. United Kingdom, 1904, p. 103. B. Anorthite-Gabbro, very rich in olivine, with bronzite and diallage. The Abtthal, Transylvania. Tschermak, Porphyrgesteine Osterreichs, 1869, p. 227. Anal, by Barber. Placed with "Picrite"by Tschermak (Ibid p. 280). See p. 236 of this book. A. B. 46-39 42-77 26-34 7'48 2-02 334 3-15 4-79 15-29 6-50 4-82 30-11 0-20 6-10 1-63 0-50 0-58 3-28 CaO MgO Na 2 . H, and loss on ignition TiO, .... MnO 100-82 98-87 234 HOLOCBY8TALLINE IGNEOUS ROCKS. II. The remarks made on the pyroxene-diorites apply equally to these rocks; but olivine must here be especially looked for. It will appear as grains of irregular form, occasionally embedded ophitically in pyroxene, and traversed by the characteristic cracks with traces of green decomposition-products. As alter- ation advances, the olivine area is converted into green ser- pentine, and it often becomes a question whether this material has arisen from olivine or from rhombic pyroxene. When the olivine is fairly ferriferous, the portion of ferrous oxide rejected during the conversion into serpentine separates out along the cracks as magnetite, and gives a characteristic appearance to the area. Very commonly, colourless patches of olivine remain in the serpentine, extinguishing together between crossed nicols and thus showing the extent of the original crystal (fig. 31, and frontispiece, ug. 1 ; also p. xiii.) When felspar surrounds the olivine, it is often split by the expansion of the latter mineral during hydration, the radial cracks set up being filled with serpentine. Varieties of Olivine-Gabbro. WITH SECONDARY ZONES (fig. 30). These are very marked around the olivines. The structure appears to arise by interaction of the minerals when subjected to earth-pressures (as in the " Flaser-gabbros " of Saxony). The constituents become divided from one another by zones of actinolite, rhombic pyroxene or rhombic amphibole, garnet, and other minerals; these zones require the microscope for their correct appreciation. GABBRO RICH IN OLIVINE (fig. 31). The " Forellenstein " of the Germans is a rock in which the dark altering olivine, set in white felspar, was supposed to resemble the markings on a trout. Little pyroxene occurs. The felspar is anorthite, and this anorthite-gabbro (with much olivine) was called "Troctolite"* by von Lasaulx. Microscopically, the felspars appear split by the expansion of the olivine during its passage into serpentine. Some authors, noting the small part played by the pyroxene, consider " troctolite as composed of olivine and felspar only. In chemical composition it is ultrabasic. Olivine-Dolerite. Structure Microgranitic and microcrystal- line. At times ophitic. The olivine is often porphyritic. Con- stituents Like olivine-gabbro. I. The rock is typically dark, with a granular appearance. Closer inspection generally reveals prismatic felspar, obscured in the total effect by the glancing points of the pyroxene and olivine. Ophitic structure and, where felspar is not abundant, * Elemente der Petrographie, p. 315. From rpw/crtj, a trout. ttOLOCtrtrSTALLINE iGNEOtJS ROCKS. 235 the " lustre-mottling " effect of olivine and pyroxene, are visible in parts of many masses. (See p. 236.) The knife usually leaves a white mark on the rock, owing to the tendency of the basic constituents to decompose. The joint- surfaces are brown with iron-rust, and weathering gives a rugged aspect like that of the gabbros. When much weathered, the olivine-dolerites become soft and greenish, and zeolites, calcite, and agates begin to accumulate in cracks and cavities. Specific Gravity. About 2*9. Lowered by alteration. Typical Analyses. A. Meissner, Hesse. Moesta, 1867 ; quoted by Roth, Beitrdge zur Petrog., 1869, p. cxxx. (one of Hatty's typical Dolerites). B. Near Valmont, Colorado. Eakins, quoted by Clarke, Bull. U.S. Geol. Surv., No. 168 (1900), p. 140. A. B. Si0 2 54-39 48-25 AL0 3 10-09 16-73 Fe 2 3 > 7-07 3-99 FeO 5-79 6-28 Ca ' . 8-89 8-32 MgO 6-49 5-77 KjO 2-17 4-08 Na-jO 4-16 3-24 H 2 0-57 1*72 Ti0 2 '89 F 2 5 '68 Other constituents, .... ... *21 99-62 100-16 II. The plagioclases, labradorite to anorthite, are rod-shaped. The pyroxene occurs in the interstices of the felspar mesh in granular forms, with commonly some sign of the eight-sided outline, or as ophitic crystals. The olivine lies scattered hap- hazard, two or three crystalline grains commonly being attached together. Porphyritic crystals of plagioclase and pyroxene are frequent. The olivine, moreover, is also commonly porphyritic, not being diffused in small granules through the groundmass. By alteration, these rocks give rise to dubious forms that are most conveniently styled "Olivine-Diabases," and sometimes be- come " epidiorites." In these secondary hornblende and biotite may occur. Both these minerals are rare in the unaltered olivine-dolerites. Varieties of Olivine-Dolerite. NEPHELINE-OLIVINE-DOLERITE. The felspar may be largely replaced by nepheline, the crystals of which appear as pale yellowish vitreous grains or rectangular and hexagonal sections on the surface amid the dark pyroxene. These rocks are naturally richer in soda than the ordinary 236 HOLOCRYSTALLINE IGNEOUS ROCKS. type, and are holocrystalline representatives of the nepheline- basalts. SUPPLEMENT. When felspar is absent, and its place is taken by a felspathoid, we have Olivine-Nephelinite, Olivine-Leucitite, &c., a small group of rocks that does not require separate description. VI. PERIDOTITE GROUP. The Peridotites, as the term is now understood, are quite exceptional when compared with the rocks of the preceding groups, being practically devoid of felspar and not rich in any aluminous mineral. They occur as segregated masses, or as veins, among ordinary basic rocks, the latter often shading into them just as granite may shade into the more highly silicated aplite. The "Picrites" of Tschermak* are rocks rich in olivine, this mineral forming about 50 per cent, of the bulk ; but some contain much felspar, and they form links with olivine-gabbro. The term cannot fairly be used in any but the general sense of its author. See analysis, p. 233. Peridotite. A name used by Cordier for a basalt or dolerite rich in olivine. Now generally adopted, following Rosenbusch, for types without felspar. "Picrite" of many authors. Structure Granitic; but very often the olivine is ophitically included in the pyroxene, amphibole, or mica, giving the " lustre-mottling " effect. Constituents 1, Pyroxene, Amphibole, or Mica; 2, Olivine. Magnetite, titanic iron ore, chromite, and other spinel- loids are common. I. The prevalent colour when fresh is a yellow olivine green, darkening with decomposition, and intermingled with black or lustrous bisilicates. In the " lustre-mottling " types, the latter minerals give the impression of forming by far the greater bulk of the rock, owing to the glancing of their cleavage-surfaces or schiller-planes ; the olivine appears set in the pyroxene, n\ 7 erted in'o hornblende-schist. [To face p. 278. PLATE II. METAMORPHIC ROCKS. Fig. 1. Fig. 2. Fig. 3. Fig. 5. Fig. 1. -Crystalline Limestone (calcite). Letterbreckan, Co. Galway. x 12. Fig. 2. Crystalline Dolomite. Near Giessen, Hesse, x 12. Fig. 3. Slate (Phyllite type), with pyrite and white mica. Easdale Id., Argyll. x 12. Fig. 4. Gneiss formed from crushing of previously fluidal granite, pro- ducing mj'lonitic bands. Carbane, Glenties, Co. Donegal. x 9. Fig. 5. Gneiss formed by intrusion of aplitic granite along the foliation- planes of biotite-schist. Craignayarrow, Co. Tyrone. x 9. METAMORPHIC ROCKS. 279 The division of foliated rocks into altered sediments and altered igneous masses is beset with such enormous difficul- ties that we must be content merely to bear in mind the possibility of either origin, and to seek diligently for elucida- tion in each case as it comes before us in the field. There is, however, a growing feeling that the great majority of amphi- bole- and chlorite-schists, a few mica-schists, and many gneisses, have their origin in igneous rocks ; while in many cases original flow, and not metamorphism, is responsible for their special structures. (Compare p. 102.) We should note that microscopic sections of foliated rocks should be taken perpendicularly to the edges of the folia. Sub-group 1 Schists. These are rocks in which the foliation is little interfered with by large crystals, and in which the differ- ence between the mineral constitution of successive layers is not so marked as in the coarser gneissic type. The folia are often intensely crumpled; but separation of the rock occurs parallel to their surfaces rather than along other divisional planes. When garnets, &c., are developed during metamorphism, they cause the foliated materials to fold over and flow round them, so that the obstacle, with the curving layers meeting again on either side of it, resembles an eye. This gives the "eye-struc- ture," which is seen on fractured surfaces perpendicular to the foliation-layers. Prof. Lapworth has styled " mylonitic " (fwXuv, a mill) those cases where, in section, the larger lenticular constituents are surrounded by a cryptocrystalline or amorphous paste, itself lying in "a flowing microscopic tissue of opaque fibres and strings," as if the whole had been ground to flour between mill- stones. Such "mylonitic rocks are compact and slate-like." Finally, we must be prepared for rocks, truly stratified, which simulate schists, from the fact that their materials are derived from the weathering away of metamorphic rocks. Mica-Schist. I. This is by far the commonest metamorphic rock. ' The lustrous folia of mica, now in broad swelling curves, now wrinkled, now bent into the sharpest folds, disguise the other constituents and appear to constitute the mass. In the field, the foliation-planes are often seen to be coincident with an original stratification. The mica is generally a pale species, and rarely appears black. Quartz can generally be detected, sometimes in great segregated nodules or in veins. Garnet, red and well seen on fracture, is almost always present, and forms little " eyes " in the foliation. The use of the thumb-nail will distinguish fine-grained mica. 280 METAMORPHIC ROCKS. schist from talc-schist, in addition to the higher lustre of the mica. With the knife the peculiar grating sound of the mica surfaces can easily be detected. Sillimanite is frequently present, giving dull white fibrous patches, in schists altered by igneous contact. Andalusite or kyanite may occur in such rocks, the latter showing its blue tint when the hammer crushes it. Typical mica-schist has a silica percentage of about 60. II. The mica is most commonly colourless, with strings of ill defined greenish and greyish matter interfoliated with it. Granular quartz and some felspar, often arranged in streams, occur. The eye- structure due to the presence of garnets is excellently seen in sections (fig. 43). Fibres of sillimanite often penetrate biotite and quartz. In fine-grained examples a double wrinkling and foliation may sometimes be traced, arising at two distinct periods of pressure and movement. rrv Fig 43. Mica- Schist. Saxony, x 7. g, Garnet, pale pink, and show- ing signs of cleavage, ra, Colour- less Mica, bent and drawn out in the direction of the foliation- layers, q, Quartz, granular, and often elongated parallel to the foliation-layers. In examining the slide we must never forget the solid; the sections of mica, for ex- ample, are cut from extended lenticular patches, the union of their basal surfaces constituting the glancing folia of the rock. Chlorite-Schist. I. This is quite a rare rock compared with the preceding. It is dark green, with black-green scales on the surfaces of foliation, and is typically rather fine in grain. The softness is characteristic, the whole having a soapy feel in the hand. In the field the absence of the glancing surfaces of mica, and the general darkness of the rock exposed, mark it out from mica-schist. Magnetite is the commonest accessory, the rock being very poor in silica (perhaps as much as 30 per cent.). The octahedra of magnetite, black and metallic, are often beautifully developed in the green scaly groundmass, and are sometimes surrounded by a spherulite of radial chlorite, which looks like a rosette when fractured. Veins and little patches of epidote may occur. II. The chlorite appears in flakes and fan-like groups ; the METAMORfHlC ROCKS. 281 cleavage of the mineral is irregular and much disturbed. Magnetite is identified by reflected light. Small brown rutiles very commonly occur. Serpentine-Schist. I. This is a common rock in some moun- tain-districts, such as the Western Alps, and is derived, in a great number of instances, at any rate, from the crushing of altered peridotites. The colour is dull green, lighter than that of chlorite-schist ; sometimes blue-green or purple. The foliated surfaces are soapy-looking, and bent in fairly broad folds ; slickensides abound. The rock, indeed, breaks in the field along joint-surfaces and slickensides quite as often as along the planes of foliation. Some few serpentine-schists can be traced into more normal types of schist, and appear to result from the permeation of aluminous schists by serpentinous matter. The percentage of silica in serpentine-schist is about 40. II. Sections generally show excellently the folding and move- ment undergone by the soft yielding rock. The whole field is a pale transparent green. Garnet, epidote, and magnetite may occur. Talc-Schist. I. A somewhat rare magnesian schist, light in colour, generally pale greenish or pure white, with a silvery and pearly lustre. The rock feels soapy to the hand, and its hardness = 1. Quartz grains and patches often occur, and needles of actino- lite may be scattered on the foliation-surfaces. The silica percentage rises at least to 55, being reduced from that of pure talc by presence of mica, &c. II. The talc is more easily distinguished in the mass than in section. Quartz granules form quite a mosaic along certain bands, and are commonly abundant. Amphibole-Schist. Many " Amphibolites " come here. I. Next to mica-schist, Hornblende-Schist is one of the com- monest metamorphic rocks, and results in very many cases from the foliation of altered basic igneous rocks. The rock is commonly green-black, with a lustre due to fibrous or somewhat plate-like hornblende, quite distinct from that of a dark mica-schist. The layers of hornblende, which are less crumpled than those of mica- schist, may alternate with thin lighter bands of felspar, quartz, and sometimes epidote. Dark mica is an accessory, and often arises from the action of invading granite on the amphibole of the schist. The rock breaks more readily along joints, and more evenly on cross-fractures, than mica-schist, since the materials are granular and idiomorphic rather than spread out into lenticles. in the field, dolerites and aphauites can be seen to pass into 282 METAMORPHIC fcOCKS. hornblende-schist, th< j rock being often only an extreme type of " epidiorite." * Even coarse gabbros, after some intermediate stages of mineral change, become rolled out into an almost mylonitic condition and form granular hornblendic schists. The silica-percentage is about 50. II. The hornblende is small ; granular or idomorphic ; some- times fibrous and coarser. The typical cleavages and pleochroism can be seen. Bands of granular clear colourless matter occur, which show between crossed nicols a mosaic of low colours. These consist, in the majority of cases, of granular felspars, as may be determined with convergent polarised light. Twinning can be seen in some of the grains, and they consist of lime- or lime-soda-plagioclase, which has recrystallised in this condition. (See also "epidiorite," pp. 226 and 229.) Prisms of yellow or colourless epidote, or of zoisite, may be abundant. Pale pyroxene, sphene, and garnet should be looked for. Iron oxides, titanic or not, and rutile, are very common. The amphibole may extend its boundaries by additions from the metamorphic mixture round it, when subjected to a new stimulus, such as the invasion of a granite magma. Other Varieties of Amphibole Schist. ACTINOLITE SCHIST. A pale or bright green variety, of limited occurrence, containing needles of actinolite. The name is sometimes given to a talc- schist with actinolite from the St. Gotthard above Airolo. GLAUCOPHANE-SCHIST. I. This rock occurs in very important masses in the southern Alpine valleys, particularly near S. Marcel, in the Val d'Aosta ; and it has been found near the Anglesey Monument, on the Menai Straits, by Prof. Blake. Probably it is of wider range, but has been overlooked. Its colour is a characteristic slate-blue grey, deepening almost to black, but distinct from the green-black of common hornblende- schist. The prismatic habit of the glaucophane gives a silky lustre when this mineral is abundant. Faint yellowish veins of epidote traverse the rock, and this mineral is also found through- out the foliation-layers. II. Glaucophane, with its beautiful pleochroism and prismatic forms, abounds. Pale yellow epidote and quartz are commonly present. Garnet, in pink grains, occurs in the " eclogite " types. Rutile is well developed at St. Marcel. Eclogite (Haiiy, 1822) and Garnet- Amphibolite. Consists of pyroxene or amphibole, or both, with garnet. Triclinic felspar * See particularly Teall, Quart. Journ. Geol. Soc., vol. xli. (1885), p. 133, arid British Petrogr., p. 198, plates xix. and xx. METAMORPHIC ROCKS. 283 and quartz are usually present in granular forms. From coarse and sometimes schistose types, these rocks shade into the granular pyroxene-diorites and pyroxene-granulites described on pp. 230 and 286. In sections, the pyroxene is usually pale green, while any hornblende present ophitically includes the other minerals. The garnet is often grossularite. The typical mode of occurrence of these rocks is in the form of blocks, large or small, entombed in gneiss or granite. Their connexion with intense thermo-metamorphism may be regarded as certain, and the original rocks, though all were probably rich in lime, may have been of very varied nature. Calc-Schist. I. This is the schistose representative of the limestones with accessory silicates, these minerals forming lustrous specks and rods upon the planes of foliation. Most commonly the rock is a schis- tose "cipollino" (see p. 274), the predominant silicate being pale silvery mica. At Shin- ness, in Sutherland, amphibole (tremolite, &c.) is developed in calc-schist. The knife readily detects the true character of the rock. Its colour is white to grey, and its general paleness makes its exposures in the field a contrast to those of the schists associated with it. Since it is far less fissile than ordinary schists, it can be quarried in regular blocks like other lime- Fig. 44. Ancient Conglomerate. Charlton Hill, Shropshire, x 7. db, Diabase, f.q, Foliated quart- zite. g, Gritty sandstone with cementing material, g, Quartzite. 8C t Schist, with characteristic out- line, unlike the adjoining pebbles, due to its breaking along the folia- tion-surfaces. This conglomerate, probably itself Pre-Cambrian, gives evidence of the existence of ma- terials which have been metamor- phosed at a still earlier date. stones. When treating the rock with acid, it must be remembered that calc-schist includes schistose dolomites. II. Nothing need here be added to what has been said under the head of crystalline limestones (see also fig. 23). The silicates may be examined separately, if necessary, after treatment of the rock with acid. Quartz -Schist. Foliated quartzite with mica, &c. (see fig. 44). See account of quartzites, p. 275. Also granulites. Sub-group 2 Gneisses. While these may be regarded as coarsely developed schists, it is the felspathic element that, by 284 METAMORPHIC ROCKS. its prominence, marks them off most distinctly from the fore- going sub-group. There can be no doubt that an immense number of occurrences of gneiss are due to the action of earth- movement upon igneous masses; and the very remarkable work of Lehmann in Saxony and Lawson* in Canada shows how this difficult question may be attacked and investigated in the field. Because a coarse gneiss accompanies and is seemingly inter- stratified with a series of schists, we must not conclude that it formed part of the original deposits of the locality, since it may result from a series of later parallel intrusions. Some gneisses owe their foliation to original conditions of consolidation (see p. 102), and are therefore not metamorphic. In sections of such rocks the larger constituents will not be surrounded by mylonitic matter, as in gneisses that have been foliated subsequently to consolidation. On the other hand, modern research seems to confirm the old view that gneisses may be formed from sediments by extreme contact-metamorphism (see Barrow, Quart. Journ. Geol. Soc. ,vol. xlix., 1893, p. 343, and Callaway, ibid., vol. liv., 1898, p. 374); in such cases, as well as in those where pressure has operated without movement, strain-shadows and mylonitic envelopes will similarly be absent. I. The gneisses, through the presence of compact bands or crystal-knots of felspar, quartz, 2). Dendriform or astrsean. In other structures like Oyathophyllum, but distinguished by presence of a well-marked columella, which is spindle-shaped in horizontal section. The longer septa are sometimes united to the columella. Carboniferous. Lonsdaleia. Like Lithostrotion, but divided by an inner wall into two portions a cellular part next the theca, and an inner more clearly septate portion. Oolumella large, elliptical in section, and built up of irregular concentric layers. Carboniferous. Omphyma (fig. 53). Simple, the cup-form often rather ex- panded, with root-like processes of the theca near the base. 308 ACTINOZOA. Numerous radial septa, four primary ones being seen, in well preserved specimens, to be set in shallow fossulae. Tabulae and tissue as in Cyathophyllum. Gotlandian. Fig. 52. Lithostrotion basaltiforme (Carboniferous). Astrsean growth. Fig. 53. Omphyma turbinatum (Wenlock Beds). Showing root- lets, and the fossilise as seen from above. Fig. 54. Calceola sandalina (De- vonian). Operculum removed. Calceola (tig. 54). Simple; shape like the pointed toe of a slipper, the calyx reaching to the base of the coral. Epitheca, ribbed on the natter side of the cup. Septa reduced to mere little bars. A lid, the Operculum, fits on the top of the calyx, though often lost in ordinary specimens. This lid resembles outwardly, with its curvilinear markings, the dor- sal valve of many brachiopods, and Cal- ceola was, indeed, long considered as a brachiopod. Devonian. Litharsea. Compound, with small, if any, areas of ccenenchyma; the hard parts are formed of a spongy non-compact calcareous tissue. Walls of the corallites pierced by apertures. Septa commonly forming three cycles only, and set with little processes on the edge and on the sides. Columella present, of spongy texture. Eocene to Miocene. Isastrsea. A typical astraean corallum, without coenenchyma. Corallites polygonal in section and united by whole length of their walls. Hard parts compact, not spongy. Septa well ACTINOZOA. 309 marked. Numerous dissepiments. Columella often present, but not strongly developed. Mesozoic ; especially Jurassic. Montlivaltia. Simple; sometimes disc-like, with a flat base covered with a concentrically wrinkled epitheca; commonly shaped like a peg-top or a curved cone, also with epitheca. Hard parts compact. Numerous septa, in 12 or more cycles, notched on the edges. Dissepiments abundant. No columella. Trias to Recent ; especially Jurassic. Thecosmilia. Branching corallum. Oorallites often com- pressed laterally at the calyx and dividing into two ; hence two adjacent calyxes often remain confluent. Epitheca present. Hard parts compact. Septa numerous, granulated at sides. Dissepiments abundant. No columella. Trias to Miocene ; especially Jurassic. Thamnastrsea. Compound ; corallum commonly like a segment of a sphere supported by an inverted cone, which bears an epitheca. No cosnenchyma, and thecse unseen, the septa of each corallite running out over the wall and uniting with those of adjacent corallites. Septa fairly numerous and granulated, i.e., set with synapticulae. Calyx shallow. Columella present. Trias to Oligocene. Cyclolites. Simple. Theca forming a flat base, with concen- trically wrinkled epitheca. The septa rise above this, forming a fair-sized roughly hemispherical skeleton. Septa thin and very numerous, notched on margin, set with synapticulse, and pierced by regularly arranged pores. The small septa are generally cemented to the larger. Typically Cretaceous. Holocystis. Astrsean; corallites united by their costse and polygonal. Septa not very numerous ; four, at right angles to one another, are well marked and larger than the rest. Tabulae are also present. This genus presents exceptional features, as compared with the more modern types with which it is associated: Lower Cretaceous. C. GENERA OP DOUBTFUL AFFINITIES. The following genera of Actinozoa are of uncertain position, while Alveolites and Ccenites, from their external aspect, have even been referred to the Polyzoa. Michelinia.* Corallum astrsean and generally top-shaped, with *In Nicholson's Manual of Palaeontology, 3rd edit., vol. i., p. 316, there is an interesting discussion of the relations of this genus and Pleurodictyum. 310 ACTINOZOA. epitheca and rootlets running from it. Corallites united by their walls, which are perforated. Calyx fairly deep. Septa represented merely by striae, so that the empty polygonal calyxes give the appearance of a honeycomb. Tabulae and vesicular dissepimental tissue present. Carboniferous. Favosites. Compound; astrsean or branching. Corallites resemble polygonal columns when the more massive specimens are broken open. The thecse show well-marked but widely set perforations. Septa represented by mere striae. Tabulae regular and well displayed. Ordovician to Carboniferous. Alveolites. Corallum spreading or branching, often of polyzoan type. Aperture of the calyx small, and like a triangle with curved sides ; the corallites lie pressed together, somewhat oblique to the surface. Thecae perforated. Septa represented by one or sometimes three ridges projecting into the cavity. Tabulate. Gotlandian and Devonian. Coenites. Closely allied to Alveolites, but with a thickening of the thecas near the outer end, so that the mouth of the calyx becomes a mere curved slit, much like the conventional flying bird drawn in landscapes. Gotlandian and Devonian. Halysites (fig. 55). Possibly an alcyonarian. Corallites tubular, elliptical in cross-section, and united by their sides in wall-like rows, so as to resemble the pipes of an organ ; these bands, each merely one corallite in width, cross one another, leaving large ir- regular interspaces in the corallum. When infilled with foreign matter, as in ordinary limestones, and broken across, the structure looks like a net- work of chains, each corallite being a link. Septa rarely traceable. Tabulae well developed. Ordovician and Gotlandian. Syringopora. Alcyonarian in type. Corallites tubular, circular in cross- section, bent and ramifying, united only by smaller horizontal tubes. The thecae thus stand well apart from one another. Septa scarcely traceable. Tabulae convex downwards. Gotlandian to Carboniferous. Fig. R.aJyttitc* catenu- laria (Ordovician). POLYZOA. 311 CHAPTER XXIII. FOSSIL GENERIC. TYPE 8. V. Polyzoa (Bryozoa). THESE minute colonial organisms leave skeletons which may be found among the washings of clays and sands, but which may otherwise be often overlooked. Many of the colonies, however, attain to a considerable size. The hard parts are built up of an external aragonite layer and an internal calcite layer (Cornish and Kendall). Almost all are marine. Terms used : Polypide or Zooid. The individual animal. Zoarium. The colonial structure formed by the ^olypides. Commonly attached or encrusting. Zocecium or Cellule. The tube-like or ovoid chamber occupied by each polypide. Ovicell. A chamber for containing one or more eggs, from which embryo-polypides develope and are set free. The ovicell forms a swelling above the aperture of certain zoo3cia in cheilostomatous colonies, and an inflation between the zoo3cia in cyclostomatous colonies. Operculum. The cover that closes the aperture of the zooecium in some polyzoa. Avicularia and Vibracula. Beak-like and whip-like appendages respectively, set on stalks and arising from little special pits on or between the zooscia (fig. 58). Used in obtaining food, or for defensive purposes. They are in reality specially modified zooecia. A. CYCLOSTOMATA. Zooecia tubular, typically not narrowing towards the aperture \ no operculum. Calcareous (aragonite with some calcite, Sorby) ; caurely horny. 