I BERKELEY 
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The Cambridge Manuals of Science and 
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 ROCKS AND THEIR ORIGINS 
 
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 *jj3^^% 
 
 o o o o 
 
 ROCKS AND THEIR 
 ORIGINS 
 
 BY 
 
 GRENVILLE A. J. COLE, 
 
 F.R.S. 
 
 Professor of Geology in the Royal 
 College of Science for Ireland 
 
 Cambridge : 
 at the University Press 
 1922 
 
r 
 O 
 
 FtVat Edition 1912 
 Second Edition 1922 
 
 <? exception of the coat of arms at 
 the foot, the design on the title page is a 
 reproduction of one used by the earliest known 
 Cambridge printer, John Siberch, 1521 
 
PKEFACE 
 
 THIS little book is intended for those who are not 
 specialists in geology, and it may perhaps be 
 accepted as a contribution for the general reader. 
 To all who are interested in the earth, the study of 
 rocks is an important branch of natural history. If 
 detailed works on petrology are to be consulted 
 later, F. W. Clarke's Data of Geochemistry (Bulletin, 
 U.S. Geological Survey, ed.4, 1920) must on no account 
 be overlooked. Its numerous references to published 
 papers, and the attention given to rock-origins, make 
 it a worthy companion to C. Doelter's Petrogenesis. 
 Many things have perforce been omitted from the 
 present essay. It seemed unnecessary to review the 
 Carbonaceous rocks, since the most important of these 
 have been admirably dealt with in E. A. N. Arber's 
 Natural History of Coal, published as a volume 
 in this series. I should like to have described 
 occurrences of rock-salt, of massive gypsum, and 
 other products of arid lands, where "black alkali" 
 poisons the surface, and the casual pools are 
 
 501050 
 
vi PREFACE 
 
 fringed with white and crumbling crusts. Rock- 
 taluses, and all the varied alluvium carried seaward 
 as the outwash of continental land, well deserve a 
 chapter to themselves. But there is really no end 
 to the subject, which embraces all the accumulative 
 processes of the earth. A few vacation -journeys, 
 judiciously planned out, teach us that text-books are 
 merely signposts to set us on the path of the pioneers. 
 Our own eyes must guide us in the quest for origins, 
 where the burst of the long sea-roller swirls the 
 pebbles on the shore, where the steam curls up among 
 volcanic clefts, or where some fire-swept clearing in 
 the forest reveals for the first time to human observa- 
 tion an acreage of antique rock. 
 
 G. A. J. C. 
 
 OARRICKMINES, Co. DUBLIN. 
 April, 1922. 
 
CONTENTS 
 
 CHAP. PAGE 
 
 LIST OF ILLUSTRATIONS viii 
 
 I. On Rocks in General ..... 1 
 
 List of the common Minerals that form Rocks 8 
 
 II. The Limestones 12 
 
 III. The Sandstones 56 
 
 IV. Clays, Shales, and Slates .... 78 
 
 V. Igneous Rocks 103 
 
 VL Metamorphic Rocks 143 
 
 REFERENCES 162 
 
 TABLE OF STRATIGRAPHICAL SYSTEMS . . 169 
 
 INDEX .... 170 
 
LIST OF ILLUSTRATIONS 
 
 FIG. PAGE 
 
 1 Surface of Limestone plateau, CausseduLarzac, A veyron 45 
 
 2 Ravine in Limestone, Canon of the Dourbie, Aveyron 47 
 
 3 Waterworn cliff of Limestone, Millersdale ... 49 
 
 4 Limestone country dissected by ravines, Hercegovina 51 
 
 5 Sand developing from Sandstone, Cape of Good Hope 59 
 
 6 Siliceous Conglomerate, Co. Waterford . . .75 
 
 7 Quartzite Cone, Groagh Patrick 77 
 
 8 Shrinkage-cracks in Clay, Spitsbergen . . . 81 
 
 9 Landslide of Limestone over Shale, Drome . . 93 
 
 10 Weathering of Shale, Isere 95 
 
 11 Boulder-clay, Crich, Derbyshire (Phot. H. A. Bemrose) 97 
 
 12 Nordenskiold Glacier, Spitsbergen .... 99 
 
 13 Sef strom Glacier, Spitsbergen . . . . . 101 
 
 14 Ash-layer of 1906 on Vesuvius . . . . . ill 
 
 15 Puy de la Vache, Puy-de-D6me 113 
 
 16 Granite invading Mica-schist, Cape Town . . . 121 
 
 17 Weathering granite, Lundy Island .... 139 
 
 18 Granite weathering under tropical conditions, Matopo 
 
 Hills 141 
 
 19 Composite Gneiss, Co. Donegal 153 
 
 20 Composite Gneiss, Angno, Sweden . . . .155 
 
 (Figs. 11 and 17 are reproduced from the Cambridge County 
 Geographies of Derbyshire and Devonshire respectively; the 
 rest of the illustrations are from photographs by the author.) 
 
CHAPTER I 
 
 ON ROCKS IN GENERAL 
 
 THE description of rocks has fallen very much into 
 the hands of lovers of analysis and classification, and 
 attention has been diverted, even among geologists, 
 from their fundamental importance as parts of the 
 earth's crust. The geographer or the general traveller 
 may often wish for closer acquaintance with the units 
 that build up the scenery around him. The characters 
 of rocks again and again control the features of the 
 landscape. When studied more nearly, these same 
 characters imply conditions of deposition or solidifi- 
 cation, and lead the mind back to still older land- 
 scapes, and to the meeting of oceans and continents 
 on long-forgotten shores. Petrology, indeed, involves 
 the understanding of how rocks " come to be where 
 we find them when we try"; but the classification of 
 hand-specimens was from the first easier than field- 
 investigation, and in later times the science was 
 threatened with the description of isolated micro- 
 scopic slides. Fortunately, a certain amount of 
 
 c. l 
 
2 ROCKS AND THEIR ORIGINS [CH. 
 
 feeling for natural history has been imported again 
 into the subject, and evolutionary principles and 
 sequences have been discussed. Experimental work, 
 moreover, has been brought to bear on the question 
 of the origins of rocks, with more success than might 
 have been expected, since it is very difficult to realise 
 in a laboratory, or even in the mind, the conditions 
 that prevail in the lower parts of the earth's crust. 
 