312 POLYZOA. Entalophora. Zoarium branching. Zooecia in the form of long curving tubes, which open all round the surface of the twig- like zoarium. Marine. Gotlandian to Recent. Fascicularia. Zoarium spheroidal, fixed at base. Zocecia tubular, often curving, united into bundles which radiate from the base, leaving hollow interspaces. Marine. Pliocene. B. CRYPTOSTOMATA. The Fenestellidse here form the most important family, in which the zocecia show considerable deviation from the tubular type ; in section, they are seen to be more complex and narrowed at the external aperture. The latter is, however, round and simple, as in the typical cyclostomata. Fenestella (figs. 56 and 57). Zoarium lamellar, the sheet-like mass being commonly folded into the shape of a funnel, often Fig. 56. Fenestella retiformis (Permian). Showing form of the zoarium. Fig. 57. Fenestella retiformis (Permian). Enlarged, to show the zooBcia and the larger interspaces. several inches across. Built up of rods which radiate from the base and are connected by little cross-bars so as to form a net- work. The minute zocecia are grouped in two rows on each of these rods. Sometimes a third central row occurs. The zocecia must be looked for with a lens ; and the far larger interspaces of the mesh are styled fenestrules. On the systematic position of this genus see Ulrich, Geol. Surv. of Illinois, vol. viii., p. 349. Marine, Gotlandian to Permian ; particularly Carboniferous. POLrzoA. 313 C. GHEILOSTOMATA. Zooecia typically ovoid, not tubular; the aperture is in the side and near the upper end, and is smaller than the diameter of the zooecium. This aperture was closed by an operculum in most forms. The pits occupied by avicularia and vibracula can often be recognised. Horny or calcareous (aragonite, with some calcite?). Eschara (fig. 58). Zoarium formed of two layers of zocecia, back to back, producing a sheet-like mass which branches as Fig. 58. Eschara mon- ilifera (Pliocene) ; after Busk. The appear- ance of the aperture varies considerably ; in this example the pitted supports of avicularia at each side of the base of the aperture are clearly seen. Fig. 59. Brachiopod (Terebratula vitrea, Recent ; after Davidson), a, The two valves united. /, Foramen in beak of ventral valve ; d, Deltidium. 6, Inte- rior of dorsal valve, showing the brachial loop, c, Cardinal pro- it spreads. Zooecia close-set, typically cheilostomatous, with pits where the appendages have fallen away ; the zocecia of one row alternate with those of the next. Marine. Jurassic to Recent. Lepralia. Much like Eschara, but forming one encrusting layer. Marine. Cretaceous to Recent. Membranipora. Zoarium encrusting. Zocecia rather flat, with raised margins, and touching one another along their borders, more commonly than overlapping. The front of each is generally lost, having consisted of a chitinous membrane, a wide shallow cavity being thus revealed. Arrangement of zocecia rather irregular. Marine. Jurassic to Recent. 314 BRACHIOPODA. Cellepora. Zoarium built up of zocecia piled irregularly on one another, and thus forming a mammi Hated aggregate fixed at the base. Sometimes branching. Zooecia fairly ovoid. Marine. Cainozoic. VI. Brachiopoda. The Brachiopoda, formerly more prominent than the Mollusca, inhabit a bivalve shell, composed of calcite, or occasionally of phos- phate of lime ; this is minutely perforated over the whole surface in almost all the families. The Rhynchonellidse form an important exception, being imperforate (impunctate). Hollow spines, often of great length, project in some genera from the surface of the shell. The valves of the shell are typically unequal ; even if ap- parently equal, their internal structure is very different. Hold the shell so that the small valve faces the observer, and the umbo.of the large valve forms the highest point of the shell ; a vertical plane passing through the umbos and the point opposite to them on the lower border divides the shell into two sym- metrical halves. Compare Lamellibranchiata. (See fig. 59.) The modern species, which are numerous and all marine, mostly inhabit deep water. The modern classification of the brachiopoda is discussed by Schuchert, Bull U. S. Geol Survey, No. 87 (1897), pp. 113-135. For our present purposes, the two large divisions of Articulata and Inarticulata will prove sufficient. Terms used : Ventral Valve. The larger valve of the shell ; or in any case that covering the ventral portion of the animal. Dorsal Valve. The smaller valve, or that covering the dorsal portion. Both valves are commonly perforated by minute canals, being then said to be punctate (fig. 65). Both show somewhat oval Muscular impressions, placed below the hinge. In addition, Vascular impressions, indicating the position of blood-vessels in the mantle, are sometimes seen as faint grooves ramifying over the internal surface of both valves ; these may appear as ridges on internal casts. Tiie ventral valve terminates posteriorly in a more or less sharp Beak or Umbo. An aperture, the Foramen, may occur in this, or just below it, serving for the exit of the fibrous pedicle by which the animal was attached. When below the beak, it is generally triangular. In the dorsal valve, the beak is usually less prominent. BRAOHIOPODA. 315 Deltidium. A triangular structure found below, and partly or wholly surrounding, the foramen in many forms. It consists of two little plates, generally meeting along part of their length, and arising from opposite sides below the beak, thus limiting the aperture (fig. 59). Pseudodeltidium. A plate formed occasionally across the foramen and spreading anteriorly from the beak. Area. The flat area often occurring between the hinge and the beak ; sometimes striated. Commonly seen in the ventral valve, rarely also in the dorsal. It stretches on either side of the triangular foramen, the deltidium, or the pseudodeltidium. Teeth. Two processes set in the ventral valve, and commonly borne by two Dental plates, which are like short septa rising from the inner surface of the valve. The teeth occur on the Hinge-line (or line along which the two valves are united during life). They fit into two sockets in the dorsal valve. The dorsal valve bears ordinarily a Cardinal Process projecting somewhat down from the centre of its hinge-line. To this the muscles that opened the shell were attached. Two plates called Crura, one on each side of the centre of the hinge-line, may occur in this valve, the " arms," or lamellar mouth-appendages of the animal, being then attached to them. In most cases they bear a Bracliial Loop, a calcareous structure of great delicacy, which supports the arms. The loop sometimes is represented by two Spires, conically coiled, their apices directed away from or towards the centre of the shell (fig. 64). Median Septum. A partition that may be found running for some distance below the hinge-line towards the shell-border, rising from the inner surface of either valve, or both. The shell-substance of the Brachiopoda (except the occasional phosphatic layers) is very characteristically built up of long curving calcite prisms, among which circular gaps, the perfora- tions, commonly appear. The obliquity of these prisms to the surface of the shell, and their curving, allow their polygonal ends and their lateral faces to be visible at once in microscopic preparations. A. AKTICULATA (Valves connected by a Hinge). Shell Calcite. Terebratula (fig. 59). Shell oval, punctate; often folded slightly at the margin ; surface smooth, with mere lines of growth parallel to the margin. Curved hinge-line. Beak pierced by a round foramen, the deltidium occurring below this and not sur- rounding it. Brachial loop short. 316 BRACHIOPODA. Devonian to Recent. Especially Ter ebratulina. Like Terebratula in all essentials, but deltidium small and surface of shell delicately striated by grooves radiating from the apex. Jurassic to Recent. Magellania (Waldheimia). Not externally distinguishable from Terebratula, but brachial loop long, and a median septum in dorsal valve. Lias to Recent. Kingena. Allied to Terebratula; hinge-line straighter, and brachial loop united to a median septum. Jurassic and Cretaceous. Pygope and Antinomia. Terebratulas in which, after a certain age, the lateral parts of the valves grow outwards and then reunite, leaving an aperture through the whole form ; in Antinomia this finally lies nearer to the beak than to the growing margin. See Buckman, Quart. Journ. Geol. Soc., 1906, p. 433. Tithonian (U. Jurassic). Stringocephalus (fig. 60). Shell punctate, and resembling a wide Terebratula, but ventral valve with distinct area ; deltidium and pseudodeltidium both present. Strongly developed median septum in ventral valve. Cardinal process long and curved, bifurcating at end to pass on each side of the septum in the Fig. 60. Stringocephalus Burtini Fig. 61. Phynchonella. Viewed from (Devonian). Deltidium missing. below, showing the plicated junc- tion of the closed valves. opposing valve. Loop curving round parallel to and near the margin of the valve. Devonian. Known also in Gotlandian. Rhynchonella (fig. 61). Shell impunctate, rather triangular, BRACHIOPODA. 317 the margin on each side of the beak being straight and the outer margin curved. Ventral valve commonly infolded down the middle line, and dorsal valve bulged out to correspond ; margins almost always bent into sharp folds, giving well marked radial ridges down the surface. Beak sharp and bent over downwards and even inwards ; foramen below it, commonly surrounded by the deltidium (compare Terebratula). No loop, the crura alone being present. Ordovician to Recent. Pentamerus (fig. 62). Allied to Rhynchonella. Shell impunc- tate, markedly inequivalve, and strongly convex ; smooth or fur- rowed. Beak curved downwards; no deltidium. Median septum in ventral valve strongly developed, dividing on its free edge into two diverging septum-like dental plates, between which a little Fig. 62. Pentamerus galeatu (Devonian). Showing on the beak the trace of the internal septum. Fig. 63. Spirifer pinguis (Carboni- ferous). chamber is thus formed, open at the end away from the beak. The dorsal valve has two septa, arising one on each side of the central line, which approach the dental plates. The remarkable size of these structures in proportion to the cavity of the shell causes it to break open easily along a surface formed by the ventral septum, one or other dental plate, and the corresponding dorsal septum. The septa can sometimes be traced as lines on the convex exterior of the shell (fig. 62). Casts show characteristic deep grooves in the place of these internal partitions. Gfotlandian and Devonian. Camarophoria. Like Rhynchonella, but with an internal structure resembling that of Pentamerus on a small scale ; one septum in the dorsal valve, dividing on its edge. Devonian to Permian ; especially the latter. 318 BRACHIOPODA. Spirifer (fig. 63). Shell impunctate, commonly with a median ventral furrow and dorsal ridge-like fold as in Rhynchonella ; generally also marked with radial grooves. Hinge-line straight, often forming the longest dimension of the shell, and even causing ear-like expansions of the margin just below it. Ventral valve with prominent sharp beak, very commonly curved over ; area triangular ; foramen triangular, closed over in part by a pseudo- del tidium. Dorsal valve with small narrow area; brachial spires present and highly developed, as may fairly often be seen on breaking open the shell (fig. 64). They occupy almost all the valve, their apices v^eing directed outwards. S. P. Woodward notes that silicified specimens occur in which Vfc^y^S^ Fig. 64. Spirifer trigoncdia (Carboni- Fig. 65. Spiriferina Walcottii ferous). Broken open to show (Lias). Showing punctate brachial spire. character. brachial spire. the spires may be freed by the use of acid from the matter that obscures them. Gotlandian to Permian. Very abundant in species in the Devonian and Carboniferous. Spiriferina (fig. 65). Like Spirifer, but punctate, and with a median septum in the ventral valve. Typically smaller than Spirifer. Perforations can easily be seen with a lens, especially on slightly rubbed specimens. Carboniferous to Lias ; typically the latter. Retzia. Shell punctate; marked by strong radial ribs. Foramen, with deltidium under it, in ventral valve. Spires in dorsal valve, much as in Spirifer. The genus, in its usual wide sense, is Gotlandian to Trias. Meristella (formerly classed with Athyris). One of the Spiri- feridse. Shell impunctate, smooth, and resembling in form a wide Terebratula, but without the foramen of that genus. Well marked median septum in dorsal valve ; spires similar to Spirifer. Gotlandian and Devonian. Atrypa. Shell impunctate, and resembling Rhynchonella, but typically with a straighter hinge-line. Foramen in beak, which BRACHIOPODA. 319 is curved over ; deltidium below ; no area. Dorsal valve with large spires, their apices directed towards the central part of the inner surface of the valve, and thus nearly touching one another. Ordovician to Trias ; especially Gotlandiau and Devonian. Koninckina. Form somewhat like Productus; dorsal valve concave. Apices of spires directed outwards. Trias of Alps. Orthis. Shell punctate, commonly approaching a rectangular shape, the valves often almost equal, and both only slightly con- vex ; marked with radial grooves in almost all cases. Hinge-line straight, but shorter than the greatest width of the valve ; each valve with an area which is notched in the centre, the two triangular notches together forming the foramen. Strongly marked muscular and vascular impressions. Cardinal p.ocess not divided (in some allied genera it is furrowed) ; brachial crura present, but small, and neither loop nor spires. L. Canibrian to Cat boniferous. An extremely abundant genus in the older Palaeozoic. Strophomena. The Strophomenidse have received of late con- siderable revision, on account of variations in the internal characters of species previously grouped under the same genus. Strophomena itself now includes shells without crura (compare Orthis), and with ventral muscular area bounded by raised margin. Ventral valve concave, dorsal convex. (Example : Slrophomena rugosa.) Ordovician. Leptasna.* Like Strophomena in general, but with dorsal valve concave, ventral convex, and broad shallow ventral muscular area. The edges of the valves are often bent sharply over in a dorsal direction. Flatter part distinctly wrinkled in con- centric folds. (Example : Leptcena rhomboidalis.) Ordovician to Carboniferous. Doubt hangs over all the species recorded from the Lias. Munier-Chalmas finds that some possess brachial spires, and has referred them to a new genus, Koninck- ella (Bull. soc. geol. France, 3me. ser., t. viii., p. 279). Rafinesquina. Like Leptsena, but without the abrupt bend in the shell, and unwrinkled. (Example : Rafinesquina (Stropho- mena) alternata.) Ordovician. Productus (fig. 66). Shell punctate, the perforations being * See Hall, Pal. of New York State, vol viii. (1892), &c. 320 BRACHIOPODA. produced in spines of various lengths. Not attached by a pedicle, as are the preceding genera, but free, or occasionally fixed by the spinose surface of the ventral valve.* Surface Fig. 66. Productus giganteiis (Carboniferous). sometimes smooth ; more commonly ribbed, with hollow spines, set mostly in the neighbourhood of the hinge. Ventral valve strongly convex, with curving beak; dorsal valve concave. No foramen; no hinge-teeth. Hinge-line straight, sometimes forming the greatest width of the shell, with ear-like expansions; sometimes shorter. Devonian to Permian. Especially Carboniferous. B. INARTICULATA. Valves not connected by a hinge, being kept closed by the adductor muscles, and moved apart in a lateral direction by "protractor sliding muscles," so that the apices of the valves are made to diverge from one another sideways, instead of approach- ing one another on opening, as in the more common brachiopods, the Articulata. Lingula. Shell formed of alternating lamellae of horny matter and phosphate of lime, the former predominating ; flexible in modern examples; punctate. Almost equivalve, each valve shaped like a flat shovel, pointed at the beak, truncated on the opposite margin. Smooth, or marked by mere delicate concentric growth-lines. A pedicle emerged between the beaks of the valves. Ordovician to Recent. Lingulella. Like Lingula, but with a vertical slit running from the beak of the ventral valve, probably to allow of the See R. Etheridge, jun., Quart. Journ, Geol. Soc. t 1876, p. 454, and 1878, p. 498. BRACHIOPODA. 321 passage of the pedicle. Muscular impressions stronger than in Lingula. L. Cambrian to Ordovidan. Obolella. Shell built up like that of Lingula, but with phos- phate of lime preponderating over the horny layers. Form ap- proaching circular, nearly equi valve; valves only slightly convex, and concentrically marked. The beak of the ventral valve is fur- rowed on the inner side by a groove for the pedicle. Differs from Obolus only in form and position of muscular impressions, those near the centre of the valve in Obolella being widest at the end away from the beak, while in Obolus this end is narrowest. Both genera have the same range, Cambrian and Ordovidan. Discina (fig. 67). Shell minutely punctate, composed mostly of horny matter. Inequivalve. Form circular, smooth or concentrically marked. Ventral valve flat or slightly conical, with beak almost central; a foramen occur* close against the Fig. 67. Discina Forbesii (Wenlock Fig. 68. Crania parisiensis Beds). (Sehonian). Interior of ven- tral valve. beak in adult forms, and from it a furrow sometimes runs externally towards the margin. Dorsal valve conical, with an excentric beak. The forms with a furrow have been called Orbiculoidea, and those with a ventral median septum Discinisca, leaving Discina only for Recent species. In its usual wider sense, Discina is Cambrian to fiecent. Crania (fig. 68). Shell showing punctation on inner surface only, the tubules breaking up into a number of much more minute ones as they near the outer surface. Calcareous and fairly thick ; sub-rectangular to circular ; surface smooth, or ribbed with ridges radiating from the beaks. Yentral valve conical and attached by the actual shell-substance of the beak, which is commonly nearly central. Dorsal valve conical, also with nearly central beak. Both valves have well developed muscular impressions and a characteristic broad flat border marked by granulations. The ventral valve is naturally often found adherent to other fossils, without the dorsal valve. Ordovidan to Recent. 21 322 LAMELLIBRANCHIATA. CHAPTER XXIV. FOSSIL GENERIC TYPES. VII. Lamellibranchiata. IN contrast with those of the Brachiopoda, the bivalve shells of these animals have typically equal valves. Moreover, hold the shell so that one valve faces the observer and the umbos form the highest point ; a vertical plane passing through fche umbos, and perpendicular to the plane of junction of the valves, will divide the shell into two unequal parts. Hence the shells are Fig. 69. Sinupalliate Lamellibranch (Cytherea incrassata, Oligocene); left valve, a, Impressions of the adductor muscles. a.., Anterior lateral tooth, e.l., Groove for the external ligament, h, Hinge, with three diverging cardinal teeth ; the middle one is divided by a groove. I, Lunule. p, Pallial line ; the infold or sinus indicates the position of the retractor muscle of the siphons. u y Umbo. said to be eguivalve but inequilateral. The longer part is in almost all cases the posterior. One or other valve may become smaller ; but in these inequi- valve genera the inequilateral character will probably betray itself (fig. 72). Similarly some genera have shells that are practically equilateral ; but a slight difference of the hinge-line on either side of the umbo (fig. 91), or the posterior position of the single internal muscular impression, will often serve as a guide. The classification here adopted seems suited to those who unfortunately have to deal with empty shells rather than with living molluscs ; but it must not be regarded in all cases as an expression of the nearest zoological alliances. The separation LAMELLIBRANCHIATA. 323 of Leda and Nucula is a marked illustration of this. (See the interesting discussion by F. Bernard, Paleontologie, pp. 541-543.) Terms used : Ant&rior Border. The end of the shell where the mouth and foot were situated. Posterior Border. The end of the shell where the cloacal aperture and, in siphonate forms, the siphons, were situated. Umbo. The beak or apex of either valve. This in the majority of forms is directed forwards i.e., towards the anterior end. Sometimes bent round or even pointing posteriorly. Hinge-line or " Hinge-border." The line along which move- ment takes place when the valves open. Ventral Border. That opposite to the hinge-line. Right and Left Valves. The shell is held resting upon its ventral border, and the anterior border of the shell is directed away from the observer. The "right" valve is then to his right, the " left" valve to his left. Ligament. The " external ligament " (by which the valves would be opened unless held closed by the muscles within) is placed, in the main, posteriorly to the umbos, and sometimes leaves an impression above this part of the hinge-line. The " internal ligament," or " cartilage," lies within, below the hinge- line, and is set in ligamental grooves or pits (figs. 84 and 93), which are seen near the hinge when the animal matter has disappeared. It may be remembered that the internal ligament becomes compressed when the valves close, and that its expan- sion causes them to open directly the muscular pull is released. Sometimes only one of the ligaments is present. Area, or Escutcheon. A generally elongated oval area seen behind the umbos in some genera, when the valves are united, and running some way along the hinge-line. Lunule. An o^al area seen, when the valves are united, in front of the umbos. Gaping. When the valves are closed, and yet leave an opening at one or both ends, the shell is said to be gaping. The following structures must be noticed on the interior of the valves : Cardinal Teeth. One or more processes, fitting into sockets in the opposing valve, and arising near the centre of the hinge- line. The teeth thus alternate in opposite valves. The true cardinal teeth arise below the umbo, but may extend back obliquely from it, so as to stimulate lateral teeth. Lateral Teeth. Similar processes commonly ridge-like, towards the anterior or posterior end of the hinge (figs. 69 and 73). Muscular Impressions, These are shallow, fairly circular, or 324 LAMELLIBRANCHIATA. pear-shaped pits representing the surfaces of attachment of the adductor muscles, or muscles used in keeping the shell closed. Sometimes only one (the posterior), sometimes two, are present in each valve. When the two impressions are fairly equal in area, the shell is that of a Homomyarian ; when the anterior impression is smaller, it is that of a Heteromyarian. The animals with only one adductor muscle are said to be Monomyarian. Pallial Line. This is a faint impressed line, parallel to the border of the valve and a little way within it, representing the line of attachment of the muscles that are placed near the edge of the mantle. If it is continuously convex outwardly, the shell is said to be integripalliate. If it is more or less indented by a pallia! sinus, the shell is sinupalliate. Pallial Sinus. An infold of the posterior portion of the pallial line, sometimes a mere shallow curve, sometimes deep and extending back even beyond the centre of the valve. This occurs only in forms which can extend and retract their siphons. The structure of the shell-substance itself exhibits two layers, the whole being covered in life by a skin, or "periostracum." The outer layer, sometimes thick, sometimes thin, consists of calcareous prisms in contact along their walls. Here and there a polygonal interspace occurs. The fibrous structure seen on cross-fracture of Inoceramus is due to well-developed prisms of this nature. The inner layer is formed of delicate, compact, and pearly lamellae, sometimes accumulated to a great thickness. These layers occasionally leave irregular interspaces or chambers of flattened and curving form, as in the thickened region near the umbos of some oysters. The shell-substance is sometimes calcite, but most commonly aragonite. Or, when both minerals are present, the outer layer con- sists of calcite, the inner of aragonite. The mineral constitution of the shells of many genera yet awaits investigation, and the usual alteration of aragonite in old forms into granular calcite pre- cludes certainty of determination in some extinct examples. The lamellibranchiata are mostly marine; the fresh -water types referred to will be specially indicated. Some genera are attached to the sea-floor by the shell itself; others by fibrous outgrowths, the byssus, issuing near the urabos ; others are free and locomotive. A few lamellibranchs, of different families, bore into mud, wood (as Teredo, the ship-worm), or into other shells, corals, or even the hardest stone (as Pholas and Litho- domus) ; the cavity thus made is called a crypt, and is increased until the animal and shell attain their full development. Thus the animal cannot leave the cavity, communication being kept up with the exterior through the narrow opening which repre- LAMELLIBRANCHIATA. 325 sents the first stage of the boring. The siphons are turned upwards, the anterior end of the animal being downwards, and a calcareous siphonal tube is sometimes developed, the small shell- valves becoming dwarfed by comparison and incorporated with the tube, and the whole shell thus appearing cylindrical. Asper- gillum is one of the most remarkable examples. Boring shells are often represented merely by casts of their crypts, which are often club-shaped, the short handle of the club being the result of the infilling of the narrow entry to the cavity. Fossil siphonate shells are occasionally found and should be looked for in the position in which they lived in the soft mud which ultimately entombed them ; their umbos are thus directed downwards, and their siphonal ends upwards, in the stratum. A. HOMOMYARIAN SlPHONATE FORMS WITH PALLIAL SlNUS (SINUPALLTATE). The adductor muscular impressions are two in each valve, one posterior, one anterior, and fairly equally developed. The animal possessed long retractile siphons. In certain exceptional families these siphons are encased in a calcareous tube projecting far beyond the limits of the valves. Cytherea (fig. 69). Shell thick, approximating to circular, umbo well forward, with lunule. Generally concentrically marked. Three diverging well developed cardinal teeth in each valve. An anterior lateral tooth in left valve. Pallial sinus acute- angled, moderately developed. Inner margin of shell smooth. Cretaceous to Recent. Venus. -Like Cytherea, but without lateral tooth, and com- monly with delicately grooved inner border. Jurassic to Recent. Tellina. Slightly inequivalve. Shell thin, elongated oval, rounded anteriorly, more acute behind. Umbos almost in centre. Concentrically marked. Hinge nar- row; in each valve two cardinal teeth, and commonly an anterior and posterior lateral tooth. Sinus very broad and deep (fig. 70). Cretaceous to jRecent; this genus is par- ticularly rich in living species. Fig. W. Tettina (Post - Pliocene). PanopSBa (Glycimeris). Shell thick, often Right valve, large, and approaching an elongated rectangle. pallTaffinus Gaping at both ends. Umbos rounded and placed well forward. Concentrically marked. One cardinal tooth in each valve. 326 LAMELLIBRANCHIATA. Note. The limits of this genus are somewhat obscure. Cainozoic. Many older forms referred to Panopaea are now placed with the Pholadomyidse (Gresslya, &c.) Pholadomya. Shell thin, elongated or obliquely oval, markedly convex ; gaping behind and sometimes in front. Anterior border a little truncated. Umbos well forward. Escutcheon some- times present. Marked with knotty radial ribs, particularly on the anterior surface ; also with more delicate concentric lines. Practically toothless, one obscure process occurring in each valve. Sinus broad and fairly deep. The thinness of the shell makes casts alone commonly met with. Lias to Recent. Characteristically Jurassic. Goniomya. Like Pholadomya, but marked with rather delicate ribs, forming Vs, the angle of which is directed towards the middle of the ventral border. Especially Jurassic. Homomya. Like Pholadomya, rather elongated, gaping at both ends, but with only concentric striations. Trias to Cretaceous. Gresslya. Also one of the Pholadomyidae. Elongated oval, much like the longer Pholadomyas, but right valve somewhat larger than left, the umbo rising higher. Umbos well forward ; lunule present, no escutcheon. Concentrically marked. No teeth. Right valve with a ridge running along the hinge-line from the umbo posteriorly, which leaves a furrow in the casts that frequently occur. Compare Ceromya. Trias to Jurassic. Ceromya (Isocardia in part). Inequi valve, sometimes the right, but more commonly the left valve being slightly the larger, the umbo rising higher, as in Gresslya, and the posterior border overlapping that of the other valve. Approximating to circular, strongly convex, slightly gaping. Umbos large and well rounded ; lunule feeble or absent. Concentrically marked. No teeth. Kidge in right valve, as in Gresslya. Commonly found as casts. Typically Middle and Upper Jurassic. Mactra. Shell fairly thick, approximately triangular, rounded in front, more pointed behind; gaping slightly posteriorly. Concentrically marked. A cardinal tooth in each valve, bifur- cating, and thus shaped like an inverted V ; behind it, and still under the umbo, a triangular pit, which marks the position of the cartilage or internal ligament. A second cardinal tooth, of lamellar shape, is sometimes present. Anterior and posterior lamellar lateral teeth well marked, those of the right valve being LAMELLIBRANCHIATA. 327 double i.e., consisting of two parallel ridges running along the hinge-line. Sinus shallow. Middle Jurassic to Recent. Especially Cainozoic. My a (fig. 71). Inequivalve, left valve the smaller. Elon- gated, somewhat oblong ; gaping markedly at both ends. Umbos Fig. 71. Mya truncata (Post- Fig. 72. Corbula pisum (Oligocene). Pliocene). Left valve, showing Showing inequivalve character, the large spoon-like process be- neath the umbo. approaching centre of margin. Concentrically marked. Left valve with a well developed spoon-like process under the umbo, for the attachment of the cartilage. Right valve with one small cardinal tooth. Sinus large and deep ; pallial line often shows strong subsidiary impressions running upwards from it. The Myas burrow into sandy mud, particularly near the shore. Miocene to Recent. Corbula (fig. 72). Ally of Mya. Inequivalve, left valve much the smaller. Shell small, oval, produced posteriorly, ending there with rather a straight border ; not gaping. Concentrically marked. One cardinal tooth in right valve ; left valve with a process much like that of Mya, which fits into a groove behind the tooth of the right valve. Sinus quite shallow. Marine or Estuarine. Trias to Recent. Especially Cainozoic. Leda. Shell small, elongated and narrowed posteriorly, with umbos directed backwards. Hinge-line bent, with numerous transverse teeth, as in Nucula, with which Leda should be com- pared (see fig. 80). Gotlandian to Recent. Teredo. One of the Pholadidse; the so-called ship-worm. Shell small, each valve three-lobed, the central lobe the longest. Concentrically striated. No true hinge or ligament, the valves being quite subsidiary to the great siphonal tube, which extends far beyond them. 328 LAMELLIBRANCHIATA. The borings of this mollusc are tubular like those of worms, but typically somewhat straighter ; they are found commonly in fossil wood, as in the London Clay, either empty or infilled with mud or crystalline deposits. Lias to Recent. Especially Cainozoic. B. HOMOMYARIAN SlPHONATE FORMS WITHOUT PALLIAL SINUS (INTEGRIPALLIATE). The adductor muscular impressions are two in each valve, as in the preceding group, and the absence of the pallial sinus makes the interior of the valves resemble those of homomyarian asiphonate forms. The siphons of the animal were not retractile. Cardium. The common Cockle. Shell fairly thick, approxi- mately circular, or elongated in a vertical direction ; sometimes slightly gaping behind. Umbos rather large and rounded. Radially ribbed, the ribs commonly ornamented with protuber- ances. Two cardinal teeth and an anterior and posterior lateral in each valve (fig. 73). Inner border notched. Forms with radial markings on the posterior part only, and concentric on the remainder, have been sometimes divided off under the name Protocardia. Rhcetic to Recent ; especially Cainozoic. Conocardium (fig. 74). Shell heart-shaped when viewed from Fig. 73. Hinge of Cardium edule Fig. 74. Conocardium aliform* (Recent). Left valve, c, The two (Carboniferous), cardinal teeth. I, The ridge-like lateral teeth. the front, but somewhat elongated behind; strongly convex. The anterior end is truncated, the umbo being close to it ; just below the umbo a tube-like prolongation of the shell runs out in continuation of the hinge-line. Gaping at the posterior end. LAMELLIBRANCHIATA. 329 Radially marked; margins sharply folded by the ridges and furrows. Almost toothless. Cfotlandian to Carboniferous. Cyrena (fig. 75). Shell thick, oval, sometimes rather acuto posteriorly. Concentrically marked. Three cardinal teeth and an anterior and posterior lateral in each valve. Sometimes a slight pallial sinus. In the sub-genus Corbicula the lateral teeth are elongated and transversely striated. Fig. 75. Cyrena cuneiformis (Lower London Tertiaries). Brackish or fresh- water at present day, but often associated at river-mouths with typically marine shells, and hence of little value as a guide to conditions of deposition. Lias to Recent. Cyclas (Sphaerium). Ally of Cyrena, but shell thin and nearly equilateral. One feeble cardinal tooth in right valve and two in left. Lamellar anterior and posterior lateral teeth. Fresh-water. The reference of Wealden species to this genus has been criticised, though very generally made. Sandberger quotes the earliest Cyclas as Eocene, referring older forms to Cyrena, &c. Cyprina. Shell thick, oval, resembling Cytherea; markedly convex. Concentrically striated. Two car- dinal teeth and one posterior lateral tooth in each valve. There are also two feeble anterior lateral teeth in the right valve ; and in the left valve one anterior lateral (Fischer). Lias to Recent. Astarte (fig. 76). Shell thick, approach- ing circular at times, at others obliquely elongated. Umbos rather pointed ; lunule generally present. Concentrically marked. Two cardinal teeth in each valve; no laterals. Lias to Recent. Especially Cainozoic. Fig. 76. Astarte elegans (Inferior Oolite). 330 LAMELLIBKANCHIATA. Cardita Shell thick, somewhat like Cardium, but occasionally elongated. Umbos well forward. Radially ribbed. Inner border notched. Two elongated cardinal teeth, and a feeble anterior lateral, in right valve ; two divergent cardinal teeth, and one feeble posterior lateral, in left valve. (As to difficulty in reading teeth in Cardita, see Fischer, Conchyliologie, p. 902.) Trias to Recent. Especially Cainozoic. Venericardia. Much like Cardita. Two oblique cardinal teeth, and a feeble anterior lateral, in right valve ; two cardinal teeth and one long lamellar posterior lateral in left valve. Interior of margin of shell distinctly notched. Eocene to Recent. Crassatella. Shell decidedly thick, oval, truncated slightly at posterior margin. Lunule present. Three cardinal teeth in right, two in left valve ; an anterior and posterior lateral in both. Pit for cartilage immediately below the umbos, and behind the middle cardinal tooth of the right valve. Cretaceous to Recent. Especially U. Cretaceous and Eocene. Cardinia. Placed by S. P. Woodward near Cardita, by other authors as an asiphonate form near Unio. Shell elongated, oval, flattened at the sides. Umbo well forward. Concentrically marked. Two cardinal teeth in left valve, one in right, their feeble development being a characteristic feature; anterior lateral tooth in right valve, elongated posterior lateral in left, well developed. Jurassic. Especially Lias. Chama. Inequivalve; commonly attached by the umbo of the left valve, the upper valve, which is therefore generally the right, being the smaller. Shell thick, almost circular. Umbos of both valves bent forward and curved over, as if about to coil spirally. Concentrically marked, the successive shell-layers protruding beneath one another with irregularly serrated edges, giving the surface a foliaceous appearance. One stout cardinal tooth in upper valve, two in lower, between which the first-named fits. Lower Cretaceous to Recent. Especially Cainozoic. Diceras. Slightly inequivalve ; attached by umbo of one or other valve. Shell thick, approximately circular. Umbos very prominent, each being spirally curved and recumbent, as it were, against the surface of the shell. Concentrically marked. Right valve with one cardinal tooth, somewhat flattened and folded j LAMELLIBEANCHIATA. 331 also a smaller tooth near the anterior end of the hinge. Left valve with one curving tooth, which is elongated parallel to the border. The curved muscular im- pressions are bounded by ridges, which leave spiral grooves on the casts that often occur; these grooves run almost vertically down towards the edge repre- senting the margin of the valves. In such casts the umbos appear still more distinctly prominent, the spiral turns of the internal moulds not being in contact with one another. M. and U. Jurassic (Tithonian). Hippurites (fig. 77). This extraordin- ary shell, a representative of an al together exceptional family, the Rudistae, is now regarded as allied to Diceras and the other Chamidse, particularly through Monopleura, in which one valve is coni- cal and the other like an operculum fitting on it. In Hippurites the shell is also very inequivalve, the lower valve, which is the right, being conical, or more often cylindrical, terminating in a cone at the base. This valve is vertically furrowed and ribbed. Left valve small and flattish, strewn over with the small apertures of canals which perforate the shell ; radially ribbed in most cases, with central umbo; resembles an operculum when closed down on the lar g e ri g ht valve. The interior of the left valve bears long vertical processes corre- sponding to teeth, which fit into deep sockets in the lower valve. The chamber in which the animal lived is quite small, the lower part of the shell being filled up by the deposit of the inner shell-layer, which produces a succession of irregularly curving partitions, with interspaces. A similar infilling occurs within the long umbos of certain oysters. In some allied forms, as Radiolites, the outer prismatic shell- layer presents on fracture a coarse structure of hollow rectangular cells. The resemblance that the shells of Hippurites, &c., bear to Fig. 77. Hippurites (Sen oman). valve remo 332 LAMELLtBRANCHIA'TA. corals is sometimes increased by their clustering together and growing up side by side in groups. Entirely Cretaceous ; particularly in the higher beds. C. HOMOMYARIAN ASIPHONATE FORMS (INTEGRIPALLIATE). The animal does not possess distinct siphons. The impres- sions of the anterior and posterior adductor muscles are practi- cally of the same size. Shell typically equivalve. Area (fig. 78). Shell thick, approaching rectangular; markedly convex. TJmbos prominent and rounded, with a triangular striated area between them and the hinge-line, forming a surface of attachment for the ligament during life. Radially marked. Straight hinge-line. Teeth very numerous in each valve, forming well-marked short transverse ridges on the broad surface of the Fig. 78. Area Noce (Recent). Left valve, Fig. 79. Cucuttcea, Bar- showing area beneath the umbo, and dingii (Devonian), the numerous hinge- teeth. hinge, the series extending on each side of the umbo nearly to the lateral margins. Pre-eocene forms are perhaps more allied to Parallelodon. Ordovician (?) to Recent. Cyrtodonta (Palaearca). Close ally of Area; umbo near anterior border. Some of the teeth lie beneath the umbo, others are set near the posterior part of the hinge. Concentrically striated. Cambrian to Devonian. Cucullaea (fig. 79). Like a stout Area in form ; but the teeth nearest the anterior and posterior margins are set parallel to the hinge-line, forming horizontal ridges. Concentrically striated. Devonian to Recent. Especially Mesozoic. Whidborne, Pal. Soc., ''Devonian Fauna," vol. iii. (1898), p. 109, includes two Devonian species. Parallelodon (Macrodon). Much like Cucullsea, but umbo more anterior, and anterior teeth transverse, posterior parallel to hinge-line and ridge-like. LAMELLIBRANCHIATA. 333 Devonian to Eocene. Especially Carboniferous. Pectunculus. Shell thick; valves almost circular and equi- lateral. Triangular ligamental surface as in Area. In most species radially and rather delicately marked. Hinge-line curved, with numerous transverse teeth, those nearest the centre being obliterated as the shell approaches old age. Inner margin of valves notched. Shell aragonite (Cornish and Kendall). Lower Cretaceous to Recent. Especially Cainozoic. Cardiola. Form intermediate between a typical Area and a Pectunculus. Umbos distinctly anterior and slightly twisted. Ligamental surface under them as in Area and Pectunculus. Surface furrowed radially and concentrically, so as to give a nodose aspect. Hinge-line straight. Believed to be toothless. The alliance of this shell with the Arcidas is fairly clear, despite the obscurity as to its teeth. Gotlandian and Devonian. Nucula (fig. 80). Shell small, somewhat triangular, the umbo forming the apex. The umbos point backwards, the smaller half of each valve being thus, by an exception, posterior.* Hinge- Fig. 80. Nucula Cobboldice (Post-Pliocene). The right valve shows the numerous hinge-teeth and the cartilage-pit. border forming two lines diverging from the umbos ; the teeth resemble those of Area, and are set along the two diverging lines of the hinge. A pit for the cartilage occurs under the umbo. Concentrically or radially marked. Compare Leda, p. 327. Gotlandian to Recent. Trigonia. Shell thick, typically rather angular at umbo, rounded anteriorly ; drawn out, but finally truncated, behind. Umbo bent slightly backwards. A large area or escutcheon is commonly formed on the posterior surface, bounded on each valve by a fold, which runs from the umbo to the lower point * On this point see Verrill and Bush, Am. Journ. Sci., ser. 4, vol. iii. (1897), p. 52. 334 LAMELLIBRANCHIATA. of the truncated border. A subsidiary escutcheon is formed within this by another pair of folds, which occur much nearer to the hinge-line. Marked with nodose radial (rarely concentric) ribs, which are generally coarsely developed. The posterior area above described is often smoother than the rest of the surface, and is marked by striae parallel to the truncated border. Two diverging plate-like cardinal teeth in right valve, with parallel grooves on their broad surfaces ; one strong central tooth in left valve, with a feebler tooth on each side of it. When the shell is lost, the fossil forms are represented by casts, in which the nodose ribs are wanting. Hence the char- acteristic form, and particularly the impressions of the furrowed teeth, must be taken as a guide. Lias to Recent. Yery rare in Gainozoic ; abundant in Jurassic. Schizodus. Form much like a small Trigonia. Surface smooth. Teeth like Trigonia, but not furrowed. Typically Permian. Myophoria. Form somewhat like Trigonia, with an external ridge and furrow running obliquely from the umbo to the junc- tion of the ventral and posterior borders. Smooth, or ribbed concentrically or radially. Teeth as in Trigonia, but the anterior one in the right valve is not furrowed. This genus is with difficulty separated from Trigonia, but the shells are typically smaller and not nodose. Nor is the umbo so angular as in Trigonia. Trias. Unio. Shell fairly thick, elongated oval, and sometimes ap- proaching a parallelogram. Umbos well forward. Commonly rather smooth, concentrically marked. Two irregular teeth in each valve ; thess are followed by one posterior lateral tooth in right valve, and by two in left, elongated parallel to the border. Fresh-water. Trias to Hecent ; especially abundant at the present day. Anodonta. Shell thin ; otherwise like Unio, but devoid of teeth. Fresh-water. Cainozoic. Archanodon. Early ally of Anodonta. Probably toothless. U. Devonian (A. Jukesii), in passage-beds to Carboniferous. Carbonicola (Anthracosia). Outwardly has some resemblance to Cardinia. One cardinal tooth in each valve; no laterals. Probably Fresh-water (Hind). Coal-Measures. Orthonota. Elongated, oval ; umbos near anterior end ; well LAMELLIBRANCHIATA. 335 marked lunule. One or two folds run obliquely from each umbo to the base of the posterior margin. Hinge-line straight, with one or two small cardinal teeth. Of somewhat doubtful affinities. Ordovician and Gotlandian. Note. Gpammysia is closely allied, but its muscular impressions would arbitrarily remove it to the heteromyaria. It is characterised by absence of teeth and by two or more grooves and ridges running from the umbo to the base of the posterior margin, and forming a broader or narrower band across the concentrically striated shell or across its casts. The form is thus much like Orthonota; also like Modiola. Gotlandian and Devonian. D. HETEROMYARIAN SIPHONATE FORMS WITHOUT PALLIAL SINUS (INTEGRIPALLIATE). For muscles, see section E below. Dreissensia (Dreissena). Though a siphonate form, this is a close ally of Lithodomus and Modiola. Shell like My til us, or approximately quadrilateral, with an anterior opening between the valves for the byssus. Hinge-line samewhat curved; umbos terminal. Often a fold, as in Modiola, runs from the umbos towards the base of the posterior margin. Smooth, or con- centrically marked. One or two feebly developed cardinal teeth, accompanied below the umbo by a plate which bears an impression of the anterior muscle. Inner shell-layer absent. Fresh and Brackish Water. JSocene to Recent. Especially Miocene and Pliocene. Congeria. A marine ally of Dreissensia, with a small process for pedal muscle behind anterior muscular impression. Cainozoic and Recent. Abundant in the shallowing Miocene and Pliocene seas of Europe. E. HETEROMYARIAN ASIPHONATE FORMS (INTEGRIPALLIATE). The impression of the anterior adductor muscle is much smaller than that of the posterior. Shell often inequi valve. Avicula. Inequivalve, the left valve more convex than the right. Typically rather small ; obliquely oval, often resembling the shape of an insect's wing. Straight hinge-line; umbo not very prominent. The shell is expanded posteriorly under the hinge-line to form a well-marked ear; a much smaller ear, notched for the exit of the byssus by which the animal was attached, occurs in front of the umbo. Commonly marked radially. One feeble cardinal tooth, and sometimes a long posterior lateral tooth, in each valve. Oartilage-pit broad. Ordovician to Recent; but characteristically Mesozoic and Cainozoic. Pterinea. Much like Avicula, but with larger posterior ear, and a number of short hinge-teeth ; three or more elongated 336 LAMELLIBRANCHIATA. posterior lateral teeth run obliquely from under the umbo towards the centre of the large posterior ear. Ordovician to Carboniferous. Mostly Devonian. Posidonomya (fig. 81). Equi valve. Shell thin and laterally compressed. Obliquely and rather broadly oval. Umbos not prominent; hinge-line short and straight, without true ears. Concentrically marked. No teeth. See Estheria (Phyllopoda). Ordovician to Jurassic. Especially Carboniferous. Monotis. Equivalve; typically small; form much like Posi- donomya, but with a small rounded anterior ear and a more marked posterior one. lladially marked. No teeth. Trias. Daonella. Ally of Monotis. Form rather semicircular, with a fairly long straight hinge-line. No ears. Umbos almost central and not prominent. Radially marked. No teeth. Trias. Inoceramus (fig. 82). Somewhat inequivalve. Varying much in size, some species measuring two feet or more across. Form obliquely oval, with a straight hinge-line. Umbos fairly prominent, sometimes twisted, and set well forward. Concentrically and boldly furrowed; rarely radially furrowed. The hinge-border bears, when viewed from within, numerous closely-set cartilage- Fig. 81. Posidonomya Becheri (Car- Fig. 82. Inoceramus Cuvieri boniferous). (Senonian). Right valve. pits, which lie transversely to the length of the hinge. No teeth. Compare Perna below. The outer layer of the shell is commonly well preserved in the species from the Chalk, though not in those from the Gault; it is easily recognised, even in fragments, by its fibrous cross- fracture (see p. 324) and is often 5 mm. thick. The inner surface of these fragments is seen to be smooth and slightly undulating. LAMELLIBRANCHIATA. 337 A sub-genus Actinoceramus (fig. 83) has been established for the singular species with deep radial furrows ; but transitional forms occur. Fig. 83. Inoceramus (Actino- Fig. 84. Perna Mulleti (Atherfield Clay). ceramus) sulcatus (Gault). Showing (a) the series of ligament-pits in the hinge-line, which is in this species exceptionally produced. Mesozoic. Especially Cretaceous. Perna (fig. 84). Sometimes markedly inequivalve. General resemblance to Inoceramus. Sometimes elongated posteriorly, but often approximately quadrilateral, with rounded ventral border. The umbo, which is acute, is set at the anterior end of the straight hinge-line. Concentrically marked. Numerous trans- verse ligamental pits. Toothless. Material of shell foliaceous. Trias to Recent. Gervillia. Inequivalve ; resembling a much elongated Avicula. Hinge-line straight, with a very small anterior and a larger pos- terior ear. Umbo terminal, like a mere anterior rounding of the hinge-line. Concentrically marked. Oblique ridge-like teeth, running posteriorly [ in sub-genus Hoernesia one cardinal tooth in each valve. Cartilage-pits conspicuous, broad, and set at some distance from one another. Trias to Eocene. Typically a Mesozoic genus. Pinna. Shell thin, elongated, each valve triangular in form ; gaping behind, so that the whole shell is wedge-shaped. Hinge- line long and straight; umbos terminal. Marked with fine concentric lines. No teeth. The inner shell-layer is thin, and is composed of ara- gonite; the prismatic (calcite) layer is thus particularly pro- minent. In the allied genus Pinnigena or Trichites (of Jurassic and Lower Cretaceous age) the prismatic fragments are found an inch or more in thickness. Devonian to Recent. Especially Cretaceous. Mytilus. The common marine mussel. Shell rather thin; 23 335 LAMELLIBRANCHIATA. elongated and approaching triangular, pointed in front, rounded behind. Smooth, or concentrically (rarely radially) marked. Hinge-line straight, umbos terminal. Sometimes one or two obscure cardinal teeth. The modern mussels live near the shore-line, becoming un- covered at low water, and are attached, often to one another, by a coarse byssus. The inner shell layer is aragonite. Trias to Recent. Modiola (fig. 85). Bears some resemblance to Mytilus, but in form approaches an elongated rectangle; the posterior end is more rounded than the anterior, and a broad fold often runs obliquely from the umbo to the base of the posterior margin. Umbos not quite terminal ; hence this -region of the shell has none of the triangular appearance so charac- teristic of the anterior end of Mytilus. Concentrically (rarely radially) marked. No teeth. The modern Modiola is burrowing in habit, or forms a nest around it of fragments of sand, shells, &c. Devonian to Recent. Especi- ally Jurassic. Lithodomus. Close ally of Modiola. Shell cylindrical, narrowed behind, not greatly elongated. No teeth. Burrows into stones (as at the famous Temple of Serapis), corals, &c., forming crypts which yield club-shaped casts (see p. 324;. Carboniferous to Recent. Note. For Dreissena, an ally of the above series, see p. 335. Fig. 85. Modiola Fittoni (Purbeck Beds). Hippopodium (fig. 86). Shell very thick and massive ; form some whatlike Modiola. Lunule present. Coarsely and concen- trically furrowed. One thick oblique cardinal tooth in each valve. The muscular impressions cause the genus to be here placed with the Heteromyaria ; but S. P. Woodward regarded Hippopodium as " a ponderous form of Oypricardia or Cardita." Lias. Fig. 86. Hippopodium ponderosum (Lias). LAMELLIBRANCHIATA. 339 F. MONOMYARIAN ASIPHONATE FORMS (INTEGRIPALLIATE). The shell is closed in the adult by one adductor muscle, which leaves a nearly central impression, placed rather towards the posterior side. It is always the anterior muscle that has dis- appeared. Except where specially mentioned, the members of this sub-group are toothless in the adult condition. Shell often inequivalve. and commonly attached by one or other valve. Hence, when loosened, the lower valve may reveal an outer scar of very various form, sometimes representing, as an external mould, another shell on which the young animal had become fixed. Occasionally in the Ostreidse the young shell lies on some surface with prominent markings, such as that of Trigonia or Cidaris, and both valves become folded to suit the curves of the support. As growth proceeds, the nacreous layer is constantly being added to within, while the shell is also spreading at the margins ; thus the original portions bearing the impress of the support become separated by new material, and form strangely marked umbos to the shell. The impressions are thus convex on the upper valve, concave on the lower; while within no trace of them is to be seen. Ostrea (fig. 87). Attached by left valve. Shell rather inequi- valve, composed of foliaceous layers ; often thick, especially near the umbos. Form rather flat, lower valve more convex. Irregularly rounded at ventral margin, more acute at dorsal, the umbos being nearly central on the hinge-line. Umbo of left Fig. 87. Ostrea expansa (Portlandian). Showing thickened character of the shell and the single muscular impression. Fig. 88.Alcctryonia frons (Cretaceous). lower) valve more prominent than that of right. Concentrically larked, with sometimes broad irregular radial foldings. A well- marked triangular cartilage-pit occurs below the umbo. Shell >mposed of calcite (Sorby). Trias to Recent. Doubtfully Carboniferous. Alectryonia (fig. 88). A genus cut off from Ostrea to include >rms with bold angular ribs and furrows in both valves, the 340 LAMELLIBRANCHIATA. margins becoming consequently acutely folded, and the space occupied by the animal being much restricted in volume. Trias to Recent ; especially Cretaceous. Gryphsea. Free, or attached only by umbo of left valve. Shell inequivalve, thick, oyster-like. Nearly equilateral. Left valve strongly convex, with umbo bent over and inwards ; right valve smaller, natter, or even concave, and sometimes reduced to the appearance of an operculum. The form of the shell varies consider- ably, being, like Ostrea, sometimes expanded and approaching circular, sometimes much narrowed. Concentrically marked. Lias to decent ; especially Jurassic to Lower Cretaceous. Exogyra (fig. 89). Much like Gryphsea, but both umbos twisted backwards almost spirally. Fixed by left valve, which is the larger. Jurassic to Cretaceous. Fig. 89. Exogyra sinuata (L. Cretaceous). Showing twisting of the umbos. Lima. Free, or attached only by byssus. Shell thin, equivalve ; obliquely oval, slightly convex, gaping at anterior border. Umbos somewhat acute, ap proaching a central position, and separ- ated from one another by a space in which a groove occurs for the ligament, which is partly external, partly internal (fig. 90). Short straight hinge-line, with a small ear on each side of umbos. The following have been divided off as sub-genera ;- Fig. 90. Hinge of Lima, showing cartilage-pit. LAMELL1BRANCHIATA. 341 Radula. Strong smooth radial ribs, with some concentric markings. Small byssal cleft under anterior ears. Plagiostoma. Smooth, or with very slight radial markings. Ctenostreon. A coarse form with strong irregularly moulded radial ribs, and a distinct anterior opening for the byssus. Lima is an important genus, ranging from Carboniferous to Recent. Pecten (fig 91). Free, or attached only by byssus. Shell almost equilateral, slightly inequivalve, the right valve being Fig. 91. Pecten ( Chlamys} islandi- Fig. 92. Janvra quinquecostata CMS (Post-Pliocene). Right valve, (Cenomanian). showing the byssal notch. the more convex ; form almost semicircular ventrally, the border becoming straighter towards the pointed umbos. Distinct ears on each side of umbos, the anterior being often the larger. Anterior ear of right valve sometimes deeply notched back for the exit of the byssus, as in the important sub-genus Chlamys. Radially ribbed, sometimes very delicately ; but commonly on a bold but regular scale. A triangular cartilage-pit appears inter- nally under each umbo. The external shell-layer exhibits the prismatic structure with marked distinctness. Both layers are calcite (Sorby). An abundant and important genus. Devonian to Recent. Janira (fig. 92). Inequivalve, the left valve flat or concave. Both valves with well marked ears, and ornamented with strong radial ribs. Inner shell-layer often lost (Zittel) and probably aragonite. Cretaceous to Recent, particularly the former. Aviculopecten. Ally of Pecten, but more inequilateral. Anterior ear small, posterior larger and broader. Radially striated. Cartilage in several grooves, which are fairly parallel to the hinge-line. Devonian to Permian. 342 SCAPHOPODA. The next two genera, although monomyarian, possess hinge- teeth. Plicatula (fig. 93). Attached by Umbo of right valve. Form much like Ostrea. Umbos rather acute, but not prominent. Concentrically marked, and some- times radially folded. Two diver- gent cardinal teeth in each valve, with a cartilage-pit between them. Trias to Recent. Especially Jurassic and Lower Cretaceous. Spondylus. Attached by right valve, not merely by the umbo. Form fairly regular, rounded ventrally, more acute dorsally, and almost equilateral. Umbos separated somewhat, with a small ear on either side. Right (lower) valve with a triangular space between the umbo and hinge-line, in which is a central groove which partly receives the cartilage. Radially ribbed ; right valve sometimes with long spines. Two curved cardinal teeth in each valve, with the cartilage-pit between them. The inner shell-layer is formed of aragonite, and is therefore easily destroyed and seldom found. Jurassic to Recent. Abundant at the present day. Fig. 93. Plicatula spinosa (Lias). Showing the teeth and carti- lage-pit. CHAPTER XXV. FOSSIL GENERIC TYPES. VIII. Scaphopoda. THE animals of this division are Marine, and are sometimes regarded as the lowest of the gastropods. The shell is tubular, and has o&en been mistaken for the calcareous case of a worm. Dentalium. Shell of varying size ; tubular, slightly curved, and tapering from the wider anterior to the narrower posterior end. It thus resembles an elephant's tusk in form, but is open at both ends. Surface sometimes smooth, sometimes longi- tudinally striated. Ordovician to Recent. Especially Cainozoic. &ASTROPODX 343 IX. Gastropoda. These include the typical univalves. The shell is spirally coiled, except in such simple types as the Limpet, and the terms used in describing it are as follows : Spire. The coiled portion of the shell above the terminal and youngest whorl. Whorl. A single revolution of the spiral coil of the shell. Suture. The line of junction of successive whorls, as seen on the surface of the shell; commonly marked by a groove. Umbilicus. The hollow sometimes left in the centre of the shell when the whorls do not touch one another internally. This separation some- times occurs only in the last whorl. Columella. The solid axis com- monly formed where the whorls come in contact in the central line of the spire. This columella is often set with one or more ridges, winding spirally up it. Apex. The point from which the spire commenced its growth. In the old age of some gastropods partitions are formed within the shell below the apex, and the earliest part of the spire finally breaks away. The shell becomes thus imperfect, and is said to be decollated. The apex forms the posterior end of the shell, the mouth the anterior. Mouth. The terminal opening, sometimes very broad, some- times even slit-like. It is entire when in no way notched or prolonged into canals. Canals. Tubular folds of the shell at the mouth, often open along their under- side. An anterior canal may occur, running out in front, and a posterior, directed up the outside of the spire, Holostomatous shells have the mouth entire (fig. 101); Siphono- a>.c. Fig. 94. Gastropod (Fusus}. a, Apex. a.c, Anterior Canal, i, Inner Lip. o, Outer Lip. sp, Spire. 