 Rocks, we have to remember, are in themselves 
 considerable masses, and have relations with others 
 far away. The coarseness of a sandstone at one 
 point, or even over square miles of country, implies 
 the deposition of finer material somewhere else. 
 The lava-flow implies the existence of mysterious 
 cauldrons in the crust. It is, however, fortunate that 
 the primary classification of rocks was promulgated 
 without regard for theories of rock-origins. The 
 work was done by men who were masters and pioneers 
 in mineralogy. At a time when a powerful school 
 regarded basalt as of sedimentary origin, and when 
 granite was generally believed to be the most ancient 
 component of the crust, rock-masses were taken in 
 hand as aggregates of certain minerals, .and were 
 reduced to an orderly scheme for arrangement in the 
 cabinets of the curious. Any system based on ideal 
 relationships would have been fatal at that time to 
 petrology as a science. 
 
 Alexandre Brongniart, in 1813, thus saw objections 
 
i] ON ROCKS IN GENERAL 3 
 
 to the classification of rocks that had been proposed 
 by Werner. In his "Essai d'une classification min- 
 ralogique des Roches melang^es," he showed the im- 
 possibility of determining the age of a rock in relation 
 to others before assigning to it a name, and the 
 absurdity of separating similar rocks on account of 
 differences in their geological age. Brongniart was 
 thus forced to rely, firstly, upon the prevalence of 
 certain mineral constituents, and, secondly, on the 
 structure of the mass. He developed this scheme in 
 1827, in his "Classification et caracteres mineralo- 
 giques des Roches homogenes et het&rogenes" ; but 
 it is clear that, even in such a system, considerations 
 of natural history and of origin will ultimately pre- 
 dominate. Brongniart was much influenced by Karl 
 von Leonhard's " Charakteristik der Felsarten," 
 published in 1823, and these two authors have been 
 regarded as the founders of petrography. 
 
 The difficulty of distinguishing between rocks laid 
 down as true sediments on the earth's surface and 
 those that have consolidated from a state of fusion 
 has been very largely removed. The assistance of 
 the microscope can now be called on to elucidate the 
 minute structure of fine-grained masses, which 
 appeared homogeneous to earlier workers. 
 
 The pioneer in microscopic methods was Pierre 
 Louis Antoine Cordier, who knew rocks as a traveller 
 knows them in the field. In 1798, as a young man 
 
 12 
 
4 ROCKS AND THEIR ORIGINS [OH. 
 
 of twenty-one, he had gone to Egypt with the famous 
 expedition under General Bonaparte. D^odat de 
 Dolomieu had charge of the geological observations, 
 and Cordier went through the hardships of the cam- 
 paign as his assistant. When Bonaparte abandoned 
 the army and withdrew to Paris, Cordier might well 
 have been lost to Europe. 
 
 However, he successfully brought home the know- 
 ledge acquired in the field, and set himself, in those 
 agitating years, to solve the problem of the compact 
 groundwork of igneous rocks. He argued that this 
 groundwork probably consisted of minerals, and that 
 these minerals were probably similar to those occur- 
 ring as visible constituents of the mass. He examined 
 the powder of these larger crystals under the micro- 
 scope, and made himself familiar with their aspect in 
 a fractured form. He then powdered the compact 
 material of his rocks, washed away the dust, and was 
 able to recognise in the coarser residue the minerals 
 that he had previously studied. He used the magnet 
 to extract the iron ore ; he determined the fusibility 
 of the particles with the blowpipe ; and he even dis- 
 covered in volcanic lavas a residual glass associated 
 with the crystalline material d). To this day, when a 
 particular mineral has to be determined in a rock, it 
 is often best to follow Cordier's method, and to extract 
 the actual crystals, however small. Various modes of 
 separation, especially those involving the use of dense 
 
i] ON ROCKS IN GENERAL 6 
 
 liquids, have been devised since Cordier's time, and the 
 specific gravity of a single crystal can now be deter- 
 mined, although it may be so small as to require 
 looking for in the dense liquid with a lens(2). 
 
 Between 1836 and 1838, Christian Gottfried 
 Ehrenberg, Professor of Medicine at Berlin, made an 
 immense step forward in the study of rocks. Being 
 keenly interested in microscopic forms of life, he 
 wished to determine their importance as constituents 
 of rocks. Using a microscope magnifying 300 dia- 
 meters, he showed the presence of organisms in flint 
 and limestone, and found in 1838 that a thin slice 
 of chalk coated over with Canada balsam became 
 practically transparent. In his " Mikrogeologie," 
 published in 1854, he gives drawings of thin sections 
 of several flints, seen by transmitted light, which are 
 thus rock-sections in the modern petrological sense. 
 His method could not have been generally known 
 until his book appeared in 1854. Meanwhile, Henry 
 Clifton Sorby, about 1845, found the naturalist W. C. 
 Williamson making thin sections of fossil plants and 
 bones. He promptly perceived the importance of 
 the method as applied to rocks in general, and 
 introduced it to the Geological Society of London 
 in 1850, in a paper on the Calcareous Grit of 
 Scarborough. Seven years later, he read his memor- 
 able paper on "The Microscopical Structure of 
 Crystals (s)/' in which he made use of slices of granite 
 