8U, Suture. 344 GASTROPODA. stomatous shells possess an anterior canal. The animals of the latter division are, at the present day, almost all carnivorous. Inner Lip. The margin of the mouth nearest to the axis of the spire. Outer Lip. The margin of the mouth away from the axis of the spire. This margin is sometimes thin, sometimes thickened, sometimes prolonged into* spines, the latter being in reality tubular folds resembling the canals. Varices Eidges or spinose bands running from the apex at intervals down the shell, and representing those stages in the growth of the spire when spines or thickenings of the outer lip took place (fig. 96). Opercuium. A horny or calcareous plate borne by the posterior part of the foot of the animal in some genera, and serving to close the mouth of the shell when the animal retracts itself. The spire is typically so coiled that, when the apex is placed above, and the mouth below and facing the observer, the mouth lies to the right of the axis. Such shells are right-handed. Left- handed shells, however, occur at times, in which the spire is coiled in the opposite direction. The representation of gastropod shells in drawings with either the apex or the mouth upwards must be a matter of indifference, the best nomenclature of the extremities of the shell being, as already stated, "anterior" and "posterior," not "lower" and " upper." Several terms have been used to describe the form of the shell as a whole. The following may require explanation : Turbinate. Shell rather broadly conical as regards the spire, and approaching hemispherical below (fig. 103). Turreted. Shell with an elongated spire, and not much pro- longed anteriorly (fig. 100). Spindle-shaped. Shell with the anterior end also produced and narrowed, so that the stoutest region lies between two taper- ing portions (fig. 94). The shell-substance in most gastropods consists of three layers, as can be seen with a lens when the shell of modern examples has been cut across. In life, it is covered in very many genera by a skin, or " periostracum." The shell is formed of calcareous prisms as in the lamellibranchs, these prisms being grouped to form lamellae which are arranged in the central layer differently from those of the outer and inner layers. See Zittel, Palceontologie, Bd. ii., p. 158 ; Cambridge Nat. Hist., Molluscs, p. 255. The material is aragonite, and hence gastropods are often represented in fossil deposits only by casts. Some few species (for the character is not even generic) have an outer layer com- GASTROPODA. 345 posed of calcite. Mr. P. F. Kendall regards the shell of Scalaria as calcite ; and he remarks that the first two whorls in those species of Fusus which possess a calcite layer consist entirely of aragonite.* The gastropoda mostly inhabit the sea. The fresh-water genera will be specially indicated. The division of the Pul- monata contains several genera that live entirely on land. With the exception of the Pulmonata, all the gastropod genera, the shells of which are here discussed, belong to the order Prosobranchiata, or, in default of complete evidence, are placed in that order on account of the resemblance of their hard parts to those of living forms. The division of the members of this order into Holostomata and Siphonostomata, according to the character of the mouths of their shells, has proved unsatis- factory,, since it separates forms otherwise closely allied. The genera are here taken practically in the order adopted by Fischer, a group of allied shells being occasionally marked out by inclusion between two black lines. A. PROSOBRANCHIATA. The animal has its branchiae placed anteriorly to the heart. Conus. Spire short; last whorl large and narrowing anteriorly, so that the shell, with its apex upwards, resembles a short cone set on a steep inverted one. Mouth slit-like and long, with a slight anterior fold. Outer lip sharp, and notched back near the suture. Oolumella smooth. Surface commonly smooth, with mere growth-lines. S. P. Woodward quotes Conus monilis with a specific gravity of 2-910 (hence aragonite), and the fossil Conus ponder osus of the Miocene as 2-713. Some species may, therefore, contain a thick calcite layer. Upper Cretaceous to Recent. Abundant at the present day. Pleuretoma (fig. 95). This genus has been greatly subdivided. Shell spindle-shaped; spire rather longer than the last whorl, and generally well indented at the suture. Mouth long, with anterior canal. Outer lip with a marked notch near the suture, which leaves a band on the shell as it closes over during growth. If the lip in a fossil form is broken away, the growth-lines may still indicate its backward curve. Columella smooth or with one or two ridges. Surface commonly ribbed vertically or horizontally, often with little spirally arranged knots. * " Aragonite Shells in the Coralline Crag." Geol. Mag., 1883, p. 489. 34:6 GASTROPODA. Cretaceous to Recent. Voluta (fig. 96). This name also covers many sub-genera. Shell thick. Shape somewhat like Conus, but more nodose and irregular. Spire fairly short; last whorl large. Mouth long, Fig. 95. Fig. 96. Pl6urotomarotata(Post- Voluta athleta (Barton Beds}. Pliocene). Showing Showing knots and varices; the notched outer lip. also folds on the columella. with slight anterior fold. Outer lip fairly thick. Columella and inner lip with several ridges. Surface often marked with prominent short spinose outgrowths, and sometimes with corre- sponding varices. Upper Cretaceous to Recent. Especially Eocene. Fusus (fig. 94). Shell thick. Spindle-shaped to ovoid, typically the former. Spire fairly long. Mouth oval, with long straight anterior canal, and no posterior notch. Columella smooth. Some late Cainozoic species have an outer calcite layer (Kendall). The genus has been much subdivided.* The sub-genus Clavella has its mouth sharply narrowed to form the canal, not tapering down as in Fusus proper. Jurassic to Recent. Most abundant in earlier Cainozoic. Clavella is Cainozoic, especially Eocene. Buccinum (the Whelk; fig. 97). Shell fairly ovoid, with few whorls, the last being large. Spire, however, prominent. Mouth oval, with a broad shallow anterior fold representing the canal. Inner lip smooth. Surface generally marked with vertical and spiral ridges. Pliocene to Recent. Nassa. Like Buccinum, but canal-fold more marked and slightly oblique. Outer lip marked with fine ribs running inwards. Upper Cretaceous to Recent. . Especially later Cainozoic. * For a revision of many species, see Grabau, Smithsonian Collections^ No. 1417 (1904). GASTROPODA. 347 Murex. Shell thick, ovoid to spindle-shaped ; whorls strongly convex. Mouth rounded, but prolonged into a well-marked and sometimes long anterior canal, the sides of which fold over so as to make it almost tubular. Outer lip thick, and sometimes ribbed. Surface set with three or more strong varices, which are often re- markably knotty or spinose. Outer layer of shell calcite (Kendall). Upper Cretaceous to Recent. Fig. 97. Buccinum undatum Fig. ^B. (Pliocene). (Pliocene). Trophon. Ally of Murex. Spindle-shaped. Canal wider and bent on one side. Yarices not set with knots. Pliocene to Recent. Purpura. Shell thick, ovoid. Spire rather short. Mouth oval, with a canal-fold scarcely more marked than that of Buccinum. Inner lip flattened down and smooth. Outer layer of shell calcite (Sorby). Miocene to Recent. Cassidaria. Shell thick and ovoid. Spire short; last whorl large and strongly convex. Mouth elongated oval, with a well- marked broad and obliquely bent canal. Outer lip expanded j inner lip often ridged. Surface variously marked. U. Cretaceous to Recent. Ficula (Pyrula in part ; fig. 98). Shell thin, ovoid, narrowed anteriorly. Spire very short, last whorl very large. Mouth large, and prolonged into a broad open anterior canal. Lower Cretaceous to Cainozoic ; especially later Cainozoic, but not abundant. Rostellaria. Spindle-shaped; spire long, commonly without much indentation at the suture. Mouth long, with a somewhat tubular elongated anterior canal, and a posterior groove-like canal running towards the apex of the spire. Outer lip rather broadly expanded and sometimes notched on edge. 348 GASTROPODA. In the sub-genus Hippochrenes (fig. 99) the shell is generally smooth. The posterior prolongation of the mouth runs up to the apex of the spire, and the outer lip is not serrated, except by the occurrence of a small anterior notch. In the sub-genus Eimella the surface is striated. The posterior groove is shorter, and the outer lip is thickened and sometimes serrated. Cretaceous to Recent. Compare next two genera. Alaria. General outline spindle-shaped ; spire fairly long. Mouth elongated, with well-marked anterior, but no posterior canal. Outer lip much expanded, and prolonged into finger-like canals. Surface often set with varices and knobs. This genus is variously limited by different authors, and some of its species are often carried over into Aporrhais. Jurassic to Cretaceous. Aporrhais (Chenopus). Distinguished from Alaria by the prolongation of the mouth posteriorly as a groove-like canal Fig. W.Rostettaria (Rippochrenes) ampla (London Clay). Fig. 100. Oerithium plicatum(left hand figure) and Cerithium elegans (Hamstead Beds). Show- ing the short oblique anterior canal. part way up the spire. The anterior canal is shorter than is common in Alaria, and there is a shallow fold in the outer lip near it. From Rostellaria it is distinguished by the prominent finger-like processes of the outer lip. Lias to Recent. GASTROPODA. 349 Cerithium (fig. 100). A very typical turreted shell, the spire being elongated and conical, and the last whorl being in no way disproportionate in bulk. Mouth oval, obliquely sloped towards a well-marked and short anterior canal, which is somewhat bent aside and backwards. The mouth is often narrowed posteriorly so as to terminate in an acute angle. Surface often set with knots. Trias to Recent. Especially Cainozoic. Potamides. Olose ally of Cerithium, with straighter canal, which is commonly not so well marked. Brackish and fresh- water. Difficult to mark off from Cerithium except in its actual habitat. In life, Oerithium has no periostracum, while Potamides has a thick one. Eocene to Recent. Melania. Much like Cerithium, but holostomatous. Form at times nearly ovoid. Apex sometimes worn away during life (decollated). Fresh-water. Wealden to Recent. Melanopsis. Ovoid rather than turreted, with short spire. Mouth with anterior notch or small canal, and a more or less marked groove-like posterior prolongation. Inner lip thickened. Apex sometimes decollated. Fresh-water. Cretaceous to Recent. Turritella (fig. 101). Shell as typically turreted as Cerithium. Spire long, often only slightly indented at the suture ; at other times with convex whorls. Holostomatous; mouth oval to round, but sometimes narrowed anteriorly. Outer lip thin. Surface marked with spiral ribs or striae, there being a striking absence of the knots and vertical ribs so common in Cerithium and Melania. Jurassic to Recent, but conspicuously Cainozoic. Pseudomelania (Chemnitzia in part). Turreted ; commonly large. Little in- dented at the suture, and hence fairly conical. Mouth oval, without canal ; wider in front and narrowed behind. Sur- face commonly marked by fine growth-lines. Trias to Miocene. Bourguetia. Shell large, turreted, and elongated. Whorls distinctly convex; longitudinally striated ; last whorl large. Mouth round in front and nar rowed behind. Fig. 101. Turritella incrassata (Pliocene). 350 GASTROPODA. Jurassic. Perhaps Carboniferous. Nerinea (fig. 102). Ally of Cerithium. Turreted ; almost conical, commonly with a spiral band-like rid^e above the suture. Mouth diamond-shaped to oval, with short oblique anterior canal. The outer and inner lip, as well as the columella, have commonly one or more ridge-like thickenings, which wind up inside the shell, so that sections parallel to the axis are very characteristic, the projection of these ridges into the cavity leaving only a remarkably constricted space for the animal. In some species (grouped under sub-genera) an umbilicus occurs in place of the columella, and distinctions are made according to the number and dis- Fi S- ^2 Nerinea Good- tribution of the thickenings on the ^. (Corallian). Worn i.rv, , . , , ~ *? , ,, specimen, with section umerent internal surfaces of the shell. to show reduction of Jurassic to Cretaceous. the internal cavity. Rissoa. Always small, about 5 mm. long. Shell thick; rather broadly conical ; spire fairly long Mouth oval, rather narrowed posteriorly, with thickened outer lip. Surface often vertically ribbed. The animal lives mostly near shore. Jurassic to Recent. Especially Cainozoic. Hydrobia. Small, and much like Rissoa ; typically rather longer in the spire. Surface smooth. Mostly brackish or fresh- water. Jurassic to Recent. Especially Cainozoic. Paludina (Vivipara). Shell thin; turb>nate; whorls strongly convex. Mouth oval to almost civcular, slightly narrowed posteriorly. Sometimes an umbilicus is present. Surface smooth or with mere growth-lines. Fresh-water. Jurassic to Recent. Natica. Shell thick; practically globular, the spire being very short, the last whorl very large and convex. Mouth semicircular (i.e., straight-sided at the inner lip and curved along the outer) or approaching oval. Outer lip sharp ; inner lip thickened. Umbilicus typically well marked, but in some species absent. Surface commonly smooth. Spiral operculum. GASTROPODA. 351 Trias to Recent. Some of the earlier forms described as Natica belong to Naticopsis. Littorina (the Periwinkle). Shell thick, almost globular, with few whorls, the last being large. Mouth rounded, more acute posteriorly. Outer lip sharp at the edge ; inner lip flattened at the columella. Surface commonly marked with growth-lines and spiral striae. Outer layer composed of calcite (Sorby). The animal inhabits the shore, sometimes forming considerable shell- banks. Lias to Recent. Nerita. Shell thick. Spire very small ; last whorl very large, and prolonged out rather more obliquely than in Natica, so that the total effect is not so globular. Mouth semicircular j outer lip commonly thickened and set with little ridges directed inwards; inner lip generally also ridged. Columella flattened. Surface smooth, or spirally ribbed. Lias to Recent. Mostly Cainozoic. The Mesozoic forms have the typical shape, but most of them have smooth lips, the outer lip being, moreover, sharp in many examples. Neritina. Close ally of Nerita, the shape being similar. Outer lip sharp, not thickened ; inner lip marked by ridges. Surface ornamented with coloured lines and spots, which are preserved even in many fossil specimens. Typically fresh-water ; sometimes brackish. The form of the shell and its mouth-characters will not serve to distinguish Neritina from some of the early Neritas, which are, however, undoubtedly marine. Eocene to Recent. Perhaps Mesozoic. Naticopsis. Form like Natica, but expanding very rapidly. No umbilicus. Mouth approaching oval Operculum convex outwardly, and not spiral. Devonian to Trias. Turbo. Form typically turbinate, sometimes approaching Littorina. Mouth round, lips not meeting to form a continuous border, part of the mouth being bounded merely by the surface of the whorl. Surface of shell often spirally ribbed. Operculum thickened with calcareous deposit till it becomes outwardly almost hemispherical, the massive examples from large living species being sometimes used as ornaments. See Trochus. Ordovician to Recent. Trochus. Allied to Turbo, but broadly conical, and somewhat flattened below. Mouth without continuous lips, but more angular than in Turbo. Operculum horny only. The genus has been much divided into sub-genera. Some fossil forms, the 352 GASTROPODA. opercula being absent, may possibly be referred to Turbo ; but the difference of outer form is characteristic. The living species are mostly shallow-water forms. Gotlandian to Recent. Phasianella. Ally of Trochus. Shell like a rather elongated Paludina; often fairly large. Mouth oval; outer lip thin. Sur- face smooth. Cretaceous to Recent. Earlier forms have now been referred to Bourguetia. Euomphalus. Shell fairly large. Almost discoidal, the spire being very low. Mouth more or less polygonal, with a slight notch not far from the suture. Umbilicus extremely wide. The earlier part of the shell is sometimes seen in sections to be cut off by partitions. Gotlandian to Carboniferous. Pleurotomaria. Shell like Trochus, but sometimes with a low spire. Mouth fairly round ; outer lip with a deep slit in it, which runs back along the whorl. As growth proceeds, this slit is closed over, and leaves a band-like mark or a ridge running spirally round the shell about the middle of the whorl. Umbilicus sometimes present. Surface of shell generally hand- somely ornamented with spiral ribs and knobs ; but no trace of these appears in casts, which are common in some formations and which are not identifiable with certainty. Cambrian to Recent. Abundant in earlier Mesozoic ; very rare at present day. Murchisonia. Shell turreted ; slit in outer lip, as in Pleuro- tomaria, and similarly closed by a band as the shell grows. Surface smooth, or with longitudinal ridges. Cambrian to Trias ; especially Devonian and Carboniferous. Bellerophon. Formerly often classed as a Heteropod. Shell shaped like a rather open Nautilus, the spired character typical of the gastropods being wanting. Mouth widely expanded and fairly circular, the shell coiling over inwards symmetrically in the centre of its inner lip. Outer lip deeply notched, a corre- sponding band running round the exterior of the shell. The shells are often distorted and compressed in the older formations, and the expanded character of the mouth becomes partly lost. Distinguish carefully from Cephalopods. Cambrian to Permian. Cralerus (Calyptraea in part). Shell rather like that of the GASTROPODA. 353 Limpet (Patella), being flatly conical ; but generally with a trace of spiral winding at the apex. Below, a spirally bent plate, which is free anteriorly and is attached along its posterior margin, crosses what would otherwise be a wide unbroken mouth. The under side of the apex is thus partly partitioned off. In Calyp- trsea proper this plate is represented by a spoon-shaped or bent process dependent from below the apex. Galerus is Upper Cretaceous to Recent. Patella. The Limpet. Shell like a flattish cone ; mouth oval ; apex of shell nearly central above it. Surface usually radially ribbed. Cretaceous to Recent. Note. Shells allied to Patella occur in the oldest Cambrian strata ; in the absence of the animal, it is difficult to refer shells of this type to existing genera. B. PULMONATA. The animal breathes by a lung-sac in place of branchiae. The group is almost entirely fresh-water or terrestrial. Delabeche (Geol. Observer, 2nd. ed., p. 122) gives the specific gravity of land-shells as 2-82 to 2-87. The material is thus probably aragonite. Helix (the common Snail j fig. 103). Shell variously shaped, but commonly rather flatly conical; at times approaching dis- Fig. 103. Helix occlusa Fig. 104. Planorbis euomphalu* (Oligocene). (Oligocene). coidal. Mouth obliquely semicircular or oval ; outer lip some- times slightly expanded and thickened ; inner lip represented, as in Turbo, partly by the surface of the whorl. Sometimes, however, a calcareous thickening occurs in the position of the inner lip (fig. 103). Rather broad umbilicus. Surface commonly smooth, with mere growth-lines. Terrestrial. 23 354 PTEROPODA. Eocene to Recent. Species very numerous at the present time. The fact that the animal lived on land may account for its apparent absence in earlier periods, sub-aerial deposits being rarely preserved. Bulimus. Elongated ovoid, or turreted, often of large size. Mouth oval, not oblique, and rather long from anterior to posterior end. Thickened and continuous lips, which are sometimes ex- panded. No umbilicus. Surface commonly smooth, or only slightly ornamented. Terrestrial See note on Helix. Upper Cretaceous to Recent. Limnsea. Shell particularly thin and fragile. Elongated oval, with large final whorl. Mouth large, rounded anteriorly, elongated from front to back. Lips thin and sharp. Oolumella twisted obliquely. Surface smooth. Fresh-water. Purbeck to Recent. Especially Cainozoic. Planorbis (fig. 104). Shell delicate, as in Limnsea ; various in form, but typically discoidal, the spire being very short, and the coils almost in one plane, as in Euomphalus. Mouth semicircular to oval. Outer lip sharp ; inner lip represented by the surface of the whorl. Umbilicus very broad. Surface commonly smooth, with mere growth-lines. Fresh- water. Jurassic to Recent. Especially Cainozoic. X. Pteropoda. While the small and commonly conical calcareous shells of several of the Thecosomata, or shell-bearing Pteropods, are found in Cainozoic deposits, some large Palaeozoic genera occur, which differ widely from the modern type. Their reference to the Pteropods must still be considered provisional, since, in the face of such strange forms as Calceola among the Corals and the Hippuritidse among the Lamellibranchs, these conical and some- times operculate shells may belong to groups, the other members of which differ from them in external aspect. Mr. G. F. Matthew has, it may be noted, referred Hyolithes to the worms (Trans, R. Soc. Canada, vol. vii., 1901, sect. 4, p. 101). Hyolithes (Theca). Shell small or large, even reaching 20 cm, CEPHALOPODA. 855 in length. Triangular in cross-section ; straight, tapering, form- ing a long steep pyramid. Aperture at the wider end of the shell, its border lying obliquely to the axis. An operculum has been observed fitting into it. Commonly marked with oblique striae. These shells are often crushed flat in early Palaeozoic shales, but the long pyramidal form remains recognisable. L. Cambrian to Permian. Almost all are early Palaeozoic. Conularia. Shaped like a straight steep-sided pyramid, as in Theca, and similarly variable in size. Some species are slightly curved at the posterior end. Four-sided, giving square, or approximately square, cross - sections. Each side bears a shallow longitudinal furrow. Oommonly marked with oblique striae. Septa have been observed, dividing off the lower part of the cavity. See Miss J. L. Slater, "British Conulariae," Pal Soc., 1907. Cambrian to Lias. Almost all are early Palaeozoic. Tentaculites. Often described as the tube of an annelid. Rather small, steeply conical ; circular in cross-section. Surface marked with ring-like ridges, which lie in planes perpendicular to the axis of the shell. The forms with longitudinal striae have a small ellipsoidal expansion at the initial end (Novak, Beitr. Pal. Oesterreich-Ungarns, Bd. ii., 1882, p. 50). Ordovician to Devonian. XI. Cephalopoda. The shell-bearing forms of these highly developed molluscs were in former ages far more numerous than at the present day ; and the species are often sufficiently constant and widely spread to serve in marking special palaeontological zones. At the present time we have a prevalence of one great dibranchiate order (which includes the ordinary cuttle-fish, Argonauta, single angular lobe. through the strong convexity and the involute arrangement of its whorls. The suture-lines are occasionally slightly folded, but are commonly zigzag or bent rather sharply, the lobes and saddles being sometimes numerous. The lobes and saddles are, however, not subdivided by notched or foliaceous boundaries, as occurs in the next sub-group (fig. 112). The siphuncle is in contact with the convex side of the whorl, and is best seen where it emerges through any septum which may be displayed terminally on the specimen. Retrosiphonate. Aptychi have been recorded. CEPHALOPODA. 363 Giyphioceras (fig. 109). Shell involute, generally globose. Suture-line with external lobe divided by a small median saddle ; external saddle narrow ; lateral lobe pointed ; lateral saddle broad and rounded. Surface practically smooth. (Ex- amples : Giyphioceras crenistria, Gl. sphcericum, GL truncatum.) Carboniferous and Permian. Gastrioceras. Shell with wide umbilicus. Suture-line with broad and deep external lobe, with small median saddle ; first lateral lobe deep and angular; second lateral lobe small and angular. Surface with longitudinal striae ; often with transverse ribs in addition, which are nodose near inner ends. (Example : Gastrioceras Listeri.) Carboniferous and Permian. Prolecanites. Shell with wide umbilicus ; whorls flattened laterally. Suture-line with several deep lobes and saddles, the lobes broadly pointed at the ends, the saddles rounded at the ends and narrowed near their bases. Surface smooth. (Example : Prolecanites compressus.) Devonian and Carboniferous. Note.. Baetrites (Ordovician to Carboniferous) is the straight form of the Goniatites. Pronorit0S (Permo- Carboniferous), with its accessory serrations at the ends of the lobes, and MedliCOttia (also Permo-Carbon- iferous), with its still more elaborate suture-lines, link the Goniatites completely to the sub-group of the Ammonitea. Sub-Group 3 PROSIPHONATE SHELLS WITH SIPHUNCLE ON THE CONVEX SIDE. These are the successors of the Goniatites, and are distinguished, apart from the character of their septal necks, by a greater complexity in the suture-lines, the main lobes and saddles being variously subdivided and broken up (fig. 112). The mouth-border is produced, not notched, on the outer side. The surface of the shell is also more strongly ornamented than in the preceding sub-groups, and is, indeed, very rarely smooth. This sub-group covers the great series of shells which are commonly styled Ammonites, the earliest forms of which are now known from the Coal-Measures of Texas.* It has become necessary to subdivide the old genus of "cornua Ammonis," and to establish a large number of new ones, each example of which may be properly styled an "Ammonite." Fischer's restriction of "Ammonites " to the members of the newer genus Arietites seems liable to cause confusion, and would destroy *J. P. Smith on the "Super-family" Arcestidae, in Monograph xlii. (1903), U.S. Oeol. Survey. 364 CEPHALOPODA. the utility of the word " Ammonite," which now, as formerly, covers a great series of shells allied to one another. New subdivisions are being, however, continually introduced, and for details larger and special works must be consulted. Ceratites. Shell umbilicated j cross-section of whorl somewhat flattened laterally. Suture-lines sometimes with auxiliary lobes and saddles. The saddles are always rounded, the curve ap- proaching semicircular; but the lobes are subdivided, their posterior border being zigzag* (fig. 110). Surface marked with Fig. 110. Suture-line of Ceratites nodosus. e.l, External lobe (which is broad when viewed from above). e.s f Ex- ternal saddle. l.U, 1st lateral lobe. Fig. 111. Natural cast of Arcestes Backhi (Trias), showing traces of successive constrictions. Part of the shell remains on the left, and the suture line can be traced on the right-hand portion of the cast. (After Mojsisovics.) ribs, which do not pass on to the outer border, but which often bear knobs as they approach it. Exclusively Triassic. Trachyceras. Shell with rather narrow umbilicus. Sutures like Oeratites in the earliest species; but in later forms both the saddles and the lobes are denticulated (i.e., bent into a zigzag form). Surface ribbed transversely, the ribs set with knots ; a furrow runs along the convex margin. (Example : Trachyceras Aon.) Trias. Arcestes (fig. 111). Shell involute, sometimes with small * A very similar type of suture recurs among Cretaceous Ammonites referred to the family of the Amaltheidae. CEPHALOPODA. 365 umbilicus; whorls markedly convex. Mouth slightly reduced by the folding over of its outer and lateral borders. Body- chamber occupying more than a whole whorl. Sutures with numerous auxiliary lobes and saddles, the line being complex and foliaceous, so that the markings look like the outlines of little branching trees. The axis of each lobe and saddle is, however, straight. Surface smooth, or with fine transverse striations. The mouth-border appears to have become thickened internally at various stages of growth, so that casts (fig. Ill) exhibit well marked and rather wavy grooves running at wide intervals from the outer to the inner side of the whorl. (Example : Arcestes subumbilicatus. ) Trias. Also Permo- Carboniferous of India, with another genus of Ammonite, Cyclolobus. Monophyllites. Shell rather flat and discoidal, with fairly wide umbilicus. The whorls enlarge rather rapidly, giving a high mouth. Suture-lines with numerous lobes and saddles, which are foliaceous ; but each saddle terminates anteriorly in a Us. - - etace- Baculites (fig. 116).-Shell straight, ^f^ffS^A^ narrowing to a point posteriorly, bilateral subdirision of the and laterally compressed. Aptychus lobes and saddles. The known. mouth lies towards the Cretaceous. ri S ht - Hamites. Evolute. Shell straight for part of its length, but curved over in a hook-like manner at one or both ends, so as to bend back parallel to its former direction. Hamites is sometimes restricted to forms which have bent thus twice or three times during their growth, while those only once bent are styled Hamulina. Surface of most species rather simply ribbed. Cretaceous. ^JBJKB$s CEPHALOPODA. 371 Crioceras. Form like an Ammonite, but evolute. Surface variously marked ; but generally with strong simple ribs. See Ancyloceras. Jurassic to L. Cretaceous. Ancyloceras. Form commencing like Crioceras; then becoming straight ; and finally curving back along its inner side like the terminal part of Hamites. Hence Ancyloceras may be a late stage in the growth of Crioceras. Jurassic to Lower Cretaceous. Scaphites (fig. 117). Form commencing as an involute shell ; then running straight for a short distance; and finally curving back along its inner side. Commonly much stouter and more rotund than Ancyloceras. Aptychus or synaptychus known. Upper Cretaceous. Turrilites. Form spiral, generally left- handed (i.e., opposite to the mode of Fig. wScaphto. C i lin f of typo* gastropods). The ^wo^(Cenomanian). whorls are sometimes not in contact. Surface commonly marked with nodose ribs. The suture-lines of course readily distinguish this form from turreted gastropods; the cross-fracture of imperfect speci- mens has generally taken place along a septum, the characteristic ammonoid folding of which can at once be seen. Cretaceous. 0. PHRAGMOPHORA, All are regarded as dibranchiate. Belemnites (fig. 118). The chambered shell in this genus is reduced to a conical body, the Phragmocone (or Phragmacone), which is divided internally by simple concave septa. The interseptal chambers are connected by a siphuncle, which runs down one side of the phragmocone, this being consequently called the ventral side. The phragmocone is closely fitted into a hollow, styled the Alveolus,* which occurs at the anterior end of a strong pencil -like calcareous body, the Guard or Rostrum. This solid guard forms the object so commonly found, and is thus popularly known as the " be- lemnite." * The Phragmocone itself has also been styled the Alveolus, 372 CEPHALOPODA. The guard is of various proportions, sometimes delicately tapering, sometimes broad and stout, sometimes thickening anteriorly for a certain distance and then decreasing in diameter, to expand again as the alveolus is neared. In cross-section, as when broken, it shows a fibrous radial struc- ture, and the calcite of which it is formed is usually stained somewhat brown. The apex of the conical alveolus is directed slightly to the " ventral " side, and determines the point from which the calcite prisms radiate in the guard; hence the axis of the guard is eccentric, and the "dorsal" or " ventral" side oi imperfect specimens can be determined by noting which part of the circum- ference is respectively farthest from or nearest to the point from which the prisms radiate. The guard has typically a smooth sur- face, on which vascular impressions, like those of ramifying rootlets, can occasion- ally be seen. A furrow sometimes runs down the ventral side, or more rarely down the dorsal, often reaching to the point of the guard. At the point itself furrows sometimes arise, extending some distance up the sides; and a common feature is the presence of two long and almost parallel grooves running thus up the dorsal side. The alveolus is often empty; and sometimes the phrag- mocone is found without the guard. In fine and carefully cleaned specimens, not only can the phragmocone be seen in place, but traces of a broad expansion of its dorsal side extend considerably above and beyond it. This thin anterior expansion is the Pro-ostracum, and covers the ink-bag, the solidified contents of which, forming a black pear-shaped body, have also been found in situ. Above this, again, im- pressions of the crown of arms about the head of the animal may be seen, the little hooked teeth with which they were set lying in rows along them. Hence the "be- lemnite " familiar to collectors formed only the posterior hard part of an animal allied to our modern unprotected cuttle-fish. Rhmtic to Albian, Fig. 118. Guard of Belemnites, cut open above to show remains of the phragmo- cone resting in the alveolus. 1 CEPHALOPODA. 373 Belemnitella (fig. 119). This form is practically only a sub- genus of Belemnites, characterised by a slit at the anterior end of the guard and parallel to its axis. This slit reaches in to the alveolus, and marks the ventral side. The phragmocone (of which several specimens, mostly preserved in silica, are known), has a low ridge running down the dorsal side, a corresponding shal- low groove occurring in the alveolus. The phragmocone, though rare, was described by Count Miinster as early as 1830. The guard shows distinct vascular markings on its ventral surface. (Compare Actino- Fig. 120. Guard of Actinocamax plenus (Belemnitella plena). Cenomanian. TJ /-/ . U PP er Cretaceous. Actinocamax (1823 ; AtractiliteS, 1807). No alveolus, or Fig. 119. Guard of Belemnitella mucro- nata (Senonian). Showing the slit and traces of vascular markings. only a shallow one, which is often four-sided rather than circular in cross-section (Actinocamax quudratus). The common charac- ter of these forms is the distinctly lamellar structure of the anterior end of the guard, so that it easily becomes broken away and injured. In apparently perfect specimens, however, as in Actinocamax plenus (fig. 120; often styled Belemnitella plena), the anterior end may be pyramidal, not hollowed out by an alveolus; in this case the phragmocone must have been surrounded by only a horny continuation of the guard. In the species quoted, a slight groove occurs at the apex, which may correspond to the slit in Belemnitella. Both in Belemnitella and Actinocamax the guard may be suddenly reduced in diameter near its point, thus terminating in a short spinose process, the "mucro." It has often been suggested that Actinocamax is only an imperfectly preserved 374 CEPHALOPODA. Belemnitella; but the uniform character of specimens at certain horizons is evidence that the phragmocone was largely above, and not included in, the true calcareous guard.* Upper Cretaceous. Note. Phragmophora with greatly elongated guards occur in the Upper Trias. In Belemnoteuthis of the Oxfordian, on the other hand, the guard is a mere short sheath about the phragmocone. BelOSepla of the Eocene has a short stout bent guard, expanded anteriorly, and protecting a curved phragmocone, a wide depression on the concave side of which does duty for a siphuncle. The pro-ostracum is large. In Spirulirostra (Miocene) the guard forms a stout short process below a curved phragmocone, which possesses a true siphuncle. In the modern Spirilla the phragmocone alone remains, in the form of a delicate evolute spiral shell, with a siphuncle on the concave side. This shell, though exposed by a cleft of the mantle, is truly internal. Sepia (Eocene to Recent) has the merest trace of a guard at the end of a phragmocone, the chambers of which are flattened, and which forms the well-known "cuttle-bone." It is important to note, however, that cepha- lopoda with a mere thin horny pro-ostracum (the "pen") over the ink-bag, and no trace of chambered shell cy guard, occur as contemporaries of even the earlier belemnites. Thus Geoteuthis of the British Lias has been placed in the same group, the Chondrophora, as LoUgo, the Squid of the present day. * As to Belemnitella and Actinocamax, see Dr. Cl. Schliiter, " Cephalo- poden der oberen deutschen Kreide," Palceontographica, vol xxiv. (1876-7)i p. 63, and plate Hi. (17), &c. ECHINODERMATA. 375 CHAPTER XXVI. FOSSIL GENERIC TYPES. XII. Echinodermata. THE Echinoderms present in their hard parts a great variety of forms, characterised, however, by the prevalence of penta- gonal symmetry. The fact that their shells or skeletons, exter- nal or internal, are built up of plates, causes their remains often to be found only in a fragmentary condition. The calcite of which these parts is composed assumes a completely crystalline structure : so that any part of the shell or skeleton cleaves across on fracture along the rhombohedral surfaces so familiar in Iceland-spar. The opaque white but gleaming cleavage-surfaces of echinodermal fragments may thus be picked out by the eye on rock-exposures from among the fractured remains of other organisms. The individual plates or block-like calcareous bodies of which the hard parts are composed are styled the Ossicles. All the Echinodermata are Marine. A. CRINOIDEA. These are the typical " sea-lilies " or " encrinites." The animal during the whole or earlier part of its existence is fixed to the sea-bottom, commonly by a flexible stalk or Stem, which bears root-like processes at its base. The principal terms used in describing the hard parts of Grinoids are as follows : Ossicles or plates. The individual calcareous bodies of which any of the hard structures are built up. Calyx. The cup-like structure, sometimes closed over above, formed of calcareous plates, and enclosing the body of the animal. Its under surface or base, which is attached to the apex of the stem (or, as in Holopvis, directly to the sea-floor), corresponds to the upper surface of most echinoderms ; its upper or oral surface bears the mouth and generally the anal aperture. The calyx and the arms are often spoken of together as the Grown of the crinoid, and the calyx is sometimes freely termed the "head." The upper (ventral) covering, or tegmen, ot the calyx may be membranous, with little plates developed in it, thus leaving a circular gap in fossil forms ; 01 it may form a dome-like structure of numerous plates in contact. There is no doubt that in some genera this dome was represented by a flexible ventral sac. 376 ECHINODERMATA. The Mouth lies centrally on or below the tegmen, and grooves lead to it from the bases of the arms. The Anus is typically excentric, and is interradial in position (see below) ; that is, it occurs between two of the arms. In some extinct genera, the dome bears one aperture, that of the anus, which then occurs almost centrally, and at times on the end of a tube or " proboscis." In these cases the mouth is concealed beneath the dome, and the brachial grooves run through the wall, and are prolonged as little canals towards the centre, where they reach the mouth. The plates composing the calyx are grouped in several series from the base upwards to the region of the mouth. The lowest plates, meeting in the centre of the base, are 2 to 5 in number (commonly 5), and are termed Basals. They are often hidden in fossil specimens by adhesion to the upper stem-joints. Sometimes, however, the base is formed of two cycles of plates, an upper one, the true Basals, in this case sometimes styled Parabasals ; and a lower cycle, alternating with the upper, and termed Infrabasals (see fig. 122). Next above the basals or the parabasals, and in either case alternating with them when the base has petagonal symmetry, is the cycle of the Radials, commonly 5 in number j vertically above these the arms of the crinoid rise. On each plate of this primary radial series one or more similar plates may stand (fig. 121), so that each arm may be supported on a vertical row of several ossicles, which are commonly entitled first, second, third, &c., radials (see "Arms" below). In several important genera these radial series are in contact laterally ; but in other types there are plates or groups of plates intercalated between them, such plates being styled Interradials. In relation to the calyx as a whole, however, the basals or the parabasals are also "interradial" in position; the infrabasals, when present, are "radial "(see fig. 122). Anal Interradial Group. This group commonly contains more plates than the others, and, on its continuation over the oral surface of the calyx, bears the anus. Frequently, interradials are found in no other portion of the calyx. Arms. The ossicles composing these are all styled Brachials. Dr. P. H. Carpenter * regards the members of the radial series * "Anatomical Nomenclature of Echinoderms, " Ann. and Mag. of Nat. Hist., 6th ser., vol. vi. (1890), p. 15; F. A. Bather, "British Fossil Crinoids,"i&id., vol. v., p. 313; and " Suggested Terms in Crinoid Mor- phology," ibid., vol. ix., p. 51. See also the terminology in Wachsmuth and Springer, "Revision of the Palseocrinoidea," Proc. Acad. Nat. Sci. Phila- delphia, 1879 (pub. 1880), p. 249. Also Bather, Qeol. Mag., 1898, p. 318. ECHINODERMATA. 377 above the first radial as all belonging to the arms. The lower cycles of his brachials thus correspond to the old second and higher cycles of radials j these Dr. Carpenter styles Costals (fig. 121). Any interradials between thBse thus become styled Interbrachials. Above the costals the arm often bifurcates, further dichotomous division taking place in many genera. The free stems and branches of the arms are sometimes formed of one vertical row of ossicles (" uniserial ;" fig. 122), sometimes of two in contact, the ossicles alternating in the two rows ("biserial;" fig. 121). On the inner surface of the arms a groove leads down from their tips to the upper part of the calyx. Pinnules. Small arm-like processes, also formed of calcareous ossicles, set in many genera on botli sides of the grooves that run down the arms. In living crinoids these bear the reproduc- tive elements. The Stem is composed of a row of ossicles, placed vertically on one another, their articulating surfaces being variously ribbed and grooved. A central canal runs down through them all. The Crinoidea form a considerable portion of some limestones, the scattered ossicles of their stems, with their circular cross- sections and often radial markings, having given rise to the name "entrochal marble." The abundance of stems at some horizons, apart from crowns, has often been remarked on, and it has been suggested that certain genera (as Actinocrinus) pos- sessed the power of casting off their stems at particular stages of their growth. On the other hand, it has been pointed out that calyxes, unless at once filled with mud on the death of the animal, run much greater risk of destruction than the more solid stems, the ossicles of the calyx being scattered too widely for easy recognition. While, on the whole, the more modern types of crinoids are marked out by the smallness of the calyx in proportion to the arms, by a general absence of interradials, and also of a pro- minent ventral sac or dome, yet the division of the group into " Neocrinoidea " and "Palseocrinoidea " can be no longer main- tained. At the present time the classifications of specialists in this refined branch of palaeontology cannot be regarded as having reached even a resting stage ; consequently the genera here selected, while showing an interesting range of structure, are not placed under any system of subdivisions. Encrinus (fig. 121). Calyx rather shallow; 5 small infra- basals and 5 large parabasals present, the former generally 378 ECHINODERMATA. hidden by traces of the stem ; 5 radial three cycles, the ossicles of the two upper cycles being now regarded by most authors as belonging to the arms (first order of brachials, styled costals). From these arise simply bifurcating arms, which have pinnules, and which are commonly formed of two rows of ossicles (an exceptional feature in a Neozoic form). Upper surface of the calyx solidly roofed over. Stem long; its ossicles are radially grooved on their articulating surfaces. Trias. Pentacrinus. Calyx very small in pro- portion to the arms ; 5 basals (sometimes 5 infrabasals and 5 parabasals) ; 5 radials, above each of which lie 2 ossicles (radials or costals). The arms are formed of one row of ossicles, and bifurcate again and again, with long and abundant pinnules. Stem long, with numerous little jointed lateral processes; in cross-section it is series composed of Fig. 121. Encrinus liiiiformis (Muschel- kalk). The infrabasals and parabasals lie al- most horizontally, and are invisible. Above these are seen the radials, each support- ing two costals. The arms bifurcate, and ultimately become biserial. sometimes rounded, but commonly appears like a five-rayed star, the indentations between the rays being deep or shallow. The articulating surfaces of the ossicles of the stem always bear a pattern of five oval markings, which radiate symmetri- cally from the central canal. These stem-ossicles form very familiar fossils. Trias to Recent. Common in the Lias. Apiocrinus. Oalyx narrowing slightly above; its plates, in- cluding tho interradials, are fitted into one another to form a solid wall. 5 basals, alternating with which are the 5 radials, each bearing two large costals (2nd and 3rd radials); the basals rest on a circular plate, perhaps formed by the union of five infra- basals. Below this plate the stem commences, at first equal in diameter to the calyx, then contracting, and then becoming very gradually wider towards its rooted base. Hence above the narrowest part of the stem rises an egg-shaped or pear-shaped body, the upper half of which is the true calyx, the lower half being formed by the highest ossicles of the stem. Stem circular in cross-section. The arms are formed of a single row of ossicles, ECHINODERMATA. 379 and bear pinnules. They bifurcate only once or twice. Ventral sac. Lias to Lower Cretaceous. Actinocrinus. Calyx, including its dome, rather ovoid; 3 basals, 5 radials, with 2 costals (2nd and 3rd radials) above each; inter- radials (interbrachials) present, with more numerous plates in the anal group. The upper surface of the calyx is formed by a fairly high convex dome of plates, sometimes bearing near the apex an anal tube. Arms repeatedly divided ; composed of two rows of ossicles, and arising from five protuberances at the base of the dome, so as to appear to emerge about midway between the apex and base of the ovoid "head." Stem round, the central canal appearing five-rayed in cross-section. Gotlandian to Carboniferous ; especially the latter. Platycrinus. Calyx formed of 3 basals, 5 fairly tall and vertical radials, 5 much smaller costals, and one interradial (or, rather, interbrachial) in each interspace between the arms ; there may be, however, 3 plates in the anal series. Calyx roofed over as in Actinocrinus, with or without anal tube. Arms repeatedly bifurcating, composed at first of one row of ossicles, and later of two. Stem-ossicles often elliptical in cross-section. Gotlandian to Carboniferous; especially the latter. Ichthyocrinus. Calyx formed of 3 small Infrabasals (sometimes not visible on ex- terior), 5 small parabasals, and 5 radials. 2 to 3 cycles of costals. A small "radianal" plate occurs under the right posterior radial. Arms numerous, uniserial, and forming at their base a seemingly solid struc- ture with the calyx. No pinnules known. Gotlandian.* Cyathocrinus (fig. 122). Calyx cup-like; 5 infrabasals ; 5 large parabasals, forming part of the side-wall of the calyx; 5 radials; interradial plates occur only in the anal area. Upper surface with a greatly elongated and probably flexible dome. Arms long and repeatedly bifurcated, composed of one row of ossicles ; no pinnules. Stem round. Gotlandian to Permian ; especially the former. Heterocrinus. Calyx small, somewhat cylindrical. 5 minute infrabasals, at times seemingly absent ; 5 parabasals ; 5 radials^ * Springer calls the Carboniferous forms with no radianal \Metiohthyo- crinus (Journ. Oeol. t vol. xiv., 1906, p. 479). Fig. 122. Cyatho- crinus (Gotlandian). Showing infrabasals, parabasals, and radials. Uniserial arms. 380 ECHINODERMATA. with 2 costals above each. One or more of the radials is formed of two plates, united by a horizontal suture. Interradials in the anal group, and prolonged upwards as a ridge. Ventral sac known. Arms long, uniserial, with strong pinnules. Stem pentagonal. Ordovician. B. BLASTOIDBA. In these forms, entirely Palaeozoic and extinct, the calyx frequently resembles a closed flower-bud, having no free arms, an ovoid contour, and only a short stem. Running from the summit of the calyx down its sides are five elongated areas, the ambulacral (or pseudo-ambulacral) areas, which remind the observer of the well-known ambulacral areas on an echinoid. These are commonly seen as depressions, leaf- like in shape, or at times straight-sided. Specimens have been found in which the areas still bear pinnules, thus resembling crinoid-arms turned back and down over the calyx. A row of pores occurs down each side of the ambulacral areas, these openings being in reality interspaces between little lateral plates. The pores communicate with delicate canals, a bundle of which, called a hydrospire, runs up internally on either side of the median line of the area, and opens at the summit of the calyx. Here there are usually five openings (spiracles), each representing two series of canals from adjacent ambulacral areas. The mouth is central, and the anus lies between the two posterior spiracles. An anal proboscis has been seen. The calyx is mainly composed of five plates (arising from a basal cycle), each of which is deeply notched above to receive the downward turned apex of an ambulacral area. These plates may therefore be regarded as radials. Pentremites. Calyx bud-shaped, narrower above; radials large. Ambulacral areas rarely reaching to the base. Short round stem. Gotlandian to Carboniferous ; especially the latter. Granatocrhms. Calyx resembling Pentremites ; but the am- bulacral areas extend down to the base, the radials are small, and the five upper plates (interradials) large. Carboniferous. The common " Pentremitef " ellipticus thus becomes referred to Granatocrinus. C. CYSTIDEA. In this group, the Cystideans, there is a spherical or ovoid calyx, sometimes with pinnulated lateral ambulacral grooves ECHINODERMATA. 381 resembling the areas in the Blastoids. These grooves, however, are often very narrow and small ; they radiate from the mouth, which is apical. Two to thirteen short simple arms are occa- sionally also present ; a short stem occurs in some genera. A lateral aperture, and sometimes even two, may be found on some part of the calyx. Each is covered by a pyramid, the valvular pyramid, formed of little triangular plates. The plates of the calyx are numerous and irregularly arranged. Some or all bear, in typical examples, minute pores, which are grouped in pairs, the members of each pair being united by a canal. In some genera these pores occur near the margins of the plates, and the pairs are made up of opposite pores on adjacent plates. Taking any one line of junction of two plates, the pores on each plate are then arranged along two sides of a triangle, the base of which is the line of junction ; hence they include a rhomboid area, half of which lies on each plate. Across these areas striae can be seen, which have caused authors to term them the pectinated rhombs; these markings are the traces of the canals by which the pores are connected. The pores are usually closed at their outer ends by a thin layer of the plate which bears them. The function of this system is unknown. A larger number of genera can be referred to the Cystideans than to the Blastoids ; but all these are also Palaeozoic. Echinosphaerites. Calyx spherical, with only rudiments of ambulacral grooves round the mouth, which has a raised border. A small opening occurs near the mouth, and is sometimes covered by a valvular pyramid. A third aperture, covered by a pyramid, is also present at some little distance from the mouth. Fixed by the base only ; no stem. Bases of arms have been iound attached to the rim around the mouth. All the plates form pectinated rhombs at their lines of junction. Ordovician. D. ECHINOIDEA. This group includes the common " sea-urchins," the spheroidal shells or "tests" of which are familiar objects, though ordinarily found denuded of their spines. They are never attached by a stem, and move about freely by means of little tube-feet pro- truded through certain of the calcareous plates of the test. The relative positions of the anal and oral apertures, and the arrange- ment of the organs of locomotion, form important points in their classification. 382 ECHINODERMATA. Test. The shell, built up of calcite plates in contact with one another, by which the animal is surrounded. Oral aperture. An opening in the base of the test, fairly circular, within which the true mouth occurred during life. This aperture is reduced in living animals by a membrane or series of little plates, in the centre of which the mouth itself opens. The beautiful masticatory structure known as the " Lantern of Aristotle," though it falls away into the hollow of the test after death, and is commonly lost piece-meal through the apertures, has been found on careful cleaning in many fossil genera. Auriculce. Calcareous arches or plates rising internally from the border of the oral aperture. They are five in number and symmetrically arranged, serving for the attachment of muscles which thrust forward the five-toothed "lantern." They may frequently be seen in fossils if the detrital matter that commonly fills the test is cleaned out from the mouth-aperture to a little depth. They together form the Perignathic girdle. Anal aperture. This appears as a second fairly large and circular opening, often diametrically opposite to that of the mouth. In life it was also reduced by a membrane bearing accessory plates, in which the anus itself occurred. Apex of the test. The highest point of the test when the flatter side, on which the mouth-aperture occurs, is placed below and horizontally. Ambulacral areas. Five areas, each composed of two rows of plates perforated by pores, which radiate from the upper part of the test. When these extend over the sides, and down to the oral aperture, as simple bands, they are said to be perfect ; when the plates bearing distinct and regularly grouped pores terminate on the sides of the test, their representatives lower down having only indistinct or no perforations, the ambulacral area is described as imperfect. The pores are grouped in pairs towards the outer margin of the area, each plate thus bearing two pores. In many forms, through the intercalation of new plates above at the apex, the lateral ones become disarranged and finally united, so that compound plates arise bearing several pairs of pores. In petaloid areas the lines formed by the pores on the surface of the test converge and completely enclose the efficient ambulacral area, which thus becomes like an elongated simple leaf. In other cases the imperfect area is " open " below, the lines of pores not converging, but simply dying out. It may be remembered that each pair of pores represents one of the little "tube-feet" of the living animal ECHINODERMATA. 383 Interambulacral areas. Between the ambulacral series of plates lie five interim bulacral series, each also typically composed of two rows of plates, which are larger and consequently less numerous than the ambulacral plates against which they abut on either side. Only in the early types of echinoids has the test more or less than 20 rows of plates, and in almost all these cases it is the interambulacral series that varies. Apical Disc. A series of plates at the apex of the test, and surrounding the anal aperture when this is apical. Each ambu- lacral area terminates here in a plate which bears a minute perforation, once connected with a sense-organ. Between these five ocular plates are five genital plates, larger and generally with a larger aperture. One of these genital plates is perforated, however, over all its surface, in addition to the principal aper- ture, and forms the madreporic plate or tubercle, which admits water into the stone-canal of the animal. The genital plates are thus interambulacral, and the madre- poric plate lies just to the right of the anterior ambulacral area. Thus, even in forms where the mouth is central and the anus is not posterior, but apical, the madreporic plate will serve to indicate the anterior portion of the test. In the sub-group of the " Irregulares," the posterior genital plate is often absent. Accessory plates may occur in the apical disc when the anus is not included in it ; and sometimes the " disc " ceases to be disc- like, the three anterior ambulacral areas (or trivium) meeting in advance of the two posterior (or bivium), the connexion being maintained by a group of smaller plates (fig. 123). The plates of the test may be ornamented with tubercles, large or minute. The principal ones, which may be handsomely developed, as in Cidaris, bear the spines, and have sometimes in their apex a circular pit, which does not perforate the test, but which causes them to be termed " perforate " or "imperforate." The beak-like appendages called " pedicellarise " are in living forms found attached to the smaller granulations. The Spines may be small and easily broken up, but in some genera are massive and even longer than the diameter of the test. Their rhombohedral calcite cleavage makes them difficult to extract entire. They are commonly found detached from the tubercles, on which they are jointed and held by ligaments during life. A common mode of ornamentation of the spines consists of granulated or serrated little ridges running longi- tudinally down them. Lastly, to form any conception of the true characters of the 384 ECHINODERMATA. echinoid test, study must be made of recent examples, when it will be seen how the great mass of spines conceals the features (ambulacral grooves, tubercles, &c.) by which the palaeontologist is accustomed to define his genera. A practical illustrative specimen may be prepared by selecting a modern Spatangus and rubbing off the spines lightly with the finger from one half of the test, leaving the other covered. Two of the petaloid ambu- lacral areas and half of the anterior one will thus be exposed, and will serve to explain the appearances seen in fossil examples.