6 ROCKS AND THEIR ORIGINS [CH. 
 
 and of Vesuvian and other lavas. Ferdinand von 
 Zirkel met Sorby by chance at Bonn in 1862, and, 
 learning his methods, proceeded to systematise the 
 examination of rock-specimens with the microscope. 
 Such studies, rapidly appreciated by Michel Levy, 
 Rosenbusch, Judd, and others, naturally led to 
 advances of the first importance in petrology. They 
 enabled workers to ascertain the relations of the 
 rock-constituents one to another, and the order of 
 consolidation of minerals from an igneous magma. 
 The broad division of rocks into those of sedi- 
 mentary and those of igneous origin has been further 
 emphasised. The rocks styled metamorphic still 
 afford the greatest difficulty, even after prolonged 
 enquiry in the field. 
 
 Seeing that some rocks are merely massive 
 minerals, that is, large masses formed of one mineral 
 species, while others consist of crystals or fragments 
 of a variety of minerals, it may be well to remind 
 ourselves of the distinction between minerals and 
 rocks. We may define a mineral as a natural 
 substance formed by inorganic action ; its chemical 
 composition is constant ; under favourable circum- 
 stances, it assumes a characteristic crystalline 
 form. 
 
 Like all definitions of natural objects, the above 
 requires some qualification. In many cases the 
 chemical composition of a mineral varies by a well- 
 
i] ON ROCKS IN GENERAL 7 
 
 defined series of atomic replacements, and we cannot 
 feel called upon to establish a new species for every 
 step away from the rigid type. Sodium thus replaces 
 potassium to some extent in orthoclase felspar. The 
 crystalline form, again, may not be specifically charac- 
 teristic, as, for instance, in the members of the garnet 
 series, which crystallise in the cubic system. The 
 homogeneity of crystalline structure throughout the 
 individual may be regarded as the most essential 
 feature of what we style a mineral species ; that is to 
 say, the atoms of the elements present are grouped 
 in definite proportions, and are arranged on the same 
 physical plan. 
 
 A rock, on the other hand, is a mere aggregate of 
 mineral particles, or of molecules that, under proper 
 conditions, would group themselves to form mineral 
 species. It may consist entirely of granules or 
 crystals of one species ; but the structures in these 
 have no common orientation, as they would have 
 in a single large continuous crystal. The rock itself 
 has no crystalline form, and any structures that 
 simulate such forms will be found on measurement 
 to have none of the regularity that characterises 
 genuine crystals. A rock, moreover, formed of several 
 mineral species in association will by no means 
 possess a constant chemical composition, and the 
 variations from point to point form a feature of 
 especial interest in the study of igneous masses, of 
 
8 ROCKS AND THEIR ORIGINS [OH. 
 
 sediments deposited on a shore, or of alluvium in a 
 valley stretching far between the hills. 
 
 In the pages that follow we hope, then, to bear in 
 mind the relations of rocks to the earth and to our- 
 selves. Like the ancient Romans, we build our cities 
 with huge blocks and slabs brought from crystalline 
 masses oversea. We now tunnel, for our commercial 
 highways, through the complex cores of mountain- 
 chains. Everywhere rocks are our foundations, 
 throughout our travels or in our settled homes. 
 They rise as obstacles against us, or they spread 
 before us fields of fertile soil. Some knowledge of 
 them is part of the general body of culture that 
 makes us, in the best sense, citizens of the world. 
 
 LIST OF THE COMMON MINERALS THAT 
 FORM ROCKS 
 
 Actinolite. See Amphiboles. 
 
 Albite. See Felspars. 
 
 Amphiboles. A series of silicates with the general formula 
 RSi0 3 , where R is magnesium, iron or calcium; in many, 
 such as the common species Hornblende, molecules occur in 
 addition in which aluminium and triad iron are introduced. 
 Hornblende thus consists of m (Mg, Fe", Ca) Si0 3 . n (Mg, Fe") 
 (Al, Fe'")2 Si0 6 . Actinolite is a non-aluminous amphibole 
 occurring in needle-like prisms. The amphiboles crystallise 
 in prisms having angles of about 56 and 124. See Pyrox- 
 
 Anatase. See Rutile. 
 
i] ON ROCKS IN GENERAL 9 
 
 Andalusite. Aluminium silicate, Al 2 Si0 5 , crystallising in the 
 rhombic system. Sillimanite consists also of Al 2 Si0 6 and is 
 rhombic, but crystallises with different fundamental angles. 
 
 Anorthite. See Felspars. 
 
 Apatite. Calcium phosphate, with fluorine, or sometimes 
 chlorine, (CaF) Ca 4 (PO^ = 3 Ca 3 ( P0 4 ) 2 . CaF 2 . 
 
 Aragonite. Calcium carbonate, CaCO 3 , crystallising in the 
 rhombic system, with a specific gravity of 2'94. See Calcite. 
 
 Augite. See Pyroxenes. 
 
 Biotite. See Micas. 
 
 Calcite. Calcium carbonate, CaC0 3 , crystallising in the trigonal 
 system, with a specific gravity of 272. See Aragonite. 
 
 Chalcedony. Crystalline silica, Si0 2 , in fibrous and often 
 mammillated forms. Flint or Chert is a concretionary 
 form, in which some interstitial opal may be present 
 
 Chert. See Chalcedony. 
 
 Chlorites. Hydrous aluminium magnesium iron silicates, re- 
 sembling green micas, but softer and with non-elastic plates. 
 