* Sub-group 1 REGULARES. In these echinoids the five ambu- lacral and the five interambulacral areas are each composed of two rows of plates, making twenty rows in all. The ambulacra are perfect, and therefore never petaloid. The oral aperture is in the centre of the base, and the anal aperture is at the apex, and is thus included in the apical disc. Echinus. Test hemispherical and thin- walled. Tubercles similar on both kinds of areas, and all fairly small and simple. Ambulacral plates formed by the union of three primary plates, and hence each bearing three pairs of pores, which are grouped across the plate, not vertically under one another. Hence three bands, each formed of a series of pairs of pores, run up each margin of the ambulacral areas (p. 382). Spines small and simple in form. Cretaceous to Recent. Cyphosoma. Test circular in horizontal section; flattened above and below. Tubercles with radial notches on the base, but without apical pit (" imperforate ") ; the principal and large ones form two rows on each of the ten areas of the test. From this cause, and in width, the two kinds of area much resemble one another. Ambulacral plates compound; but the pairs of pores form a single band, except near the apex and the mouth. Apical disc generally lost, the upper aperture being consequently large, and the test, as found fossil, almost annular (compare Oidaris). Spines long. Jurassic to Eocene. But almost entirely Upper Cretaceous. Acrosalenia Test small; form depressed spheroidal. Inter- ambulacra rather larger than the ambulacra, both kinds of areas bearing two rows of " perforated " tubercles with radially notched bases. Pairs of pores forming only one row on each ambulacral margin. The interambulacral tubercles are the larger. A dis- tinguishing point is the intercalation of several firm plates in * For an important revision of the Echinoidea, see P. M. Duncan, Journ. Linn. Soc., Zoology, vol. xxiii. (1890). p. 1. ECHINODERMATA. 385 the central area of the apical disc, whereby the anal aperture becomes thrust to the posterior side. Border of mouth-aperture notched. Spines rather thin. Lias to Lower Cretaceous. Cidaris. Test fairly large; form flattened spheroidal, not perceptibly conical towards the apex. Ambulacral areas very narrow, forming wavy curving bands, with only a single row of pairs of pores on each margin. Commonly two small tubercles on each ambulacral plate. Interambulacra wide, with boldly developed tubercles, which are commonly " perforated," and are sometimes notched at the base. Apical disc commonly lost, a large aperture, like that of the mouth, being left (compare Oyphosoma). Spines thick, massive, of very various form, long or short, species of Cidaris having been named from these peculiarities. Permian to Recent, cfimmishing throughout the Cainozoic systems. Sub-Group 2 TRREQULARES. In this sub-group the radial symmetry that prevails, except in minute details of the apical disc, throughout the sub-group of the Regulares, gives place to a distinctly bilateral symmetry, the plane of symmetry passing through the oral and anal apertures, the anterior ambulacral area and the apex. The anus is not included in the apical disc, and occurs sometimes even on the basal surface. The apical disc shows irregularities, the posterior genital plate being often absent. The oral aperture itself may be excentric. There are, as in the Regulares, only 20 rows of plates in all ; but the pore-bearing parts of the ambulacral series frequently form petaloid areas. Echinoconus (Galerites). Conical ; flat at base, which has an outline approaching pentagonal. Oral aperture in centre of base, without masticatory apparatus ', anal aperture also on the base, but close to the posterior margin. Tubercles " perforate," minute; and numerous over all the test. Ambulacral areas perfect, narrow, the pairs of pores forming single marginal rows except on the base, where they become crowded so as to form three rows on each side. The posterior genital plate is im- perforate. Spines small, rarely seen. Cretaceous. Discoidea. Hemispherical, sometimes flattish ; base flat. Apertures as in Galerites. Tubercles "perforate," small. Am- bulacra perfect, narrow, with only one row of pairs of pores on each margin. Posterior genital plate imperforate in most species. 25 386 ECHINODERMATA. The essential character is the occurrence of ten low ridges radi- ating on the interior of the base from the oral aperture to the lateral walls of the test, and leaving corresponding grooves on casts. Cretaceous. Pygaster. Depressed hemispheroidal, with flat base, which is roundly pentagonal in outline. Oral aperture central ; anal aperture large and on upper surface, behind the apex, being narrower at its anterior and broader at its posterior end. Tubercles small, "perforate." Ambulacral areas much as in Discoidea. Posterior genital plate absent. Jurassic to Cretaceous ; also one Recent species. Scutella. This is an example of the extremely discoidal and flattened echinoids prevalent in some Cainozoic deposits. Base flat ; upper surface only slightly convex ; internal space much reduced. Posterior margin straight, or with a central notch. As the test grows, an indentation on its edge may increase in importance, until at last the test re-unites on either side of it, leaving it as a perforation. This remarkable feature is paralleled by Pygope among the brachiopods. Mouth central ; anus small, and on the posterior margin. Tubercles minute. Ambulacra petaloid. Posterior genital plate absent. Oligocene and Miocene. The next six genera, like Galerites, were unprovided with a masticatory apparatus. Echinobrissus. Form approaching hemispherical, but rather depressed ; base slightly concave, with an almost straight bor- der between the two posterior ambulacra. Tubercles small. Oral aperture slightly in advance of the centre; anal on the upper surface, just behind the apex, and lying in a groove that widens posteriorly. Ambulacra imperfect, with nearly parallel sides, open below ; outer pore of each pair elongated and slit-like. The posterior genital plate is im perforate. Jurassic to Recent ; characteristically Middle Mesozoic. Clypeus. Close ally of Echinobrissus. Test large, flattened. Tubercles small. Apertures as in Echinobrissus; but anal groove sometimes wanting. Ambulacral areas rather broad, im- perfect, open below, but contracting near the base. Outer pore of each pair long and slit-like. Apex slightly posterior; pos- terior genital plate imperforate ; madreporic plate central in the apical disc, while the posterior ocular plates are extended so a* to reach the anal area. Jurassic. ECHIffODERMATA. 387 Collyrites (fig. 123). Test ovoid, with rather flattened base. Tubercles minute. Oral aperture rather in front of the centre; anal aperture on posterior lateral surface. The striking character lies in the extension of the apical disc and its accessory plates, so as to form an elongated band running along the line of symmetry; hence the three anterior ambulacra (styled the "trivium") become divided from the other two (the " bivium"), which enclose on the posterior surface an area around the anus. Ambulacra narrow Fig. 123. Collyrites and perfect bicordata (Coralline Jurassic and Lower Cretaceous. Oolite). Showing Echinocorys (Ananchytes). Test convex *: 2S3SS at above, with rather vertical aides; oval in the apex. horizontal section; base flat. Tubercles minute. Oral aperture near the anterior margin ; anal aperture near the posterior margin, and also within the base. Ambulacral areas fairly wide; perfect. Posterior genital plate absent. Upper Cretaceous. Holaster. Form heart-shaped, i.e., oval when viewed from above, with a broad notch anteriorly and a sharper posterior termination. Upper surface convex ; base flat. Tubercles small. Oral aperture near anterior margin ; anal aperture on the pos- terior lateral surface. Ambulacra perfect (compare Micraster, which has a similar form); the anterior ambulacral area lies in a well-marked groove, which continues round to the mouth- aperture. Posterior genital plate absent. Cretaceous. Micraster. Form much like Holaster; typically heart-shaped ; sometimes more acute, sometimes slightly truncated, at the pos- terior end. Tubercles small. Apertures as in Holaster ; the test projects forward from behind the mouth-aperture so as to form a short covering below it. Pore-bearing areas set in grooves, the paired ones petaloid ; the three anterior areas are longer than the posterior. An anterior groove, in which the unpaired ambulacrum lies, runs from apex to mouth as in Holaster. Posterior genital plate absent. Upper Cretaceous to Miocene. Note. SpatangUS, a Gainozoic and common living form, resembles Micraater, but has larger inter ambulacral tubercles on the upper surface, while the anterior unpaired ambulacrum is only feebly represented in its groove. 388 ECHINODERMATA. Sub-group 3 PAL^ECHINOIDEA.* The remains of early echinoids are rare, and the various genera have been collected by Zittel into this division, to which he opposes the " Euechi- noidea," subdivided into Regulares and Irregulares. Some Palseechinoids seem to have had tests with plates that moved slightly on one another, so that the whole could be deformed by pressure without fracture, as is the case in the exceptional living euechinoids, Asthenosoma and Phormosoma. Moreover, their most striking characteristic is a deviation from the normal number of twenty rows of plates. Thus Palaeeehinus (Gotlandian to Carboniferous) is a spherical form with five normal and perfect ambulacral areas; but the interambulacrals are wide, and each is formed of five to nine rows of plates. Echinocrinus (Archasocidaris), again, of the Carboniferous and Permian, has fair-sized tubercles on the interambulacra, and wavy narrow ambulacra, suggesting those of Cidaris; but there are from four to eight rows of plates in each interambulacral area. Melonites, also Carboniferous, has several supernumerary rows of plates in both the ambulacral and the interambulacral areas. Bothriocidaris (Ordovician and Gotlandian) has normal ambu- lacral areas, but only one row in each interambulacral area. E. ASTEROIDEA. The members of this group ^are of less assistance to the geo- logist than is the great group of the Echinoids, owing to the ease with which their hard parts become separated and dispersed. Passing over the allied Ophiuroidea, in which the long arms contain no prolongations of the viscera, we may note that the arms of the Asteroidea, or true Star-fishes, contain numerous skeletal ossicles, which give them at times considerable solidity. There are thus the little ambulacral ossicles, which, by meeting in pairs so as to form a ridge, cover the ambulacral vessel that runs down the under side of each arm. Beneath this ridge, and thus in the groove formed by it, the tube-feet of the star-fish lie during life. At the lateral margins of each arm are often two rows of marginal ossicles, one above and one below, each pair in contact. These ossicles are typically convex outwardly, often ornamented with granules or spines, and flat-sided where they abut against their neighbours, whether of the same or the adjoin- ing row. At the base of the ambulacral ossicles a row of adam- bulacral ossicles always occurs. Accessory plates may be formed on the back of the arms, or between the marginals and the * See Mary J. Klein, Trans. Acad. Sci. 8t. Louis, vol. xiv. (1904). ANNELIDA. 389 adambulacrals. The marginal ossicles are not unfrequently found in fossil deposits. Palseaster. Form like the common Star-fish (Asterias or Asteracanthion), with deep ambulacral grooves. The ambulacral ossicles, unlike any modern form, are alternate, not opposite, on the two sides of the groove. Marginal and dorsal ossicles present. Cambrian to Carboniferous. Protaster of the Gotlandian and Carboniferous is an Ophiuroid. Astropecten. Form like the common Star-fish, but with strongly developed marginal ossicles. Lias to Recent. Goniaster. In this type the form closely approximates to a pentagonal disc, through the extreme shortness of the arms ; the notch between one arm and the next is represented merely by a shallow concavity, along which marginal ossicles form a firm border. Between these ossicles and the small ambulacral areas, with their adambulacral and ambulacral ossicles, are abundant accessory plates, thus covering five intervening triangular areas. The dorsal surface is also covered with accessory plates. This genus is consequently represented by fairly coherent specimens. Jurassic to Recent. Fairly common in the Cretaceous. XIII. Annelida. The division of the Annelida is largely represented by the borings of marine genera in sands, which have become converted into cylindrical casts by the deposition of material during subsequent rising of the tide. Sometimes the infilling, as in the very early examples in the quartzites above the Torridon Sand- stone, is conspicuous by consisting of a sand either more or less ferruginous than that into which the animal bored. If this infilling becomes consolidated more firmly than its surroundings, the cross-sections of the casts may stand out on weathered surfaces of the rock as little circular discs.* The specimens of such borings from Sutherland and Ross-shire, as above described, and from the quartzite of the Wrekin ridge, may claim to be among the very oldest fossil remains. The tube-building worms naturally leave abundant traces. * For figures and descriptions of such objects see Sir J. W. Dawson, Quart. Journ. Geol. Soc., 1890, p. 695. 390 OSTRACODA. Thus Serpula has a calcareous tube, often irregularly corrugated on the surface, and very variously curved ; the tube commonly appears as if creeping forward in the fashion of a moving worm. It is usually fixed to other bodies, being thus often seen, for example, on the tests of Chalk echinoids. Common from the Jurassic to the present day. Ditrupa forms an unattached simply curved tube, open at both ends, which closely resembles the scaphopod Dentalium. From this it may sometimes be distinguished by irregularity of curva- ture, and by being ornamented only on the side that was uppermost during life (Zittd). Cretaceous to Recent. CHAPTER XXVII. FOSSIL GENERIC TYPES. XIV. Arthropoda. FROM a stratigraphical point of view, the Arthropoda become less important in Cainozoic deposits than they are in the Palaeozoic, and hence the earlier types of Crustacea or " Arachno-Crustacea," which are often grouped together in the heterogeneous division of the Entomostraca, must claim our chief attention. Their common character is a variable number of body-segments (Somites), coupled with a simple type of organisation. A. OSTRACODA. These little crustaceans have never more than 7. pairs of limbs, and are enclosed in a bivalve Shell, which corresponds to the shield formed by the union of the segments of the head and thorax in the Malacostraca. This shell is kept closed by a muscle. Its surface is smooth or variously marked, often with hemispherical knobs, and its small oval form is characteristic. The valves of ostracods are seldom liable to be confused with those of young lainellibranchiata (see fig. 124). The shell if chitinous or calcareous. OBTfcACObA. Hinge-border. The line of junction of the two valves along which they remain united. Little teeth sometimes occur upon the hinge (fig. 124, c). Ventral border. The lower border, towards which the limbs of the ostracod are directed during life. Eye-spot. A fairly hemispherical tubercle occurring in some genera on the anterior part of the valve, and indicating the position of an eye. The Ostracods are mostly Marine ; fresh or brackish water forms will be specially indicated.* Cypris (fig. 124, a). Shell small, partly horny, and thin. Left valve the larger. Oval, or rather bean-shaped ; ventral border commonly somewhat concave. No Fig. 125. Bey- richia (Ordo- vician). Fig. 126. Leperditia inflata (Carbonifer- ous). Natural size shown in centre. Fig. 124. a, Cypris purbeck&n- teeth. Surface generally smooth, six (Lowfir Knifautk TCorl^ a.nrl riArr>P^ wit.li rrivi/ V>^.l/% sis (Lower Purbeck Beds). The left valve is towards the observer. 6, Cypridea punc- i*ta, var: gibbosa (Middle Purbeck). Left valve. c, Cy there retirugata, var: rugu- lata (Lower Purbeck). In- terior of left valve, showing an anterior socket for the tooth of the right valve; then a tooth, followed by a bar- like ridge, which terminates in a posterior socket for the posterior tooth of the right valve. (The three figures after Prof. Rupert Jones. ) and pierced with minute holes. Fresh-water. Purbeck to Recent. Cypridea (fig. 124, b). Like Oypris, but shell bearing a little beak-like process, with a notch behind it, at the anterior end of the ventral border. These are the common " Cyprids " of the Weald. Fresh-water. Purbeck to Wealden. Cypridina. Shell small, thin, horny or calcareous. Oval, with a prolongation near the middle of the anterior border, beneath which a notch occurs. * For interesting descriptions and figures of Purbeck and Wealden forma see T. Rupert Jones, Quart. Journ. Geol. Soc., 1885, p. 311. 392 PHYLLOPCXDA Carboniferous to Recent. Common in Carboniferous. Cythere (tig. 124, c). Shell thick, oval, or somewhat rect- angular ; often very highly ornamented with knots or spines. Right valve with a tooth at each end of the hinge-line, and a pit and horizontal groove between them; left valve with terminal pits, a ridge, and an anterior tooth. Gotlandian to Recent. Primitia. Shell small, thick, elongated oval; but hinge-border straight. Not always equivalve. A furrow runs on the surface from the hinge-border vertically towards the ventral border, sometimes reaching as far as the centre of the valve. Cambrian to Carboniferous. Especially Ordovician and Got- landian. Beyrichia (fig. 125). Shell thick, surface distinctly convex; straight hinge. Somewhat truncated anterior and posterior borders, and convex ventral border. Surface divided by strong fairly vertical furrows into markedly convex lobes, which com- monly unite below; the marginal area is smoother. Cambrian to Carboniferous. Especially Ordovician and Got- landian. Leperditia (fig. 126). Shell large (attaining 2 cm. in length), thick, rather bean-shaped ; straight hinge-border, and convex ventral border. Anterior border shorter than posterior. Eight valve larger than, and lapping somewhat over, the left. A small eye-spot occurs near the hinge-border. Cambrian to Carboniferous. Especially Ordovician and Got- landian. B. PHYLLOPODA. The animal is more distinctly segmented than in the Ostracoda, and the thoracic limbs, often numerous, are flattened and leaf- like, each dividing into two flaps at the end. Many genera have a covering that recalls the ostracod shell. Compare also the Phyllocarida. Estheria. Shell bivalve, thin, sometimes partly calcified, but commonly horny, with a polished appearance. A small rounded umbo occurs near the anterior margin; hinge-line toothless and straight. Ventral border convex. Surface in most species concentrically ribbed ; sometimes smooth. Prof. T. Rupert Jones* remarks that, while Estherise have * Fossil Estherice. Palaeont. Society, 1862, pp. 12 and 13. TRItOBTTA. 393 often been mistaken for small forms of Avicula and Posidonomya, their horny appearance will distinguish them from the former, while there is also an absence of any bending out of the con- centric markings towards an ear, such as Avicula possesses. The valves, moreover, are rarely so quadrate in form as those of Posidonomya. Fresh and Brackish water; but early forms are found associated with Marine fossils, perhaps owing to floods, perhaps through difference of habit. Devonian to Recent. Fairly common in the Trias. C. TRILOBITA. Owing to the absolute extinction of the whole order, the rela- tions of the Trilobites to the well-defined Crustacea and to the Arachnida have remained somewhat obscure. Thanks to the work of Messrs. E. Billings,* 0. D. Walcott,t and J. Mickle- borough, | the appendages of the genera Asaphus, Oalymene, Cheirurus, and Acidaspis, are now fairly known; while Mr. Beecher has examined them in Trinucleus and, with signal suc- cess, in Triarthrus. Dr. Oehlert has recently summarised the results of these researches (Bull. Soc. geol. de France, 3me ser., t. xxiv., p. 97); and it has been pointed out that Linnaeus observed antennae in a specimen of Parabolina as far back as 1759 (see Geol. Mag., 1896, p. 142). The verification of these in Triarthrus has led to the retention of the trilobites among the Crustacea; and Mr. H. M. Bernard sees in Apus their nearest living repre- sentative. Some valuable considerations on the segmentation and limbs of trilobites are given by O. Jaekel (Zeitschr. deutsch. geol Gesell, Bd. liii., 1901, p. 133). The hard covering of the trilobites, which is all that ordinarily remains to us can be clearly divided into three parts, the nomen- clature of which has depended upon the classificatory position taken up by successive palaeontologists. The supporters of the arachnid view employ " cephalo-thorax," "abdomen," and "post- abdomen" or "pygidium"; the rival school uses "head," "thorax," and "pygidium." To avoid confusion, and to carry forward the same nomenclature when writing of the Merostomata, we pro- pose to speak of the "head-shield," "body," and "pygidium." This is practically the plan adopted by Mr. H. M. Bernard. Head-shield (fig. 127). This portion is approximately semi- circular, and is not broken up in the adult into transverse * Quart. Journ. Oeol. Soc., vol. xxvi. (1870), p. 479. t Bulletin Museum Comp. Zoology, Cambridge, U.S.A., 1881. $ Reprinted in Geological Magazine, 1884, pp. 80 and 162. I Am. Journ. Sci., 1895 and 1896, and 4th ser., vol. xiii, (1902). 394 TRILOBITA. fr.c. segments. It has curved anterior (or frontal) and lateral borders, forming an outwardly convex margin ; while it has an almost straight posterior or oc- cipital border, where it joins the first body-seg- ment. An occipital furrow often occurs in the head- shield parallel to and near this border. The shield is folded over below its front margin, so as to extend a little way back towards the mouth. In young forms, the original segments compos- ing the head-shield have been traced. O. Jaekel regards these as always eight, including the hypo- stome as the first. Glabella. The convex elevated portion of the head-shield reaching from the centre of the occipital border nearly to the an- teriorborder. Theglabella varies in form, and some- times bears lateral fur- rows, which run approxi- mately at right angles to its sides; they thus divide its edges into lobes, and probably represent traces of original segmentation. Facial Sutures. Fine lines of junction between the two parts into which the area on either side of the glabella can be divided. Each of the two facial sutures arises at some posterior point of the border of the head-shield, runs forward between the glabella and the eye, and either terminates by cutting across the anterior border (fig. 128), or by meeting its fellow-suture in front of the glabella (fig. 132). Occasionally in the former case an additional suture, the marginal suture, runs from one facial suture to the other along the anterior part of the head-shield, as in Calymene, Paradoxides, and Illsenus. Fi 127. Trilobite (Dalmania caudata ; Wenlock Beds). H, Head- shield ; T, Body-segments ; P, Pygidium. e, Eye. fx.c, Fixed cheek, fr.c. Free cheek, fs, Facial suture (starting in this example from the lateral margin and finally passing round in front of the glabella). g, Glabella (bearing lateral furrows), pi, Pleura. TRILOBITA. 395 Fixed Cheeks. The areas on each side of the glabella between it and the facial suture. Free Cheeks. The portions of the head-shield between the facial sutures and the outer margin. These two parts may become detached after the death of the animal, and are often seen to have shifted away slightly from the fixed cheeks. Eyes. These are sometimes absent (fig. 130), sometimes represented only by papillae ; but they are commonly present as two somewhat crescentic elevations, occasionally supported on a stalk, and are covered with numerous facets, each of which was a lens (figs. 127 and 135). As above hinted, they occur on the free cheeks, close against the facial suture, where it approaches the side of the glabella. Hypostome. The anterior portion of the head-shield is bent over, and may form a broad crescentic plate-like surface on the under side. A small plate of various form, often occurs before the mouth, with its anterior border in contact with the edge of the folded-over head-shield, and its other borders free. This is the Hypostome (fig. 133) representing the labrum that overlaps the front of the mouth in higher Crustacea. Two sense-organs (1 eyes) occur on it in some genera. (See W. K. Spencer, Geol. Mag., 1903, p. 489.) Body. The portion between the head-shield and the pygidium. It consists of a very variable number of segments, which were movable on one another, so that in some genera the animal could coil itself up after the manner of a wood-louse. Two longitudinal furrows or depressions run down the body, one starting on each side of the glabella, and corresponding, in fact, to the depressions which divide the glabella from the cheeks. Each body-segment is thus marked out into a central convex part, the annulus, and a natter and commonly broader portion on each side. The latter areas form the pleura;* they are generally marked with a groove, or a ridge, from the annulus outwards, and often terminate in spines (fig. 128). The convex ridge formed by the series of annuli, running from the glabella over the body, and commonly on to the pygidium itself, is termed the rachis. Pygidium. The shield covering the posterior part of the trilobite. Its outline often repeats that of the head-shield (figs. 129 and 133), and it consists of permanently united segments. Sometimes the traces of the original segmentation are perfectly clear (fig. 134), and the rachis generally persists on it for some distance. * Pleura and pleurae have alike been used ; their respective singulars are pleuron and pleura, both of which are good Greek forms. 396 TRILOBITA. Appendages. Of the five pairs of head-appendages known, the first is a pair of antennae, their bases inserted on the edge of the hypostome. These are long in Triarthrus, and consist of one ray. The bases of the remaining four pairs were probably used in mastication. Each body-segment bore a pair of appendages, and the pygidium had also a number of pairs corresponding to its original segmentation. All these limbs were fixed to the inside edges of the rachis. They were simple biramous types ; and the pygidial pairs, and possibly those on the body, served for respiration as well as for swimming. There is a gradual change in type, from the anterior body-appendages, which resemble those on the head-shield, to the posterior ones, which resemble those on the pygidium. Lastly, we should note that the more resisting character of the head-shield and the pygidium often allows of their being found isolated in rocks, when the body-segments have become parted asunder and lost. It has been suggested that the "trilobite," as found, may often be a mere " skin " cast off by the animal during life. The Trilobites were all Marine. Paradoxides. Form elongated ; sometimes large (70 cm. or so in length), tapering fairly uniformly from the front to the pygidium. Head-shield semicircular, with a long curving spine running backwards from each of its posterior angles. Glabella rather flat, rounded and broad in front, narrowing posteriorly, with lateral furrows. Facial sutures running from posterior to anterior border, without bending in any great degree towards the glabella. A marginal suture is present. Body with numerous (16 to 20) segments, and with well-marked trilobed character. Pleura furrowed and prolonged as spines. Pygidium very small, the rachis being continued on to it for a short distance; a long spine often runs out posteriorly on each side. Exclusively Cambrian. Typically Middle Cambrian. Olenellus. Resembles Paradoxides, with narrower glabella; facial sutures obscure or absent. Third body-segment often larger than the others. Pygidium at times styliform. 