 Chromite. Iron chromium oxide, FeCr 2 4 . Magnesium may 
 replace part of the dyad iron, and aluminium and triad iron 
 some of the chromium. 
 
 Diallage. An altered augite with a shimmery submetallic 
 lustre. 
 
 Diopside. See Pyroxenes. 
 
 Dolomite. Magnesium calcium carbonate, MgCa (C0.s) 2 . 
 
 Enstatite. See Pyroxenes. 
 
 Epidote. Calcium aluminium iron silicate, Ca 2 (A10H) (Al, Fe"')2 
 (Si0 4 ) 3 . 
 
 Felspars. A series of silicates of aluminium with potassium or 
 sodium or calcium, or all of these. Orthoclase, KAlSi 3 8 , 
 and the corresponding sodium form, Albite, NaAlSi 3 8 , lie 
 at one end of the series, and the calcium felspar Anorthite, 
 CaAl>(Si0 4 ) 2 , at the other. While Orthoclase crystallises in 
 
10 ROCKS AND THEIR ORIGINS [CH. 
 
 the monoclinic system, a triclinic form, Microcline, with the 
 same composition, is also common. All the other felspars 
 are triclinic, and, with microcline, are often styled plagio- 
 clases. The principal felspars between Albite and Anorthite 
 are Oligoclase, the "soda-lime felspar," and Labradorite, 
 the "lime-soda felspar." 
 
 Flint. See Chalcedony. 
 
 Garnets. A series of silicates with the general composition of 
 R 3 "R 2 '"(Si0 3 ) 4 , R" being Ca, Fe", or Mn, and R'" being Al 
 or Fe'". The common red garnet in mica-schists is 
 Almandine, Fe 3 Al 2 (Si0 3 ) 4 , while that in altered limestones 
 is Grossularite, Ca 3 Al 2 (SiO 3 ) 4 . 
 
 Grlauconite. A hydrous iron potassium silicate, with some 
 aluminium, magnesium, and calcium. 
 
 Gothite. Iron hydroxide, FeO(OH). 
 
 Gypsum. Hydrous calcium sulphate, CaS0 4 +2H 2 0. 
 
 Hornblende. See Amphiboles. 
 
 Hypersthene. See Pyroxenes. 
 
 Ilmenite. Titanium iron oxide, m FeTi0 3 + n Fe 2 3 . 
 
 Iron Pyrites. Iron disulphide, FeS 2 . A cubic species, 
 Pyrite, and a less common rhombic species, Marcasite, 
 occur. 
 
 Kaolin. Hydrous aluminium silicate, H 4 Al 2 Si 2 9 . 
 
 Kyanite. Aluminium silicate, Al 2 Si0 6 , crystallised in the tri- 
 clinic system. See Andalusite. 
 
 Labradorite. See Felspars. 
 
 Leucite. Potassium aluminium silicate, KAl(Si0 3 )2. 
 
 Limonite. Common massive iron hydroxide. 
 
 Magnetite. Magnetic iron oxide, Fe 3 4 . 
 
 Marcasite. See Iron Pyrites. 
 
 Micas. A series of aluminium silicates, with potassium, and 
 often with magnesium, or iron, or both. The two marked 
 types are Muscovite, aluminium potassium mica (some varie- 
 
i] ON ROCKS IN GENERAL 11 
 
 ties containing lithium), H 2 KA1 3 (SiO^, with a silvery aspect, 
 and Biotite, the common dark "ferro-magnesian" mica, (H, K>2 
 (Mg,Fe")2(Al,Fe'") 2 (Si04)3. 
 
 Microcline. See Felspars. 
 
 Muscovite. See Micas. 
 
 Nepheline. Sodium aluminium silicate, with some potassium, 
 the pure sodium type being NaAlSi0 4 ; the types with 
 potassium contain slightly more silica. 
 
 Oligoclase. See Felspars. 
 
 Olivine. Magnesium iron silicate, (Mg, Fe)2 SiO4. 
 
 Opal. Uncrystallised silica, SiO 2 , with some water. 
 
 Orthoclase. See Felspars. 
 
 Pyrite. See Iron Pyrites. 
 
 Pyroxenes. A series of silicates corresponding in composition 
 to the Amphiboles, but crystallising in prisms which have 
 angles of about 87 and 93. On the whole, the pyroxenes 
 are richer in calcium than the amphiboles. The formula of 
 Wollastonite is CaSi0 3 . Diopside consists of Ca (Mg, Fe) 
 (Si0 3 )2. Augite, the commonest form, is aluminous, cor- 
 responding to Hornblende among the amphiboles ; but the 
 change from Augite into Hornblende, which often occurs, 
 may imply a loss of calcium. Enstatite and Hypersthene are 
 species crystallising in the rhombic system; the former 
 consists of MgSi0 3 , while in Hypersthene iron replaces some 
 of the magnesium. 
 
 Quartz. Silica, Si0 2 , crystallised in the trigonal system. 
 
 Eock-Salt. Sodium chloride, NaCl. 
 
 Eutile. Titanium dioxide, Ti0 2 , crystallised in the tetragonal 
 system. Anatase has the same composition, and is tetragonal, 
 but has different fundamental angles. 
 
 Serpentine. Hydrous magnesium iron silicate, H 4 (Mg, Fe)3 
 Si 2 O 9 . 
 
 Siderite. Iron carbonate, FeC0 3 . 
 
12 ROCKS AND THEIR ORIGINS [CH. 
 
 Sillimanite. See Andalusitc. 
 
 Talc. Hydrous magnesium silicate, H 2 Mg 3 (SiOaV 
 
 Tourmaline. A borosilicate of aluminium with various other 
 elements, R'gA^BOH^Si^g. R represents H, Na, Al, 
 Mg, Fe. 
 