13 to 26 body-segments. Lower Cambrian.* * See " The Story of Olenellus," Natural Science, vol. i., p. 340. Holmia and Mesonacis are subgenera. TRILOBITA. 397 Olenus (fig. 128). Form broader and more oval than in Paradoxides. Head-shield broad and semicircular, with a spine running back from each posterior angle. Gla- bella rather conical, narrowing anteriorly, and with lateral furrows. A little ridge runs out from it to each eye, at right angles to the axis. Facial suture running from posterior to anterior border, sometimes closely approaching glabella. Body with 12 to 15 segments, which have short sharp backward terminations. Pleura broad. Pygidium small, broad, with rachis well marked on it for some distance. Exclusively Cambrian. Typically Upper Cambrian. Conocoryphe (Conocephalus). Form much like Olenus. shield semicircular, without posterior spines. Fig. 128. Olenus micrurus (Lin- gula Flags). Head- Glabella conical, furrowed ; somewhat truncated at its narrower (anterior) end, and divided by deep depressions from the cheeks. Eyes rather near the anterior end of the glabella, and facial sutures running from the posterior margin, near the angles of the shield, inwards almost to the glabella, and then out, widely diverging, across the anterior border. Body with fourteen or fifteen segments; rachis well marked ; pleura furrowed, and rounded at the ends. Pygidium small, with distinct rachis, and with traces of segmentation. L. Cambrian to Ordovician. Angelina. Proportions much as in Olenus, and head-shield with posterior spines. Glabella narrowed anteriorly, but smooth and rounded. Body with fourteen or fifteen segments ; pleura furrowed. Cambrian, Agnostus (fig. 129). Form small, elliptical, the head-shield * c Fig. 129. Agiiostus (Cambrian). Fig. 130. Trmucleus concen- tricus (Bala Beds). In various stages of growth. Fig. 131. Harpes Flanngani (Bala Beds). and pygidium being almost similar; both are rounded at the outer end, with convex or straightish sides. Glabella distinct. 398 TRILOBITA. No eyes or facial sutures. Body with only two segments. Pygidium with fairly marked rachis, which terminates broadly ; a little process often runs out posteriorly from each lateral border of the pygidium ; but it is otherwise difficult to distin- guish detached pygidia from head-shields. Cambrian. Trinucleus (fig. 130). Head-shield large and predominant, projecting laterally beyond the body ; semicircular in front, and often with long spines from the posterior angles. Glabella and cheeks smooth, and forming three strongly convex elevations, which leave a broad flat semicircular border beyond them. This border is pierced with minute holes. No eyes or facial sutures. Body with six segments ; rachis rather narrow and distinct, con- tinued on to the small pygidium ; pleura furrowed. The body and pygidium are together smaller in area than the head-shield. Ordovician. Harpes (fig. 131). This remarkable form bears some re- semblance to Trinucleus, having a similar broad perforated border to the head-shield, prolonged backwards in this case almost as far as the pygidium. Eyes present, but no (or very indistinct) facial structures. A little ridge sometimes runs from the glabella to each eye, as in Olenus. Numerous body segments (about twenty-five). Pygidium very small. The flat border of the head-shield is sometimes found detached and isolated. Ordovician to Devonian. Calymene. Form oval, rather broad. Often ornamented with little tubercles. Head-shield broad, rounded anteriorly; posterior angles generally without spines. Glabella convex, with three strong pairs of furrows, the most posterior pair in some species bifurcating at the end. Facial sutures running from the posterior angles obliquely inwards to the eyes, and then across the anterior border, where they are connected by a marginal suture. Body with thirteen segments ; rachis well marked. Pygidium rounded and scarcely distinct from the body, the rachis reaching to the end, and traces of segments being clearly marked. This genus is one of those most frequently found in a rolled up condition, as in the specimens from the Wenlock Beds at Dudley. The hindmost of the pairs of limbs on the head-shield is larger and broader than the others, suggesting its differentia- tion into a special pair of paddles. Ordovician and Ontlandictn. TRILOBITA. 399 Homalonotus (fig. 132). Form fairly long; sometimes orna- mented with spines. Head-shield rather broad, either rounded or pointed anteriorly ; no pos- terior spines. Glabella com- monly only feebly marked off from the cheeks, and unfurrowed. Facial sutures much as in Caly- mene, but commonly meeting without intersecting the anterior border. Body with thirteen Fig. l32.Homalonotu8 (Gotlandian). Fig. 133. Asaphus Powisii (Qrdo- Showing facial suture continuous vician). With hypostome. in front of the glabella. segments ; rachis not sharply marked off. Pygidium with rachis and traces of segments ; pointed posteriorly, sometimes ending in a short spine. Ordovician to Devonian. Ogygia: Form often la rge, roundly oval, and rather flat. Head- shield semicircular, sometimes with posterior spines. Glabella rather straight at sides, widening in front, with four pairs of lurrows. Hypostome not notched on its posterior border. Facial sutures running from the posterior border, near the angles, obliquely to the large crescentic eyes; thence they sometimes cross the anterior border, but generally unite in front of the glabella. Body with 8 segments; rachis well marked; pleura broad, furrowed, not spinose at the ends. Pygidium about the same size as the head-shield, and nearly semicircular, slightly 400 TRILOBITA. elongated; its rachis is distinct, with numerous segmental markings on it and on the lateral areas. Ordovician. Asaphus (fig. 133). A close ally of Ogygia, and often large. Like Ogygia, but rather more convex; 8 body-segments. The traces of segmentation on the pygidium are confined to its rachis or altogether absent. The head-shield may be sharply pointed in front, or semicircular; glabella commonly not furrowed. Hypostome with a deep notch on posterior border. The glabella on the head-shield and the rachis on the pygidium may appear merely as broadly convex folds. Ordovician. Illaenns. Ally of Asaphus, but commonly more strongly convex, and more broadly elliptical in outline. Glabella only feebly indicated, externally without furrows. Facial sutures connected by a marginal suture; 8 to 10 body-segments (typically the latter) ; pleura smooth. Pygidium with slight trace, if any, of rachis, and with no external signs of segmentation. Ordovician and Cfotlandian. Phacops (fig. 134). Form elongated, oval, or elliptical. Head- shield almost semicircular, without posterior spines. Glabella Fig. 134. Phacops (Chafimops) conophthalmus (Bala Beds). In the sub-genus Chasmops the an- terior lobes of the glabella are Fig. 135. Bronteua flabellifer exceptionally expanded. (Devonian). much widened anteriorly, distinct ; only feebly furrowed, except in the posterior part. Facial sutures arising on the lateral margins, almost opposite the eyes, and uniting in front of the glabella. Body with 1 1 segments. Pygidium semicircular, with marked rachis and signs of segmentation. See Dalmania, Grotlandian to Devonian. MEROSTOMATA. 401 Dalmania (fig. 127). Like Phacops, of which it is often re- garded as a mere sub-genus ; but the glabella is distinctly fur- rowed, and not so markedly widened anteriorly. Long posterior spines to the head-shield; the pygidium also commonly terminates in a spine. Ordovician and Gotlandian. Bronteus (fig. 135). Form broadly oval. Head-shield semi- circular ; free cheeks often detached ; glabella much widened anteriorly, and sometimes furrowed. Ten body-segments; pleura with longitudinal ridges. Pygidium large, rounded posteriorly, with very short rachis, from which somewhat broad furrows radiate to the margin. Ordovician to Devonian. The last family, the Proetidse, contains the latest surviving trilobites. Proetus. Form small ; oval or elliptical. Head-shield semi- circular, with a distinct thickened marginal rim ; posterior spines sometimes occur. Glabella convex, somewhat narrowed in front; unfurrowed. Facial sutures running rather straightly from the posterior to the anterior border. Eight to ten body-segments ; rachis distinct, pleura furrowed. Pygidium with a semicircular border like that of the head-shield, the whole form being thus simple and elliptical. Ordovician to Carboniferous. Typically Lower Palaeozoic. Phillipsia. Close ally of Proetus; but the glabella bears three pairs of furrows, and is bounded by nearly parallel sides. Nine body- segments. The smooth elliptical outline of Proetus is maintained in Phillipsia. Gotlandian to Permian ; typically Carboniferous. Griffithides. Like Phillipsia, of which it may be regarded as a mere sub-genus ; but glabella distinctly widened in front, with one pair of furrows only, these being posterior. Carboniferous. D. MEROSTOMATA. This group of arthropods, represented at present by the King- Crab (Limulus), attains importance on account of the great size of many of its forms. Aglaspis of the American Cambrian is regarded as an early limuloid, and Neolimulus and Hemiaspis 2(3 402 MEROSTOMATA. are allies from the British Gotlandian. The Eurypterida range from the Cambrian to the Carboniferous. Here, again, the nomenclature of the parts of the animals depends upon the classification adopted. The arachnid view gives us " cephalo-thorax," "abdomen," and "post-abdomen"; the crustacean view, "head-shield," "thorax," and "abdomen." We propose to adopt "head-shield," "body," and "posterior region," in order to avoid debatable ground, and to compare the Merosto- mata fairly with the Trilobites. (i) XIPHOSURA. Belinurus is an early ally of Limulus, with hemispherical head- shield, 5 unfused body-segments, and 3 fused posterior segments, this region terminating in a long spine. Having a glabella, rachis, and furrowed pleura, it has a decidedly trilobitic character. Devonian (U. Old Red Sandstone) and Coal- Measures. Prestwichia (fig. 136) is like Belinurus, but its body-segments are fused, as well as those of the posterior region. U. Devonian to Permian. Prolimulus, resembling a larval Limu- lus, occurs in the Permian of Bohemia. Limulus itself, with its fused body- . ' - 6 segments, and no other representative of Prestwichia rotundata the posterior region than a spine, occurs (Carboniferous). as early as the Trias. The larva of Limulus, with its separate head-shield, marked with a glabella, and its segmented body, presents a striking resemblance to Prestwichia, and is to the geologist one of the most interesting of living creatures. (ii) EURYPTERIDA. The Eurypterida include animals some four or even six feet long, the appendages attached to the head-shield being highly developed and of very various form. The body has six unfused segments, with branchiae, which are connected with plate-like appendages, occurring on their under sides; there are seven pos- terior segments, also movable, the last consisting of a telson, as in the familiar Crayfish. The whole surface preserves only traces of a trilobed character, and is more or less folded over in the body and posterior regions. The whole form is long, LEPTOSTRACA. 403 but somewhat pear-shaped. For a general review, see Laurie, Trans. R. Soc. Edinburgh, vol. xxxvii. (1893), p. 509. Eurypterus has a somewhat semicircular head, rather straight at the sides, with a large pair of eyes set well within the mar- gin ; " ocelli " or eye-spots occur in addition in the centre of the shield. The work of Dr. Fr. Schmidt* has revealed an additional small pair of appendages, not reaching to the margin of the head-shield, and in advance of those previously known. This first pair is now known to be provided with prehensile claws (chelae), and behind it come four larger pairs of simple limbs. The sixth and last pair consists of large flattened swimming- paddles. Schmidt brings forward evidence to show that the body- segments bend over comparatively slightly at the sides, as in ordinary trilobites, and that the plates which seem to continue them on the under-side, thus covering the branchiae, belong in reality to the branchial (phyllopodous) appendages themselves. The telson, in opposition to that of Pterygotus, is a long spine. Uppermost ffotlandian to Carboniferous. Pterygotus is generally similar in form to Eurypterus, but its two large eyes lie on the anterior margin of the head-shield. The first appendage on each side is long, and terminates in a claw like that of the lobster. Schmidt f finds four smaller simple pairs of appendages behind this one not three as usually figured; the sixth, as in Eurypterus, is a pair of broad swimming- paddles. The telson is also broad and paddle-shaped. Ordovician (Bohemia) to Devonian, (Old Red Sandstone). Stylonurus resembles Eurypterus, and has similarly a spine for the telson ; but its two posterior pairs of limbs are very long, resembling jointed rods. Uppermost Gotlandian to Devonian. E. LEPTOSTRACA (PHYLLOCARIDA). This division of the Crustacea has been formed to include the small living genus Nebalia, which has characters intermediate between some Entomostraca and the Malacostraca. As in the latter division, the head and thorax together include thirteen * "Die Crustaceenfauna der Eurypterenschichten von Rootzikiill auf Oesel." Mem. Acad. imp. des Sciences de St. Petersbourg, s^r. 7, tome xxxi. (1883), p. 51, &c. t/6id.,p. 64. 404 MALACOSTRACA. segments; but Nebalia is peculiar in having eight abdominal segments. The thoracic limbs ally Nebalia to the Phyllopods ; and it has, like Apus in that group, a thin dorsal shield, folded over laterally, and covering the cephalic and thoracic segments. On the other hand, it has a characteristic appendage, the rostrum^ in front of the shield. Nebalia is Marine. Several well-known Palaeozoic genera have been transferred here from the Phyllopoda, on account of the fairly constant number of their segments, the presence of a rostrum, &c. But much caution must necessarily be exercised in dealing with their thin and fragmentary remains. Ceratiocaris. Dorsal shield bivalve, somewhat rectangular when viewed from the side. Fourteen or more segments, of which sometimes as many as seven are free, and project beyond the dorsal shield. Surface of shield finely striated parallel to its length. Rostrum known. Abdomen terminating in a telson, which is formed of a large and two shorter spines. Ordovician to Carboniferous. Hymenocaris. Dorsal shield composed of one piece folded over, distinctly convex at the ventral border. Telson with several spines. Cambrian (Lingula Flags). F. MALACOSTRACA. In this division, including the modern types of highly-organ- ised Crustacea, the animal has typically six head-segments (some authors, reckoning in the eye, have counted seven), seven thoracic segments, and seven abdominal segments, including the telson. While fossil remains of Malacostraca occur scattered fairly freely through Mesozoic and Cainozoic rocks, they are scarcely to be regarded as of importance in characterising special horizons. Archseoniscus may be mentioned as an early representative of the Isopoda, the order that includes the Woodlouse (Oniscus). The isopods have the head distinct from the thoracic segments ; the form is broadly oval, and the branchiae are borne by the fused abdominal segments. In Archaeoniscus there are thirteen thoracico- abdominal segments, including a rounded telson. Milne-Edwards assigns seven of these to the thorax. Purbeck. As an example of the Macrurous Decapoda, which include the Lobster, we may mention Hoploparia, in which there is the CHARACTERISTIC INVERTEBRATE FOSSILS. 405 characteristic fusion of the anterior segments into a cephalo- thoracic shield. One of the two great anterior clawed limbs is more slender than the other. Lower Cretaceous to Eocene (London Clay). Lastly, ffalseocorystes (Gault to Eocene), and Xanthopsis of the Cretaceous and Eocene, are familiar Brachyurous Decapods; they are crab-like, therefore, in form, with the abdomen, unlike that of the Macrura, folded under the broad cephalothoracic shield. CHAPTER XXVIII. SUGGESTED LIST OF CHARACTERISTIC INVERTEBRATE FOSSILS. As already mentioned (p. 292), this list consists in great part of forms familiar in the British Isles, and must be modified to suit the needs of observers in any special area. Some rare forms are included, where they mark important zones, or where they are a distinct addition to the fauna as displayed by other genera. Attention is particularly directed to the generic names, since these give an idea of the faunae of successive periods, in whatever country the student may be placed. On going over the list in front of the specimens in a public collection, notes may conveniently be added as to specific characters. A few such notes are given here; but to ascertain the real points of difference between one species and another of the same genus, reference must be made to the original descriptions, or to publi- cations such as those of the Palseontographical Society. Dr. E. Koken, in his Leitfossilien (C. H. Tauchnitz, Leipzig, 1896), enters usefully into the specific details of a large 'number of characteristic fossils. Where synonyms exist, the more familiar names of genera have been adopted; and where a new genus has been established out of a subdivision of an old one, the older name is often also given. Abbreviatipns used : Hydro. = Hydrozoa. Actin. = Actinozoa. Brack. = Brachiopoda. Lam. -- Lamellibranchiata. Gast. = Gastropoda. Ceph . = Cephalopoda. A m. = A mmonites. 406 CHARACTERISTIC INVERTEBRATE FOSSILS. I. CAMBRIAN. Lower Series (Olenellus Series; Taconian, Lapworth, 1891). Spongise. Protospongia. Brach. Lingulella primceva. ^ Pteropoda 1 Hyolithes antiquus. Trilobita. Olenellus. Middle Series (Menevian ; Paradoxides Series). Spongise. Protospongia fenestrata. Brach. Discina pileolus. Obolella sagittalis. Pteropoda ? Hyolithea corrugatus. Trilobita. Paradoxides Davidis. Conocoryphe coronata. Ag~ nostus scutalis. Upper Series (Olenus Series). LINGULA FLAGS STAGE. Brach. Lingulella Davifii. Or this lenticularis. Trilobita. Agnostus pisiformis. Phyllocarida. Hymenocaris vermicauda. T RE MA DOC STAGE. Hydro. Dictyonema sociale. Brach. Orthis Carausii. Lingulella lepis. Lam. Cyrtodonta and Gflyptarca, an early ally of Area (rare). Pteropoda. Hyolithes. Gonularia. Trilobita. Olenus. Conocoryphe depressa. Angelina Sedgwicki. II. ORDOVICIAN (LOWER SILURIAN). Arenig Series. Hydro. Didymograptus. Diplograptus. Llandeilo Series. Hydro. Didymograptus Murchisoni. Brach. Orthis striatula. Gast. Bellerophon perturbatus. Trilobita. Ogygia Buchii. Asaphus tyrannus. Trinucleut. Calymene. CHARACTERISTIC INVERTEBRATE FOSSILS. 407 Bala Series, Hydro. Diplograptus. Climacograptus. Brach. Or this calligramma. Orthis flabellulum. Ceph. Orthoceras vagans. Cystidea. Echinosphcerites. Trilobita. Trinucleus concentricus. Illcenus. Phacops Brong- niarti. III. GOTLANDIAN (UPPER SILURIAN). Llandovery Series. Hydro. Diplograptus vesiculosus. Monograptus Sedgwicki. Rastrites peregrines. Brach. Pentamerus oblongus (common as casts in England). Trilobita. Proetus Stokesii. Wenlock Series. Hydro. Monograptus priodon. Stromatopora. Actin. Heliolites inter stinctus. Holy sites catenularia (also in Bala Series). Omphyma turbinatum. Cyathophyllum angustum. Favosites gothlandica. Alveolites. Gcenites. Polyzoa. Fenestella. Brach. Rhynchonella borealis. Orthis elegantula. Leptcena rhomboidalis. Atrypa reticularis. Meristella tumida. Lam. Orthonota amygdalina. Gast. Euomphalus rugosus. Pleurotomaria. Murchisonia. Pteropoda ? Tentaculites. Ceph. Orthoceras annulatum. Phragmoceras. Gomphoceras. Crinoidea. Actinocrinus . Cyailwcrinus. Trilobita. Calymene Blumenbachii. Homalonotus delphino- cephalus. Dalmania caudata (Phacops caudatus). Ludlow Series. Brach. Pentamerus Knightii (Aymestry Limestone). Lam. Cardiola interrupta. Orthonota. Grammy sia. Ceph. Orthoceras ludense. Lituites. Eurypterida. Eurypterus. Pterygotus. Stylonurua. IV. DEVONIAN. Lower Series. Brach. Spirifer speciosus. Lam. Grammysia marginata. 408 CHARACTERISTIC INVERTEBRATE FOSSILS. Ceph. Orthoceras. Trilobita. Bronteus. Homalonotus. Middle Series. Actin. Gyathophyllum helianthoides. Favosites cornigera. Calceola sandalina. Brach. Stringocephalus Burtini. Spirifer elegans. Pentamerus 'galeatus. Gast. Bdlerophon striatus. Pleurotomaria. Murchisonia. Orinoidea. A ctinocrinus. Trilobita. Phacops latifrons. Bronteus flabellifer. Upper Series. Hydro. Stromatopora. Brach. Atrypa reticularis (passes up from Silurian). Rhyn- chonella cuboides. Spirifer Vemeuili. Lam. Cucullcea unilateralis (Hardingii). Cardiola. Ceph. Clymenia. Prolecanites (Goniatites). Old Red Sandstone (Fresh-water Devonian). Lam. ArcJianodon Jukesii (passage-beds to L. Carboniferous). Phyllopoda. JSstheria. Eurypterida. Eurypterus. Pterygotus. Stylonurus. V. CARBONIFEKOUS. Foraminifera. Endothyra. Fusulina. Saccammina fusulini- formis. Actin. Cyathophyllum regium. Zaphrentis (Caninia) cylindrical. Lithostrotion basaltiforme. Lonsdaleiafloriformis. Dibunophyllum (ally of Lonsdaleia, but not compound). Michelinia Javosa. Syringopora ramulosa. Polyzoa. Fenestella. Entalophora. Brach. Spirifer striatus. Productus semireticulatus. Pro- ductus giganteus. Orthis resupinata. Rhynchonella pugnus. Terebratula hastata. Lam. Posidonomya Becheri. Aviculopecten (Pterineopecten) papyraceus. Conocardium aliforme. Carbonicola aquilina and other species (freshwater beds). Gast. Euomphalus penlangulatus. Bellerophon. Naticopsis. Pleurotomaria. Pteropoda. Conularia. CHARACTERISTIC INVERT KBRATE FOSSILS. 40$ Oeph. Orthoceras. Discites. 'Goniatites : Glyphioceras crenistria; Gastrioceras Listeri; Pronorites cyclolobus ; Prolecanites gompressus (Pendleside series). Crinoidea. Actinocrinus. Platycrinus. Cyathocrinus. Blastoidea. Granatocrinus ellipticus. Pentremites. Trilobita. Griffithides seminiferus (Phillipsia seminifera). Phillipsia gemmulifera. VI. PERMIAN. Polyzoa. Fenestella retiformis. Brach. Productus horridus. Camarophoria multiplicata* Lam. Schizodus truncatus. Gast. Bellerophon. VII. TRIAS. Lower Series (Bunter and Werfen Series). Lam. Myophoria costata (Bunter). Modiola hirundi/ormis (Bunter). Monotis (Pseudomonotis) Clarai (Werfen). Ceph. Trachyceras, and other allies of Ceratites (Werfen ; Beneckeia, among these, occurs also in Bunter). Phyllopoda. Estheria Albertii (Bunter). Middle Series (Muschelkalk Series). Brach. Spiriferina Mentzeli. Terebratula vulgaris. Retzia ( Tetractinella) trigonella. Lam. Daonella Sturi (Alps). Lima (sub-gen. JRadula) striata. Gervillia (Hcernesia) socialis. Myophoria vulgaris. Ceph. Orthoceras campanile (Alps). Ammonites : Trachy- ceras. Ceratites nodosus. Crinoidea. Encrinus liliiformis (E.fossilis). Upper Series (Keuper and Upper Alpine Series). Brach. Koninckina Leonardi (Alps). Lam. Daonella (Halobia) Lommeli (Alps). Cardita crenata (Alps). Gervillia subcostata. Myophoria Goldfussi. Myophoria raibliana (Keuper and Alps). Gast. Turbo solitarius (Alps). Ceph. Orthoceras elegans (Alps). Ammonites: Arcestes subumbilicatus ; Arcestes Gaytani ; Trachyceras Aon (Alps). 410 CHARACTERISTIC INVERTEBRATE FOSSILS. Echinoidea. Cidaris (Alps). Phyllopoda. Estheria minuta (Keuper). Rhaetic. Lam. Monotis decussata. Avicula contorta. Pecten valoniensis. Protocardia rhcetica. VIII. JURASSIC. Lower Jurassic. LOWER LIAS. Brach. Spiriferina Walcottii. Lam. Avicula cygnipes (also Middle Lias). Cardinia Listeri. Cardinia ovalis. Hippopodium ponderosum. Gryphcea incurva. Lima (sub-gen. Plagiostoma) gigantea. Gast. Pleurotomaria anglica. Ceph. Ammonites : Psiloceras planorbis; Schlotheimia angu- lata (jEgoceras angulatum) ; Arietites Conybeari ; Arietites Euck- landi; Arietites obtusus ; Oxynoticeras (Amaltheus) oxynotus ; Arietites raricostatus ; jEgoceras planicosta. Crinoidea. Pentacrinus. MIDDLE LIAS. Brach. Terebratula punctata. Rhynchonella tetra/iedra. Lam. Pecten cequivalvis. Ceph. Ammonites: Amaltheus margaritatus ; Amaltheus spinatus; JEgoceras capricornus ; ^Egoceras Henleyi ; JZgoceras armatum. UPPER LIAS. Lam. Leda ovum. Ceph. Ammonites : Phylloceras heterophyllum ; Cceloceras commune; Harpoceras (Hildoceras) bifrons ; Harpoceras serpen- tinum. Belemnites often abundant. Middle Jurassic. MIDFORD SANDS. Brach. Rhynchonella cynocephala (ventral margin strikingly plicated). Lam. Pholadomya Jidicula. Ceph. Lytoceras jurense (Ammonites jurensis). This species has several auxiliary lobes, contrary to the rule in Lytoceras. CHARACTERISTIC INVERTEBRATE FOSSILS. 411 INFERIOR OOLITE. Actin. Montlivaltia. Brach. Terebratula fimbria. Rhynchonella spinosa. Lam. Lima (sub-gen. Ctenostreon) proboscidea. Trigonia^ numerous species ; e.g., Trigonia costata. Ceromya bajociana. Gast. Pseudomelania (Chemnitzia). Nerinea. Ceph. Ammonites : Parkinsonia (Cosmoceras) Parkinsoni ; Stephanoceras humphriesianum. Echinoidea. Clypeus Plotii. Pygorhytia ringens (Pygorhytis is one of the Oollyritidae). BATHONIAN. Brach. Terebratula maxillata (note the range of form in this species). Waldheimia digona. Rhynchonella concinna. Lam. Gresslya peregrina. Gervulia acuta. Homomya gibbosa. Crinoidea. Apiocrinus Parkinsoni (A. elegans). Echinoidea. Echinobrissus clunicularis. Upper Jurassic. OXFORD CLAY. Lam. Alectryonia (Ostrea) Marshi. Gryphcea dilatata. Tri* gonia elongata (also in Kimeridge Clay). Oeph. Ammonites : Cosmoceras Jason ; Cosmoceras callo- viense (Kellaways Rock). Nautilus hexagonus. Belemnites Oweni (=puzosianus). CORALLINE OOLITE. Actin. Thecosmilia annularis. Thamnastrcea arachnoides. Lam. Trigonia clavellata. Goniomya v-scripta. Gast. Bourguetia (Phasianella) striata. Pseudomelania (Chem- nitzia) heddingtonensis. Nerinea Goodhalli. Ceph. Amaltheus (Cardioceras) vertebralis (also in upper zone of Oxford Olay). Belemnites abbreviatus. Echinoidea. Cidaris Jlorigemma. KIMERIDGE CLAY. Brach. Rhynchonella inconstaw. Lam. Ostrea deltoidea. Exogyra virgula. Ceph. Holcostephanus pallasianus* * See Miss Healey, Quart. Journ. Geol. Soc., vol. Ix. (1904), p. 60. CHARACTERISTIC INVERTEBRATE FOSSILS. PORTLAND BEDS. Actin. Isastrcea oblong a (best known by its casts in flint, in which the white parts represent the infilling of the calyx, and the darker more transparent portions the replacement of the septa and wall. In this species there is no columella, and the septa are strongly marked with lateral granules). Lam. Trigonia gibbosa. Gast. Ceritkium portlandicum (best known by its screw-like casts). PURBEGK BEDS. Lam. Cyrena. Ostrea distorta. Gast. Paludina. Limncea. Crustacea. Cypridea punctata. Cypridea granulosa. Cypria purbeckensis. Archceoniscus Brodiei. TITHONIAN STAGE (Basin of the Khone). Brach. Pygope and allied varieties of Terebratula. IX. CRETACEOUS. Lower Cretaceous. NEOCOMIAN of France and Switzerland (Lower Neocomian of many authors). Compare with Lower Speetoii Beds in England. Lam. Exogyra (Ostrea) Couloni. Perna Mulleti (see Ather- fielg Clay). Janira atava. Ceph. Ammonites : Hoplites neocomiensis ; Holcostephanus (Olcostephanus) astierianus. Crioceras (Ancyloceras) Duvalii. Belemnites lateralis. Echinoidea. Toocaster complanatus. (Toxaster is a close ally of Micraster ; the ambulacra are open below, and the pores are slit-like.) WEALDEN (FRESH-WATER PASSAGE-BEDS FROM UPPER JURASSIC}. Lam. Cyrena media. Unio valdensis. Gast. Paludina elongata. Melania strombiformis. Ostracoda. Cypridea valdensis. CHARACTERISTIC INVERTEBRATE FOSSILS. 413 BARREMIAN* or UEGONIAN (Atherfield Clay). Actin. Holocystis elegans. Lam. Gervillia anceps. Perna Mulleti (found also in the Upper Speeton Beds). Panopcea plicata. APTIAN (Hythe Beds and most of Folkestone Beds). Spongise. Numerous spicules in the cherts. Brach. Terebratula sella (also in the Atherfield Clay and Speeton Beds). Lam. Exogyra sinuata (often large). Plicatula placunea. Ceph. Hoplites (Am.) Deshayesii (in this species the median furrow common in Hoplites is absent ; characteristic also of the Upper Speeton Beds). ALBIAN or SELBORNIAN} (Gault, with uppermost part of Folkestone Beds). Brach. Kingena lima. Terebratula biplicata. Lam. Exogyra conica. Inoceramus concentricus. Inoceramus (sub-gen. Actinoceramus) sulcatus. JVucula pectinata. Janira quinquecostata. Pecten orbicularis and asper (also in Oeno- manian). Scaphopoda. Dentalium decussalum. Gast. Alaria carinata (often called Aporrhais. This species has a narrowed tongue-like, and not broad, expansion of the outer lip). Ceph. Ammonites : Acanthoceras (Douvilleiceras) mamil- latum (near base) ; Hoplites interruptus ; Hoplites lautus ; Hoplites splendens ; Schlcenbachia injlata ( = rostrata). Hamites. Ancyloceras. Belemnites minimus. Upper Cretaceous. CENOMANIAN (Lower Chalk Stage). Spongise. Plocoscyphia mceandrina. Lam. Alectryonia (Ostrea) frons. Pecten asper. * See De Lapparent, TraiM de Gtologie, 3me. e"d. (1893), pp. 1098 and 1118. Barr^mian, due to Coquand in 1862, is the preferable term. t See Jukes-Browne and Hill, as to correlation of beds styled " Upper Greensand" in England, Mem. Geol. Surv. t " Cret. Rocks of Britain" (1900), pp. 14-31. 