 Tridymite. Silica, Si0 2 , crystallised in doubly refracting six- 
 sided plates. Its specific gravity is 2'3, that of Quartz being 
 2-65. 
 
 Wollastonite. See Pyroxenes. 
 
 Zeolites. A series of hydrous aluminium silicates, with potassium, 
 sodium, calcium, and sometimes barium. 
 
 Zircon. Zirconium silicate, ZrSi0 4 . 
 
 CHAPTER II 
 
 THE LIMESTONES 
 INTRODUCTION 
 
 THE term Limestone covers, by common consent, 
 rocks consisting mainly of calcium carbonate. 
 Dolomite (properly Dolomite-rock), in which half 
 or nearly half the molecules consist of magnesium 
 carbonate, is, however, generally included. The 
 convenience of limestones as building materials has 
 given them a world- wide interest. Their stratified 
 and jointed structure appealed to the early Egyptian 
 architect, when he sought blocks for his pyramids. 
 The ease with which limestones could be carved, 
 combined with a reasonable resistance to decay, 
 
n] THE LIMESTONES 13 
 
 gave them a pre-eminence with the designers of 
 our rich cathedrals. The Romans found in the 
 stained and altered varieties colour-schemes for 
 basilicas and baths, and their luxurious taste in 
 limestone has been inherited by the modern builders 
 of hotels. 
 
 The rock suffers, however, from its solubility in 
 water containing even a mild acid. In the gases 
 dissolved by rain- water from the atmosphere, carbon 
 dioxide assumes a far larger proportion than that 
 which it possesses in the air itself. The surface of 
 limestone slabs becomes in consequence pitted and 
 corroded by every rain that falls. The sulphuric 
 acid in the air of modern coal-consuming cities is,- 
 however, still more deadly in its action. J. A. Howe, 
 in his recent work on building stones, is of opinion 
 that limestone is unsuitable for towns. Limestones 
 may broadly be recognised by their solubility in cold 
 dilute acids, with brisk evolution of carbon dioxide. 
 Dolomitic varieties require hot acid for brisk action. 
 
 Limestones divide themselves into types produced - 
 by chemical precipitation and those due to the 
 accumulation of the hard parts of organisms ; but in 
 many of the latter types chemical precipitation also 
 plays a part. Organic action, moreover, frequently 
 promotes the deposition of the chemical types. 
 Detrital limestones, that is, limestones formed 
 from the debris of older ones, are comparatively 
 
14 ROCKS AND THEIR ORIGINS [OH. 
 
 unimportant. They occur in certain zones of the 
 Chalk and of the Carboniferous Limestone in our 
 islands, and record the breaking up in shallow water 
 of beds that had already become consolidated. The 
 Miocene Nagelfluh conglomerates of the north side 
 of the Swiss Alps are often formed of pebbles of the 
 far older Mesozoic limestones. Similar conglomerates, 
 cemented by calcium carbonate, are now being formed 
 in the river-beds of the limestone karstland of 
 Hercegovina. Limestone, however, as a rule goes 
 to pieces before the buffetings sustained by mixed 
 rocks on a shore. Even if it survives for a time in 
 gravels, percolating waters ultimately dissolve it, and 
 only a porous skeleton, formed of its impurities, 
 remains. 
 
 LIMESTONES DEPOSITED FROM SOLUTION 
 
 Though calcium carbonate is far less soluble than 
 calcium sulphate, large quantities are carried invisibly, 
 owing to the presence of carbon dioxide, in river 
 waters, and thus accumulate in inland seas that have 
 no outlet except by evaporation. Here Calcareous 
 Tufa may be deposited as a crust upon the shores 
 and on the growing islets, as the water shrinks away, 
 and before the more soluble gypsum and rock-salt 
 can separate out. Hot springs of volcanic origin, 
 like the Sprudel of Karlsbad in Bohemia, may 
 deposit calcium carbonate as the water cools and 
 
n] THE LIMESTONES 15 
 
 is relieved from pressure. At Karlsbad, little grains 
 of granite, or of the minerals of granite, serve as 
 centres, and encrusting layers are formed round 
 them, until pea-like bodies are produced. These 
 become cemented together, giving rise to the well- 
 known freshwater pisolitic limestone or roestone. 
 
 On the shores of the Great Salt Lake of Utah, 
 calcareous tufa occurs also in the form of grains 
 resembling little eggs. These are the oolitic grains 
 that were first known as constituents of fossil lime- 
 stones. The calcium carbonate of oolitic grains at 
 Karlsbad, from the Great Salt Lake, and from the 
 sea, is deposited in a form that gives the reaction of 
 aragonite when boiled in cobalt nitrate. W. A. Herd- 
 man (Pres. Address Brit Assoc., 1920) has emphasised 
 the part played by the Bacillus calcis of C. H. Drew 
 in precipitating small lumps of calcium carbonate 
 from the destruction of nitrates and nitrites in sea- 
 water. 
 
 Travertine is a tufa laid down on twigs and other 
 vegetation, where springs emerge laden with calcium 
 carbonate. In a massive form, it builds tufa-basins, 
 as in the Mammoth Hot Springs of the Yellowstone 
 Park. Both here and at Karlsbad, it appears that 
 vegetation of humble type, multiplying under warm 
 conditions, materially assists the deposit by with- 
 drawing carbon dioxide from the water. The unstable 
 calcium bicarbonate is thus converted into the 
 
16 BOCKS AND THEIR ORIGINS [CH. 
 
 carbonate, which is thrown down as a quickly in- 
 creasing crust. 
 
 Among the limestone regions of the Dinaric Alps, 
 calcareous tufas or travertines, laid down by ordinary 
 streams, form massive beds that tend to choke the 
 hollows of the hills. The basin of Jajce in Bosnia is 
 thus partially filled up, and the town is built on 
 materials brought in solution from the mountains. 
 The modern waters are still adding to this deposit, 
 and Fr. Katzer(4) has pointed out that the falls of the 
 Pliva are prevented from cutting their way down to 
 the level of the Vrbas ravine, into which they plunge, 
 by the mass of tufa which they build up in their own 
 course. 
 
 Another type of limestone deposited from solution 
 is of considerable interest in arid lands, or lands with 
 onl}' a seasonal rainfall. Where evaporation goes on 
 steadily at the surface, while water is brought up by 
 capillary action from below, calcium carbonate may 
 * form a cement to the soil, or to the crumbling rock 
 near the surface, and a solid calc-tufa may arise by 
 continued transference of matter in solution from 
 lower levels. In the Cape of Good Hope such 
 formations are conspicuous(5). 
 
 In a careful series of experiments, G. Linck(6) 
 showed in 1903 that sea-water at 17 C. can only 
 hold '0191 per cent, of calcium carbonate in solution. 
 Though this quantity is not realised in the open 
 
n] THE LIMESTONES 17 
 
 ocean, yet near shores rivers may bring down an 
 excess. The Thames, though flowing for a long < 
 distance over a limestone area, contains only *0116 j 
 per cent, of calcium carbonate ; but springs traversing ' 
 limestone often carry *03 per cent, or ten times as / 
 much as that found in ordinary seas. Hence a; 
 precipitation of calcium carbonate from the bi-' 
 carbonate state may take place not far from land. 
 The mineral deposited is calcite in temperate climated 
 and aragonite under warm tropical conditions. That 
 such a precipitation actually occurs is proved by the 
 massive grey limestones, containing modern shells, 
 which have been recorded for our islands from the 
 sea-floor off the Isle of Man and off the coast of Mayo. 
 In the case of the Irish Channel, the excess of calcium 
 carbonate may be supplied by springs rising through 
 the glacial gravels, which contain abundant pebbles 
 of limestone. 
 
 Ammonium carbonate, again, derived from the 
 decay of organisms, or sodium carbonate, will pre- 
 cipitate calcium carbonate as aragonite from the 
 calcium sulphate and chloride, but not from the 
 calcium bicarbonate, of salt water. Films of aragonite 
 are at present accumulating by this process on the 
 floor of the Black Sea, and marine oolitic grains, also 
 consisting of aragonite, are produced by the same 
 reaction. 
 
 In the case of oolitic grains, deposition is no doubt 
 
 c. 2 
 
18 ROCKS AND THEIR ORIGINS [CH. 
 
 helped by evaporation, since they seem to arise in 
 shallow waters. The Oolitic Limestones that have 
 ^proved so admirable as building stones, whether from 
 the quarries of Caen or Portland, are cemented repre- 
 sentatives of the loose deposits formed in modern 
 tropical seas. De la Beche long ago compared their 
 grains with those from West Indian coral-reefs. 
 These small egg-like bodies develop round fragments 
 J of foraminiferal and other shells, round the ossicles 
 of echinoderms, and round broken bits of coral. At 
 first they have the general form of the nucleus ; but, 
 as they are rolled by the waves during their growth, 
 they become more and more spheroidal as they 
 enlarge. Boring algae make tubular passages in them, 
 and these have led to the view that algse of thread- 
 like form actually originate oolitic structure. Doelter, 
 Linck, and others conclude, with much reason, that 
 the mode of deposition is inorganic. When the grains 
 are unusually large, they are often flattened and 
 irregular, as in the marine Pisolites or Pea-grits. 
 
 For building purposes, the fine-grained oolites 
 without large fossils are much sought after, since 
 they can be trimmed equally in any desired direction. 
 
 Before leaving the question of the inorganic de- 
 position of limestone, we may note that R. A. Daly (7) 
 has suggested that the pre-Cambrian and early Cam- 
 brian limestones were entirely products of chemical 
 precipitation. He believes that the continental 
 
ii] THE LIMESTONES 19 
 
 areas were at first relatively small, and that the 
 abundance of decaying soft-bodied organisms on 
 the sea-floor led to a continuous precipitation of such 
 calcium carbonate as was available. Hence the ocean 
 was limeless, and it was only when continental land 
 became more extended that a sufficient quantity of 
 lime salts was brought in by rivers to counterbalance 
 that thrown down by ammonium carbonate and 
 sodium carbonate on the sea-floor. Daly urges that, 
 on this account, the earlier organisms could not form 
 calcareous shells or skeletons, and he also believes 
 that pre-Cambrian and Cambrian limestones, even 
 when unaltered, show no signs of having originated 
 from fragmental organic remains. Linck's researches 
 (p. 17) show that limestones thus precipitated must 
 have originally consisted of aragonite. 
 