414 CHARACTERISTIC INVERTEBRATE FOSSILS. Ceph. Ammonites: Acanthoceras rothomagense ; Schlosnbachia various. Scaphites cequalis. Turrilites costatus. Actinocamax pleuus (Belemnitella plena ; at too of series). Echinoidea. Discoidea cylindrica. Holaster subglobosus. TURONIAN (Middle Chalk). Brach. Terebratulina gracilis. Lam. Inoceramus labiatus. Echinoidea. Holaster planus. SENONIAN (Upper Chalk). Spongise. Doryderma. Ventriculites. Cliona cretacea (known as borings, or casts of borings). Brach. Terebratula carnea. Terebratulina striata. RJiyn- chonella plicatilis. Crania. Lam. Pecten nitidus (a small almost smooth form). Spondylus spinosus. Inoceramus Cuvieri. Inoceramus Brongniarti. Hip purites. Ostrea vesicularis. Ceph. Belemnitella mucronata (in higher beds). Echinoidea. Gidaris sceptrifera. Cyphosoma Kcenigi. Galerites albogalerus (Echinoconus conicus). Micr aster coranguinum. Anan- chytes ovatus (Echinocorys vulgaris). DA NIAN. Lam. Ostrea vesicularis. Hippurites. Oeph. Baculites Faujasi. X. EOCENE. Lower Series (London Series). LOWER LONDON TERTI ARIES. Lam. Ostrea bettovacina. Pectunculus terebratularis. Cyrena cuneiformis. Cyprina Morrisii. Gast. Cerithium funatum. Melania inquinata. Natica sub- depressa. LONDON CLAY. Lam. Pectunculus brevirostris. Pholadomya Teredo (common in fossil wood). CHARACTERISTIC INVERTEBRATE FOSSILS. 415 Gast. Turritella imbricataria. Aporrkais Sowerbyi. Pleuro- toma teretrium (and several other species). Galerus(Calyptrcea) trochiformis. Cassidaria nodosa. Ceph. Nautilus imperialis. Annelida. Ditrupa plana. Crustacea. Hoploparia Belli. Xanthopsis Leachi. Middle Series (Bracklesham Series). Foraminifera. Nummulites Icevigatus. Nummulites variolarius. Actin. I/itharcea Websteri. Lam. Venericardia (Cardita) planicosta. Tellina speciosa (and several other species). Gast. Cerithium giganteum. Murex minax. Pleurotoma attenuata. Conus diadema (and several other species). Upper Series (Barton Series). Lam. Crassatella sulcata. Cardita sulcata. Chama squamosa. Gast. Rostellaria (sub-gen. Nippochrenes) ampla. Fusus (sub- gen. Clavella) longcevus. Valuta luctatrix. Valuta ambigua. XI. OLTGOCENE. Lower Series (Fluvio-marine Series of Isle of Wight). Lam. Cytherea incrassata (the typical fossil of the marine bands). Cyrena obovata. Cyrena semistriata. Ostrea vectensis. Corbula pisum. Gast. Melania muricata. Melania costata. Melania turri- tissima. Melanopsis carinata. Cerithium mutabile. Cerithium plicatum. Cerithium elegans. Rissoa Chasteli. Neritina concava. Potamides concavus. Limncea caudata. Limncea longiscata. Planorbis euomphalus. Planorbis discus. Paludina lenta. Bulimus ellipticus. Helix globosa. Ostracoda. Cypris. Upper Series (Aquitanian of Paris Basin and North Germany). Gast. Limncea cornea. Potamides Lamarcki. Planorbis cornu. Helix Defrancei (and several other species). 416 CHARACTERISTIC INVERTEBRATE FOSSILS. XII. MIOCENE. Burdigalian * (Faluns of Bordeaux). Gast. Melania aquitanica. Hydrobia acuia (in Mayence Basin). Helvetian (Faluns of Touraine and Anjou in great part; Swiss Marine Mollasse). Lam. Ostrea crassissima. Lima squamosa. Area turonica. Gast. Trochus incrassatus. Erhinoidea. Scutella. Tortonian (of N. Italy, &c.). Gast. Pleurotoma (numerous species). Conusantiqvius. Voluta varispina. Helix turonensis (in Touraine). XIII. PLIOCENE. Pannonian (Sarmatian and Pontian of De Lapparent's Miocene System). Lam. Panopcea Menardi (lowest beds). Venus inultilamella. Congeria (several species). Placentian (Coralline Crag). Polyzoa. Fascicularia aurantium, Eschara monilifera. Cells' pora edax. Brach. Terebratula grandis (an unusually large species ; found also in the Lenham Beds). Lingula Dumortieri. Lam. Pectunculus ylycimeris (also found in the Lenham Beds). Astarte Omalei. Cardita senilis. Cyprina islandica (also in higher Pliocene). Gast. Turritella incrassata. Voluta Lamberti. Cassidt bicatenata. Ficuta {Pyrula) reticulata. Echinoidea. Echinus Woodwardii. * This term is due to M. Dep^ret ; see De L apparent, TraiU de Ctoi(. 3me. ed., p. 1294. CHARACTERISTIC INVERTEBRATE FOSSILS. 417 Astian to Sicilian. RED CRAG. Lam. Pecten opercularis. Cardium Parkinsoni. Tellina obliqua. Mactra ovalis. Gast. Buccinum undatum. Nassa reticosa. Purpura tetra- gona. Purpura lapillus. Natica mnltipunctata (and several other species). Trophon (Chrysodomus) antiquus. Trophon (Chrysodomus) contrarius (a common " left-handed" form). NORWICH CRAG. Gast. Turritella terebra ( = communis). Trophon scalariformis. CHILLESFORD BEDS. Lam. Mya truncata. Cyprina islandica. Gast. Littorina littorea. 27 419 INDEX. Abney's level, 8. Absorption of light, 144. Acanthoceras, 369. Acetylene tetrabromide, 31. Aridaspis, 393. Acmite, 181. Acrosalenia, 384. Actinocamax, 373. Actinoceramus, 337. Actinoceras, 359. Actinocrinus, 379. Actinolite, 155. -Schist, 282. Actinozoa, 304. ^Egirine, 181. ^Egoceras, 366. Agate mortar, 41. Agglomerate-lavas, 99, 197. Agglomerates, volcanic, 195. Aggregate-growth, 149. Aglaspis, 401. Agnostus, 397. Alabaster, 208, 274. Alaria, 348. Alaskite, 219. Albite, 84, 173, 275. Alcyonaria, 306. Alectryonia, 339. Algae, action of, 203, 204, 208. Allotriomorphic crystals, 99. Allport, on Wrekin rocks, 265. Alum in trachyte, 245. Aluminium plate, 40. tests for, 60. Alunite, 77. Alveolites, 310. Amaltheus, 366. Amazon-stone, 84. Amblystegite, 178. Amethyst, 155. Ammonites, 363. Ammonoidea, 360. Amphiboles, 155, 181, 190, Amphibole- Schists, 281. Amphibolite, 273, 275, 281, 282. Amygdaloidal structure, 100. Analcime, 155. Analysis, chemical, of rocks, 107. Ananchytes, 387. Anatase, 155. Ancyloceras, 371. Andalusite, 89, 155, 272. Andesine, 84, 173. Andesite, 249, 250. -Glass, 266. Angelina, 397. Angles of crystals, 16, 136 ; of ex- tinction, 145. Anglesite, 66. Anhydrite, 66. Annelida, 389. Anodonta, 334. Anomalous double refraction, 150. Anorthite, 66, 84, 173. -Gabbro, 234. Anorthoclase, 168. Anthophyllite, 156. Anthracite, 213. Anthracosia, 334. Antimonite, 66. Antimony, tests for, 6 1 . Antinomia, 316. Apatite, 67, 156. Apiocrinus, 378. Aplite, 219. Apophyllite, 67. Aporrhais, 348. Aptychus, 357. Apus, 393, 404. Arachnida, 393, 402. Araeometer, 25. Aragonite, 67, 156, 201. Acra, 332. Arcestes, 364. Archaeocidaris, 388. Archaeoniscus, 404. Archanodon, 334. Arenaceous foraminifera, 294, 297. 420 INDEX. Arfvedsonite, ISO. Argentite, 67. Argonauta, 355. Arietites, 367. Arkose, 193. Arsenic, tests for, 61. Arthropoda, 390. Articulata, 315. Asaphus, 400. Asbestos, 72. Ashes, volcanic, 195, 277. Asiphonate Lamellibranchs, 332, 335, 339. Assilina, 296. Astarte, 329. Asteroidea, 388. Asthenosoma, 388. Astropecten, 389. Atacamite, 67. Atractilites, 373. Atrypa, 318. Augite, 67, 157. -Andesite, 252. -Diorite, 227. -Syenite, 223. Augitite, 262. Avicula, 335. Aviculopecten, 341. Axes of elasticity, 87. Azurite, 67. Baetrites, 363. Baculites, 370. Baked shale, 272. Banded structure, 97, 241. Barium, tests for, 61. Barkevikite, 181. Barrandeoceras, 358. Barytes, 67, 192. Basalt-Glass, 268. Basaltic Andesite, 252. Basanite, 255, 260, 261. Bastite, 157. Bathmoceras, 358. Bauxite, 199. Becke on refractive index, 142. Belemnitella, 373. Belemnites, 371. Belemnoteuthis, 374. Belinurus, 402. Bellerophon, 352. Belosepia, 374. Beneckeia (Bunter), 409. Bertrand eye-piece, 146. Beyrichia, 392. Biotite, 158. Birefringence, 144. Bisectrices, 88. Bismuth, test for, 61 ; native, 68. Bismuthine, 68. Black Band, 209. Blastoidea, 380. Blende, 77. Blowpipe, 37- ,, -examination of minerals, 37-38 ; works on, 43. ,, -flames, 44. ,, -lamps, 38. ,, -reagents, 42. Blue glass, use of, 49. Bolton, on use of organic acids, 34. Bone-beds, 207. Bonney, on siliceous cements, 192. Borax, reactions in, 49. Boring molluscs, 324. Bornite, 68. Boron, tests for, 61. Borotungstate of cadmium, 30, 116. Bothriocidaris, 388. Bourguetia, 349. Brachiopoda, 314. Brecciated lavas, 99, 1 97 ; limestone, 206, 207. ,, structure, 94. Bromoform, 31. Bromyrite, 73. Bronteus, 401. Hronzite, 178. Brown Coal, 212. Brucite, 68. Bryozoa, 311. Buccinum, 346. Bulimus, 354. Bytownite, 84, 173. C. Cadmium, test for, 61. Calamine, 68 ; " Electric," 72. Calcareous Algae, 203. Calceola, 308. Calciphyre, 273, 274. Calcite, 68, 158, 201. Calcium, tests for, 61, 83. Calc-Schist, 283. Calymene, 398. INDEX. 421 Calyptrsea, ,352. Camarophoria, 317. Camptonite, 232. Caninia, 307. Carangeot, contact goniometer, 16. Carbon dioxide, tests for, 61. Carbonates, examination of, 34, 105. Carbonicola, 334. Card trays, 292. Cardinia, 330. Cardioceras, 411. Cardiola, 333. Cardita, 330. Cardmm, 328. Carpenter, P. H. , on crinoids, 376. Carstone, 193. Cassidaria, 347. Cassiterite, 68, 159. Celestine, 68. Cellepora, 314. Cephalopoda, 355. Ceratiocaris, 404. Ceratites, 364. Cerithium, 349. Ceromya, 326. Cerussite, 68. Chalcedony, 159, 192, 210. Chal copy rite, 70. Chalcosine ( = Redruthite), 75. Chalk, 201, 202. Chalybite, 69. Chama, 330. Characteristic invertebrate fossils, 405. Charcoal for blowpipe-work, 39. ,, reactions on, 55. Chasmops, 400. Cheilostomata, 313. Cheirurus, 393. Chemnitzia, 349. Chenopus, 348. Chert, 200, 210. Chiastolite, 159, 272. Chlamys, 341. Chloanthite, 69. Chlorine, tests for, 61. Chlorite, 160. Chlorite-Schist, 280.- Chloritoid, 172.. Chondrophora, 374. Chromite, 69, 160. Chromium, tests for, 61. Chrysocolla, b9. Chrysodomus, 417. Cidaris, 385. Cinnabar, 69. Cipollino, 274, 283. Citric acid, use of, 35. Clavella, 346. Clay-ironstone, 209 (see Chalybite). Clays, 197. Cleavage, of minerals, 20, 138 ; of rocks, 96, 276. Climacograptus, 303. Clinometer, 5. Clinozoisite, 185. Cliona, 299. Clymenia, 362. Clypeus, 386. Coals, 212. Coates's balance, 27. Cobalt, nitrate, use of, 55, 58. ,, tests for, 61. Cobaltine, 69. Coccolite, 160. Coccoliths, 202. Cceloceras, 368. Ccenites, 310. Collyrites, 387. Colour of minerals, 15. Columnar structure, 97. Compact Syenite, 224. Concretionary rocks, 209. Concretions, 207, 209. Cone-in-cone structure, 209. Congeria, 335. Conglomerates, 213, 283. Conocardium, 328. Conocephalus, 397. Conocoryphe, 397. Contact Goniometer, 1 6. ,, -metamorphism, 271. Contour lines, 8. Conularia, 355. Conus, 345. Convergent polarised light, use of, 151. Copper, tests for, 61. Glance, 75. Pyrites, 70. Coral-Limestones, 203. Corallines, 203. Corbicula, 329. Corbula, 327. Cordier, researches on constitution of rocks, 110, 132. Cordierite, 160, 284. Cornish and Kendall, on shells, 201. 422 INDEX. Corrosion of crystals, 137. Corsite, 102. Corundum, 70. Cosmoceras, 369. Crania, 321. Crassatella, 330. Crinoidea, 375. Crioceras, 371. Crustacea, 390. Cryolite, 70. Cryptocrystalline structure, 99, 244. Cryptostomata, 312. Crystalline sandstones, 192. Crystallites, 99, 243, 264, 207- Ctenostreon, 341. Cucullaea, 332. Cupellation of lead ores, 56. Cuprite, 70. Cyathocrinus, 379. Cyathophyllum, 307. Cyclas, 329. Cyclolites, 309. Cyclolobus, 365. Cyclostomata, 311. Cyphosoma, 384. Cypridea, 391. Cypridina, 391. Cyprina, 329. Cypris, 391. Cyrena, 329. Cyrtoceras, 360. Cyrtodonta, 332. Cystidea, 380. Cystideans, 380. Cythere, 392. Cytherea, 325. D. Daeite, 250. Dalmania, 401. Dalton, method of determining dip, 6. Daonella, 336. Decapoda, 378, 379. Decollated shells, 343. Delesse, on rock-constitution, 121, 132. Dense liquids, 29, 116. Dentalina, 295. Dentalium, 342. Derived fossils, 292. Desert-sand, 189. Deemine, 182. Diabase, 227, 230, 235, 253, 258. Diallage, 161. Diatomaceous deposits, 213. Dibunophyllum, 408. Diceras, 330 Dichroscope, 89. Dick, Allan, petrological microscope, 126. Dictyonema, 304. Didymograptus, 303. Diffusion-column, 31. Diopside, 161. Diorite, 227. Dip, determination of, 5. Diplograptus, 303. Dipyre, 180. Discina and Discinisca, 321. Discites, 358. Discoidea, 385. Displacement-apparatus for specific gravities, 103. Ditroite, 223. Ditrupa, 390. Dolerite, 230, 234. Dolomite, 34, 70, 105, 161, 206, 274. Dolomitic Limestone, 105, 206, 274. Domite, 247. Doryderma, 300. Double refraction, 144 ; anomalous, 150. Dreissensia (Dreissena), 335. Drusy structure, 97. Dunite, 239. E. EchinobPissus, 388. Echinoconus, 385. Echinocorys, 387. Echinocrinus, 388. Echinodermata, 375. Echinoidea, 381. Echinosphserites, 381. Echinus, 384. Eclogite, 282. Elseolite, 74, 169. -Diorite, 229. -Syenite, 223. Elvan, Elvanite, 220. Enclosures in minerals, 139, Encrinites, 375. Encrinus, 377. INDEX. 423 Encrustations on charcoal, 57. Endothyra, 297. Enstatite, 178. Entalophora, 312. Entomostraca, 390. Entrochal marble, 377. Eozobn, '273 (footnote). Epidiorite, 226, 228, 229, 235, 282, 285. Epidote, 70, 161. Epsom salt (Epsomite), 71. Erubescite (Bornite), 68. Eschara, 313. Estheria, 392. Euechinoidea, 388. Euomphalus, 352. Euphotide, 227. Eurite, 220. Eurypterida, 402. Eurypterus, 403. Exogyra, 340. Extinction, angles of, 145. Extraction of minerals, 14. Eye-structure, 96, 279, 284. F. Faseieularia, 312. Favosites, 310. Felsite, 220, 242. Felspars (see the various species). in sands, 190. , flame-reactions of, 78. Felstone, 242. Fenestella, 312. Ficula, 347. Field-observation, works on, 2. Flaggy Gneiss, 276. Flame-colouration, 47, 78. Flaser-gabbro, 234. Flint, 189, 200, 210. Flow-Breccia structure, 99. Fluidal gneissic structure, 102. ,, structure, 99. Fluorine, tests for, 62. Fluor-spar, 71, 162. Foliation, 96, 278. Foraminifera, 294. ForeUenstein, 234. Form of minerals, 15, 134. Formations, geological, 293. Fossils, study of, 287 ; mode of pre- servation, 290 ; derived, 292. Fouque, use of electro-magnet, 115; of hydrofluoric acid, 120. Foyaite, 223. Franklinite, 71. Fusibility of minerals, 45, 81 ; of rocks, 104. Fusion-place, 45, 79. Fusulina, 297. Fusus, 34G. G. GabbPO, 227, 233, 285. ,, -Gneiss, 285. Galena, 71. Galerites, 385. Galerus, 352. Gallinace, 266. Garnet, 71, 163. -amphibolite, 282. Gastrioceras, 363. Gastropoda, 343. Gedrite, 156. Gelatinisation on treatment with acid, 33. Genera, range of, 293 ; importance of, 405. Geoteuthis, 374. Gervillia, 337. Glass Tubes, reactions in, 53. Glassy igneous rocks, 263. Glauconite, 191, 299. Glaucophane, 181. -Schist, 282. Globigerina, 295. Glucina (see Aluminium), 61. Glycimeris, 325. Glyphioceras, 363. Glyptarca, 406. Gneisses, 283. Gneissic structure, 102, 284. Gomphoceras, 359. Goniaster, 389. Goniatites, 362. Goniometers, 16. Goniomya, 326. Gothite, 71. Grammysia, 335. Granatocrinus, 380. Granite, 217, 225. Granitic structure, 100. Granitite, 218. Granophyre, 102, 220, 221, 227. Granophyric structure, 102. 424 INDEX. Granular structure, 100, 229. Granulite, 286 ; igneous, 100, 230. Graphic granite, 219. ,, structure, 101, 219. Graphite, 71. Graptolites, 303. Gravels, 213. Greisen, 219. Gresslya, 326. Griffithides, 401. Grits, 191. Grossularite, 163. Gryphsea, 340. Gypsum, 72, 163, 199, 208. H. Haanel, plaster plates, 55. Haematite, 72, 163. Halleflinta, 242, 286. Hallirhoa, 300. Halobia, 409. Halysites, 310. Hamites, 370. Hammer, geological, 3. Hamulina, 371. Hardness of minerals, 21 ; of rocks, 93. Harpes, 398. Harpoceras, 367. Hauenschild's apparatus, 1 J 9. Haiiyne, 84, 163. Haiiynophyre, 255. Heavy liquids, 29, 116. Heliolites, 306. Helix, 353. Hemiaspis, 401. Hemicrystalline structure, 99. Hemimorphite, 72. Heterocrinus, 379. Heteromyaria, 335-338. Hexacoralla, 304. Hexactinellidse, 300. Hildoceras (Harpoceras), 410. Hippochrenes, 348. Hippopodium, 338. Hippurites, 331. Hcernesia, 337. Holaster, 387. Holcostephanus, 368. Holmia, 396. Holocrystalline igneous rocks, 100, 217. Holocystis, 309. Holopus, 375. Holostomatqus shells, 343. Homalonotus, 399. Homomya, 326. Ho.nomyaria, 325-335. Hoplites, 369. Hoploparia, 404. Horn Silver, 72. Hornblende, 72, 164. -Schist, 281. ,, -Granite, 217, 226. Hornstone, 220. Hutchings, on clays, 199. Hydrobia, 350. Hydrometer, 25. Hydrozoa, 302. Hymenocaris, 404. Hyolithes, 354. Hypersthene, 178. Hypersthenite, 228. I. lehthyoerinus, 379. Idiomorphic crystals, 99. Igneous rocks, 215; collection of specimens of, 216 ; table of, 270. Illsenus, 400. Ilmenite, 76, 183. Inarticulata, 320. Indices of refraction, 87. Inoceramus, 336. Integripalliate Lamellibranchs, 328. Intermediate igneous rocks, 225. Intersertal structure, 255. Iron, tests for, 62 ; native, 72, 165. Iron Pyrites, 72, 165, 291. Ironstones, 209 ; pisolitic and oolitic, 206, 210. Irregulares, 385. Isastrsea, 308. Isocardia, 326. Isolation of rock-constituents, 110. Isopoda, 404. Isotropism, 150. J. Janira, 341. Jolly's spring-balance, 28. Judd, on volcanic glass, 105 ; on growth of crystals, 137 ; on Schil- lerisation, 139 ; on Gabbro and Dolerite. 232 ; on constitution of lavas, 249. INDEX. 425 Kaolin, 72, 166, 190. Kendall, on gastropod shells, 345. ,, and Cornish, on shells, 201. Kerargyrite, 72. Keratophyre, 222, 224. Kersantite, 225, 226, 232. Kersanton, 225. Kingena, 316. Klein's solution, 30. Koninckella, 319. Koninckina, 319. Kupfernickel, 73. Kyanite, 166. Labechia, 302. Labelling of specimens, 11. Labradorite, 73, 84, 173. Lagena, 295. Lamellibranchiata, 322. Laminated structure, 94. Lamprophyre, 224, 232. Lantern of Aristotle, 382. Lapis Lazuli, 164. Lapworth, on metamorphic rocks, 278, 279. Laterite, 193, 199,210. Lawson, on Malignite, 223 ; on Laurentian, 283. Lead, tests for, 62. Leda, 327. Left-handed shells, 344. Lemberg's test, 36. Leperditia, 392. Lepralia, 313. Leptaena, 319. Leptostraca, 403. Leptynite, 286. Leucite, 84, 166. ,, -Andesite, 255. -Basalt, 261. Leucitite, 232, 236, 256, 261. Leucoxene, 183. Lhorzolite, 239. Lima, 340. Limburgite, 262, 268. Limestones, 199; concretionary, 209; crystalline, 273, 274, '283. Limnsea, 354. Limonite, 73, 167. Limulus, 401, 402. Linck, on limestones, 204. Lingula and Lingulella, 320. Liparite, 240. Litharaea, 308. Lithistidae, 300. Lithium, tests for, 62. Lithodomus, 338. Lithoidal rocks, 99, 240. Lithophyses, 98. Lithostrotion, 307. Littorina, 351. Littorinella ( = Hydrobia), 350. Lituites, 359. Loams, 198. Loligo, 374. Lonsdaleia, 307. Luken's balance, 27. Lustre of minerals, 15. -mottling, 101, 236. Luxulyanite, 219. Lydian stone, 212, 273. Lytoceras, 370. Maerodon, 332. Mactra, 326. Madreporaria, 306. Magellania, 316. Magmabasalt, 262. Magnesite, 73. Magnesium, tests for, 62. Magnet, use of, in isolating consti- tuents, 115. Magnetic characters, 22. Magnetite, 22, 73, 167, 190. Malachite, 73. Malacostraca, 404. Malignite, 223. Manganese, tests for, 62. Manganite, 75. Marbles, 274. Marcasite, 73, 165, 291. Marekanite, 105. Marialite (Scapolite), 179. Marls, 198. Medlicottia, 363. Meigen's test, 36. Meionite (Scapolite), 179. Melania, 349. Melanite, 163. Melanopsis, 349. Melaphyre, 253, 258, 260. Melilite, 262. -Basalt, 261. 426 INDEX. Melonites, 388. Membranipora, 313. Mercury, tests for, 62. Meristella, 318. Merocrystalline structure, 99. Merostomata, 401. Mesonacis, 396. 1 58, 59. Metal, reduction to, with blowpipe, Metamorphic Rocks, 271. Methylene iodide, 30. Metichthyocrinus, 379. Miarolitic structure, 97. Miascite, 223. Micas, 167, 190. Mica-Schist, 279. ,, -Trap, 224. Michelinia, 309. Micraster, 387. Microchemistry, 36. Microcline, 168, 173. Microcosmic salt, reactions in, 51. Microcrystalline structure, 99. Microgranite, 220. Microgranitic structure, 100. Microgranular structure, 101. Microgranulite, 220. Microgranulitic structure, 100. Micrographic structure, 102. Microlites, 243, 265. Microlitic structure, 99. Micropegmatitic structure, 101. Microscope, petrological, 123. Miliola (Miliolites), 295. Miller, reflective goniometer, 17. Millstone-Porphyry, 241. Minerals, aspect in rock-sections, 134, 154. Minette, 222, 224. Mispickel, 73. Modiola, 337. Mohr's displacement-apparatus, 103. Molybdenite, 73. Molybdenum, tests for, 62. Monactinellidae, 299. Monograptus, 303. Monomyaria, 339. Monophyllites, 365. Monopleura, 331. Monotis, 336. Montlivaltia, 309. Mounting of microscopic objects, 128. Murchisonia, 352. Murex, 347. Muscovite, 169. Mya, 327. Mylonitic structure, 279. Myophoria, 334. Mytilus, 337. N. Nassa, 346. Natica, 350. Naticopsis, 351. Natrolite, 74, 169. Nautiloidea, 358. Nautilus, 358. Nebalia, 403 Negative and positive crystals, 88. Neocrinoidea, 377. Neolimulus, 401. Nepheline, 74, 84, 169. -Andesite, 255. -Basalt, 260. -Diorite, 229. -Dolerite, 231, 235. -Syenite, 223. -Trachyte, 247. Nephelinite, 232, 236, 256, 261. Nerinea, 350. Nerita, 351. Neritina, 351. Niccolite, 73. Nicholson's araeometer, 25. Nicholson, on Graptolites, 303 ; on Heliolites, 306. Nickel, tests for, 63, 69. Nickeline, 73. Nitre, 74. Nodosaria, 295. Norite, 227. Nosean, 84, 170. Noseanite, 232. Nothoceras, 358. Novaculite, 211. Nucula, 333. Nullipores (corallines), 203. Nummulites, 296. 0. Oblique extinction, 147. Obolella, 321. Obolus, 321. Obsidian, J >, 264. Octocoralla, 304, 306. Ogygia, 399. INDEX. 427 Olcostephanus (Holcostephanus), 368. Olenellus, 396. Olenus, 397. Oligoclase, 74, 84, 113. Olivine, 74, 170; nodules, 260. -Basalt, 257. -Diabase, 235, 258. -Dolerite, 234. -Gabbro, 232. -Leucitite, 261. -Nephelinite, 261. -Rock, 239. Omphyma, 307. Oniscus, 378. Oolites, 203. Oolitic structure, 95, 203 ; in flint, 212. Opal, 171. Ophicalcite, 274. Ophite, 101, 237. Ophitic structure, 101, 229, 260. Ophiuroidea, 388, 389. Optical sign of minerals, 147. Orbicular structure, 102, 220. Orbiculoidea, 321. Orbitoides, 296. Orthis, 319. Orthoceras, 359. Orthoclase, 74, 84, 171. ,, -porphyry, 224. Orthogneiss, 278. Orthonota, 334. Orthophyre, 224. Ostracoda, 390. Ostrea, 339. Ottrelite, 172. Oxidising flame, 45. Oxynoticeras, 366. P. PalsBarea, 332. Palseaster, 389. Palseechinoidea, 388. Palseechinus, 388. Palseocorystes, 405. Palseocrinoidea, 377. Palaeontology, works on, 289. Palagonite, 266, 269. Paludina, 350. Panopsea, 325. Pantellerite, 246. Parabolina, 393. Paradoxides, 396. Paragneiss, 278. Paragonite, 280. Parallelodon, 332. Parish's balance, 27. Parkinsonia, 369. Patella, 353. Pecten, 341. Pectunculus, 333. Pegmatite, 219. Pegmatitic structure, 101. Petes Hair, 268. Pentacrinus, 378. Pentamerus, 317. Pentremites, 380 ; P. ellipticus, 380. Peridotite, 236, 263, 268. Perisphinctes, 369. Perlite, 264. Perlitic structure, 98, 265. Perna, 337. Peronidella (Peronella), 302. Petrography, works on, 133. Petrosilex, 220, 242. Phacops, 400. Pharetrones, 301. Phasianella, 352. Phenocrysts, 97. Phillipsia, 401. Phlogopite, 173. Pholadidse, 327. Pholadomya, 326. Pholas, 324. Phonolite, 247, 248, 249. Phormosoma, 388. Phosphatic deposits, 208. Phosphoric acid, test for, 36. Phosphorus, test for, 63. Phragmoceras, 360. Phragmophora, 371. Phyllade, 277. Phyllite, 277. Phyllocarida, 403. Phylloceras, 365. Phyllopoda, 392. Picrite, 236. Finite, 173. Pinna, 337. Pinnigena, 337. Pisolitic structure, 95, 203; ore, 210. Pistacite, 161. Pitchblende, 74. Pitchstone, 263. Plagioclases, 173. Plagiostoma, 341. 428 INDEX. Planorbis, 354. Plaster of Paris plates, 55. Platycrinus, 379. Pleochroism of minerals, 89, 143. Pleurotoma, 345. Pleurotoraaria, 352. Plicatula, 342. Plocoscyphia, 301. Poikilitic structure, 101. Polished surfaces, examination of. 132. Polyzoa, 311. Porcellanea, 295. Porcellanite, 272. Porifera, 298. Porphyrite, 230, 250, 251, 252, 253, 255. Porphyritic structure, 97. Posidonomya, 336. Positive and negative crystals, 88. Potamides, 349. Potassium, tests for, 63, 81, 85. Poterioceras, 360. Prestwichia, 402. Primitia, 392. Productus, 319. Proetus and Proetidae, 401. Prolecanites, 363. Prolimulus, 402. Pronorites, 363. Proportions of rock - constituents, 121. Propylite, 255. Prosiphonate shells, 363. Prosobranchiata, 345. Protaster, 389. Protocardia, 328. Protogine, 284. Protospongia, 301. Proustite, 74. Pseudo-hypersthene, 157, 228. Pseudomelania, 349. Psiloceras, 367. Psilomelane, 74. Pterinea, 335. Pterineopecten, 408. Pteropoda, 354. Pterygotus, 403. Pulmonata, 353. Pumiceous structure, 99. Purpura, 347, Pygaster, 386. Pygope, 316. Pygorhytis (Inferior Oolite), 411. Pyrargyrite, 74. Pyrite, 72, 165, 291. Pyrolusite, 74. Pyromeride, 241, 265. Pyromorphite, 75. Pyrope, 163. Pyroxenes, 177, 190. Pyroxene-Andesite, 252. ,, -Diorite, 227. Pyrrhotine, 22, 75, 177. PyruU, 347. Quartz, 75, 177, 189. -Andesite, 249. -Aphanite, 226. -Diabase, 226, 227. -Diorite, 225. -Felsite, 220. -Keratophyre, 222. -Pantellerite, 245. plate, 146. -Porphyrite, 250. -Porphyry, 220, 242. -Rock, 194. --chist, 276, 283. -Trachyte, 240. wedge, 125, 148. Quartzites, 194, 275, 283. R. Radiolaria, 297; in albite, 275. Radiolites, 331. Radula, 341. Rafinesquina, 319. Rastrites, 303. Redruthite, 75. Reducing flame, 45. Reflective Goniometers, 17-19. Refractive index of minerals, 141, Regional Metamorphism, 27-3. Regulares, 384. Retrosiphonate shells, 362. Retzia, 318. Rhaphidonema, 31)2. Rhizopoda, 294. Rhodonite, 75. Rhombic Pyroxenes, 178. Rhynchonella, 316. Rhyolite, 240. -Glass, 264. Rhyolitic Andesite, 249. INDEX. 429 Riebeckite, 181. Right-handed shells, 344. Rimella, 348. Rings and crosses in convergent polarised light, 151. Rissoa, 350. Roasting of minerals. 45. Rock-Salt, 75, 208. ,, -structures, 94. Rocks, study of, in field, 92. Rohrbach's solution, 30. Rostellaria, 347. Rotalia, 296. Rudistae, 331. Rupert Jones, T., on Ostracoda, 391 ; on Estherise, 392. Rutile, 75, 179, 187, 191, 277. Rutley, on Novaculite, 212. Saccammina, 297. Sal-ammoniac, 76. Sand, 186. ,, -grains, characters of, 187. Sandstone, 191, ? 73. Sanidine, 171. Saussure, H. B. de, determinations of fusibility, 46. Saussurite, 177, 227, 285. Scaphites, 371. Scaphcpoda, 342. Scapolites, 179. Schalstein, 258. Schiefer and Schiste, 278. Schillerisation, 20, 139. Schists, 278-283. Schizodus, 334. Schkenbachia, 366. Schlotheimia, 366. Schliiter, on Belemnitella, 374. Schmidt, on Eurypterida, 403. Schorl, 183. Sclerography, 288. Scoriaceous structure, 99, ICO. Scutella, 386. Scyelite, 237. Sea-urchins, 381. Secondary devitrification, 241, 244, 264. Secondary growths of crystals, 137. Sections for the microscope, 129. Sedimentary Rocks, 186-214. Selenite plate, 146. Separating apparatus, 113-120. Sepia, 374. Septarian structure, 95. Sericite, 169. [281. Serpentine, mineral, 180; rock, 237 -Schist, 281. Serpentinous limestone, 274, 275. Serpula, 390. Shale, 197. Sheila, constitution of, 201. Shelly Limestones, 201. Siderite (= Chalybite), 69, 180. Sieves and sifting, 113, 187. Silicates, decomposition of, 33. Silicon, tests for, 63. Sillimanite, 180. Silver, tests for, 63. Sinter, 208. Sinupalliate Lamellibranchs, 325. Siphonate Lamellibranchs, 325, 328, 335. Siphonia, 300. Siphonostomatous shells, 343. Skeleton-spherulites, 244. Slate, 276. Smaltine, 76. Smaragdite, 164. Smeeth, on specific gravity of small grains, 24 ; separating apparatus, 119. Smithsonite, 68. Soda-Amphiboles, 181. , -Microcline, 168, 173. , -Nitre, 76. , -Orthoclase, 74, 84, 172. , -Pyroxenes, 181. , -Rhyolite, 245. , -Trachyte, '246. Sodalite, 84, 181. Sodium, tests for, 64, 81. Soils, 10, 115, 187. Sollas, on diffusion - column, 31, 120. Solubility of minerals, 32. Sonstadt's solution, 29. Sorby, on microscopic mounting, 128 ; on sand -grain s, 188 ; on Stones - field slate, 200; on constitution of shells, 201 ; on oolite, 204 ; on travertine, 208 ; on corals, 304. Spatangus, 387. Specific-gravity bottle, 24. 430 INDEX. Specific gravity of minerals, 23-32 of rocks, 103; of substances lighter than water, 24. Specular iron, 72. Sphene, 76, 182. Sphaerium, 329. Spheroidal structure, 97. Spherulitic structure, 98, 241, 265, 267. Spilite, 258. Spinel, 76. Spinelloids, 182. Spirifer, 318. Spiriferina, 318. Spirula, 374. Spirulirostra, 374. Spondylus, 342. Sponge-spicules, 298 ; in chert, 211. Sponges, 298; siliceous, 298; cal- careous, 301. Spotted Shale, 272. Stalactites, 207. Stalagmites, 207. Stephanoceras, 368. Stibnite ( = Antimonite), 66. Stilbite, 182. Straight extinction, 147. Streak, 15. Stream Tin, 68. Strike, 7. Stringocephalus, 316. Stripe in slate, 276. Stromatopora, 302. Strontianite, 76. Strontium, tests for, 64. Strophomena, 319. Stylonurus, 403. Sulphur, tests for, 60, 64; native, 76. Syenite, 222 ; Compact, 224. Sylvine, 76. Syringopora, 310. Szabo on Flame-reactions of the Felspars, 78-84. T. Taehylyte, 266. Talc, 76, 183. -Schist, 281. Teall, on slate, &c., 277, 282. Tellina, 325. Tellurium, 48. Tentaculites, 355. Te'phrine and Tephrite, 255. Terebratula, 315. Terebratulina, 316. Teredo, 327. Tetracoralla, 304. Tetractinella (Retzia), 409. Tetractinellidse, 300. Textularia, 295. Thallium silver nitrate, 120. Thamnastraea, 309. Theca, 354. Thecosmilia, 309. Thecosomata, 354. Theralite, 229. Thoulet's separating apparatus, 114, Thread-lace scoria, 268. Tin, tests for, 64. Tinguaite, 225. Tinstone ( = Cassiterite), 68. Tiree Marble, 274. Titanic iron ore, 76, 183. Titanium, tests for, 64. Tonalite, 225. Topaz, 76, 183. Tourmaline, 77, 183, 190. Tourmaline-Granite, 219. Toxaster (Neocomian), 412. Trachyceras, 364. Trachyte, 245. -Glass, 265. Trachytic Andesite, 251. Trap-Granulite, 230, 286. Travertine, 208. Trays for specimens, 292. Tremacystia, 302. Tremolite, 184. Triarthrus, 393, 396. Trichites (crystallites). 265. ,, (mollusc), 337. Tridymite, 184. Trigonia, 333. Trilobites, 393. Trinucleus, 398. Trocholites, 358. Trochus, 351. Troctolite, 234. 238. Trophon, 347. Tuffs, 195. Tungsten, tests for, 64. Turbo, 351. Turrilites, 371. Curritella, 349. Twinning, 16, 137, 1 W. INDEX. 431 U. Unio, 334. Uralite, 157. Uranium, tests for, 65. V. 268. Variolite, 258, 267, Venericardia, 330. Ventriculites, 301. Venus, 325. Verde di Corsica, 227. Vibration-traces, 86. Vitrea, 295. Vivianite, 77, 89. Vivipara, 350. Vogesite, 225. Volcanic agglomerates, 195. Voluta, 346. w. Waldheimia, 316. Wet reagents, tests with, 32. Wethered, on oolitic structure, 204. Witherite, 77. Wolfram, 77. Wollastonite, 77. Worm-borings, 389. X. Xanthopsis, 40f>. Xenocrysts, 97. Xiphosura, 402. Zaphrentis, 307. Zeolites, 184. Zinc, tests for, 65. Zinc-Blende, 77. Zircon, 77, 185, 187, 191. Zirconium, tests for, 65, 77. 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