 LIMESTONES FORMED OF ORGANIC REMAINS 
 
 These limestones present an immense variety, 
 according to the nature of the originating organisms, 
 and the amount of foreign material brought down 
 into the water where they accumulated. The cal- 
 careous remains of Chara may form a white deposit 
 on the floors of freshwater lakes. The part played 
 by calcareous algse in the formation of marine lime- 
 stones has long been recognised ; but the detailed 
 exploration in 1904 of the atoll of Funafuti in the 
 Pacific showed that Halimeda may be responsible for 
 
20 ROCKS AND THEIR ORIGINS [OH. 
 
 a considerable portion of an ordinary "coral-reef." 
 Lithothamnium occurs in immense quantities, associ- 
 ated with molluscan remains, near many shores. No- 
 dules of limestone (Cryptozoon &c.) have been ascribed 
 to algee in early Paleozoic and even Huronian strata 
 Animal, not vegetable, activity, however, is re- 
 sponsible for the majority of our limestones, and the 
 humbler organisms, by reason of their abundance, 
 play a prominent part in rock-formation. Analogies 
 between the Globigerina-ooze of deep waters and the 
 groundwork of the soft white limestone known as 
 Chalk have been freely pointed out. Early in the 
 nineteenth century, Ehrenberg, in a series of re- 
 searches with the microscope, proved the organic 
 origin of the compact ground of marine limestones. 
 The occurrence of foraminifera from the shore out- 
 wards to truly oceanic waters provides a fine-grained 
 calcareous material which forms deposits at very 
 various depths. The milioline types, often with a 
 surface like that of glazed porcelain, are common in 
 the sandy beds formed near a coast. Few rocks are 
 more fascinating under the microscope than those in 
 which such types are seen in section, associated with 
 detrital grains of quartz, washed down from the land, 
 and perhaps with bright green grains of the marine 
 mineral, glauconite. In Ireland white chalks occur, 
 speckled throughout with glauconite, which looks 
 dark in the rock-mass, but which reveals its green 
 

 nj THE LIMESTONES 21 
 
 tint when streaked out by the hammer. When 
 formed still farther from land, pure chalk arises from 
 the consolidation of foraminiferal ooze, and the 
 probable depth in which it accumulated must be 
 judged from the nature of the associated organisms. 
 A white limestone may, however, arise in a compara- 
 tively shallow sea, where the rivers bring down b'ttle 
 solid matter from the land. A coast formed of pure 
 limestone, with clear streams flowing from a land of 
 similar rock behind, may allow of the development of 
 pure limestone on its shores. It is generally agreed 
 that the Upper Chalk of the British Isles and of 
 northern France was laid down in water one thousand 
 fathoms or more in depth ; yet the corresponding 
 white limestone of northern Ireland in places follows 
 rapidly on conglomeratic and glauconitic deposits, 
 and seems to owe its purity to the comparative 
 absence of rain and rivers on the highland of crystal- 
 line rocks which stretched westward from its shore. 
 
 There are two epochs of the earth's history in 
 which foraminifera were remarkable for their size as 
 well as their abundance. The first gave us the grey 
 Fusulina limestone of Upper Carboniferous times, 
 when this spindle-shaped shell spread freely from the 
 United States through the arctic regions to the east 
 of Asia. The second gave us, in the Eocene period, 
 the great beds formed of Nummulites and allied forms, 
 which we meet with in Europe on the Lake of Thun, 
 
22 ROCKS AND THEIR ORIGINS [OH. 
 
 but which are far more important in Lower Egypt. 
 The disc-like forms of the numrmilites in the white 
 limestone of the Pyramids are familiar to hundreds 
 of travellers, and forms are recorded up to four and 
 a half inches across. 
 
 The foraminiferal origin of many compact lime- 
 stones can often be appreciated on smooth surfaces 
 with a pocket-lens. The older examples have 
 commonly become stained and darkened, and 
 crystallisation of calcite throughout the ground 
 has in part destroyed the original organic struc- 
 tures. This tendency to crystallise affects even the 
 larger fossils, and brachiopods and molluscs have 
 sometimes disappeared from our Carboniferous lime- 
 stones, without the intervention of " metamorphic " 
 heat or pressure.' In most limestones older than the 
 Eocene period, the shells and other fossils, such as 
 corals, that were originally formed of aragonite have 
 passed into the calcite state, without the destruction 
 of their characteristic shapes. Shells, however, have 
 been found still preserved as aragonite in beds as old 
 as the Jurassic periodfe). 
 
 The lamellibranchs, the ordinary bivalves, came 
 into prominence as limestone-builders with the 
 Carboniferous period, and are now rivalled by the 
 univalve gastropods, which displayed no widespread 
 activity until Eocene times. The most massive exist- 
 ing shell, however, is a lamellibranch, the giant 
 
n] THE LIMESTONES 23 
 
 Tridacna of Australian seas, a single valve of which 
 may weigh 250 Ibs. The cephalopods, though lying 
 far nearer to the crown of molluscan development, 
 became important from the Silurian Orthoceras on- 
 wards, and nautiloids of various forms are common 
 fossils in the Carboniferous limestone. Their large 
 size attracts attention from our present point of view. 
 The cephalopods, however, swell the bulk of many 
 limestones, not by the thickness of their shells, but 
 through their chambered character, which has pre- 
 vented complete infilling of the shell, and which thus 
 allows of cavities in the mass. 
 
 This is notably the case with the ammonites, which 
 contribute so largely to Jurassic limestones. Crystal- 
 line calcite has often been deposited by infiltration 
 on the septa and on the inner layer of the shell, thus 
 reducing the hollow spaces. The massive calcite 
 guards of the belemnites form a considerable part 
 of many limestones. 
 
 Even freshwater lakes possess molluscan deposits, 
 producing a white limestone of their own. Where 
 streams flow over pure pre-existing limestone, there 
 is no alluvial mud to choke the basins. In the hard 
 lake-waters, gastropods such as Limnsea and Planorbis, 
 and a few bivalves, can then flourish freely, and a 
 "shell-marl" accumulates at the bottom, unmixed 
 with sediment. Limestone of this type is con- 
 spicuous in hollows in the Dinaric Alps, which were 
 
24 ROCKS AND THEIR ORIGINS [OH. 
 
 once occupied by lakes, and is often found beneath 
 peat in the limestone lowland of central Ireland. 
 
 In older days, two groups of organisms, now 
 relatively unimportant, had a powerful place. The 
 brachiopods, including in early Palaeozoic times an 
 interesting series of thin shells largely composed of 
 calcium phosphate, were for long the predominant 
 shell-bearing organisms. The stout Spiriferidse and 
 the well-known Productus gigantem of the Carboni- 
 ferous period illustrate their dominance. The group 
 became much restricted in variety in Jurassic times ; 
 but even then Terebratula and Rhynchonella occurred 
 so abundantly that they now fall out of many rock- 
 faces like pebbles from a loose conglomerate. 
 
 The sea-lilies have similarly lost their place as 
 limestone-builders, though their "ossicles," notably 
 from their stems, furnish crinoidal or "encrinital" 
 masses from Silurian to Carboniferous times. The 
 broken portions of their stems, resembling tubes of 
 tobacco-pipes, are conspicuous when they are weathered 
 out on rock-surfaces or revealed in polished slabs of 
 marble. The fact that each joint or ossicle, as is the 
 universal case in the echinodermata, consists of a 
 single crystal of calcite causes the fragments to break 
 with the characteristic cleavage of that mineral. The 
 smooth glancing surfaces thus seen on fractured 
 specimens readily call attention to them in a rock. 
 
 Those humble colonial organisms, the compound 
 
n] THE LIMESTONES 25 
 
 corals, have so special a place as limestone-formers 
 that they have been reserved for more detailed treat- 
 ment. The accumulation of their skeletons, and the 
 fact that they may form large continuous masses by 
 their very mode of growth, promotes the formation 
 of solid rock at an unusual rate. Von Richthofen 
 long ago pointed out how foraminifera and other 
 drifted material became caught in the interstices of 
 coral, producing even a stratified structure in the 
 hollows of a reef; and subsequent research has shown 
 the composite character of reefs in various portions 
 of the tropic seas. Calcareous algse, as already re- 
 marked, and the massive and often encrusting 
 skeletons of hydrozoa, such as Millepora, are freely 
 associated with the products of true corals. 
 
 Charles Darwin, in his famous theory of the for- 
 mation of atolls and barrier-reefs, showed how, in 
 a subsiding area, corals might keep pace with the 
 downward movement. Hence reefs might arise of 
 great vertical thickness, although the polypes them- 
 selves could flourish only in the upper twenty fathoms 
 or so of water. This conclusion, which appears 
 strictly logical, has met with much opposition from 
 Karl Semper, Alexander Agassiz, and Sir John 
 Murray. Murray in particular urges the importance 
 of banks of calcareous organisms in building up 
 platforms on which corals may ultimately dwell. 
 The extension of reefs outward into deep water has 
 
26 ROCKS AND THEIR ORIGINS [CH. 
 
 been attributed to the rolling down of wave-worn 
 coral debris over submarine mountain-slopes. From 
 this point of view, an apparently thick atoll may be 
 formed as a comparatively thin mass of limestone 
 at the summit of a volcanic cone that fails to reach 
 the sea-level. 
 
 The opponents of the view that thick coral- 
 limestones are formed at the present day in the 
 Pacific have been unwilling to accept the results even 
 of the deep boring in the atoll of Funafuti^), which 
 penetrated materials like those of the superficial 
 layers of the reef to a depth of 1114 feet. They have 
 also refused to see in the huge dolomitic rocks of 
 Tyrol the remains of Triassic reefs four thousand feet 
 in thickness. None the less, most geologists regard 
 the Funafuti boring as a strong support for Darwin's 
 contention ; but recent research does not support his 
 view of a fairly uniform subsidence of the oceanic floor. 
 Atolls and barrier-reefs are now shown to have arisen in 
 very many cases on the seaward edges of rock-platforms 
 planed by marine erosion ; but it must be allowed that 
 the sinking and warping of these platforms has caused 
 reefs to increase considerably in thickness. The same 
 problem arises wherever shore-deposits have accumu- 
 lated to an exceptional thickness. Darwin, at the 
 end of Chapter V of his "Structure and distribu- 
 tion of Coral-Reefs," gives a vivid account of the 
 features that would appear in a section of an atoll 
 
H] THE LIMESTONES 27 
 
 that has grown large through subsidence of its 
 inorganic floor, and he emphasises the occurrence 
 of conglomerates of broken coral-rock on the outer 
 zone. The stratification of material by wave-action 
 in this zone, and the horizontal deposition of finer 
 material in the lagoon, would give to the dissected 
 mass a general sedimentary aspect. Darwin con- 
 cluded that the ring of solid coral, the true reef, 
 might be denuded away during an epoch of elevation, 
 and that only stratified portions might remain. He 
 does not seem to have discussed the contemporaneous 
 deposition of pelagic material from foraminiferal and 
 other sources against the outer surface of the reef 
 whereby an interlocking of two facies of limestone 
 might arise. 
 
 These features, together with those predicted by 
 Darwin, have been recognised by von Richthofen and 
 Mojsisovics in the Tyrol dolomites, and have afforded 
 Austrian geologists good evidence that large parts of 
 these limestones originated as coral-reefs. Faulting, 
 however, has undoubtedly taken place in this region, 
 producing here and there a subsidence of the lime- 
 stone blocks among the surrounding more normal 
 sediments. Rothpletz, Ogilvie Gordon do), and other 
 critics of von Richthofen's view have seen in this 
 faulting the cause of the abrupt change from a facies 
 of massive dolomite to one of normal sedimentation 
 on the same horizontal level. They have also urged 
 
28 ROCKS AND THEIR ORIGINS [OH. 
 
 that shell-banks may accumulate locally so as to 
 simulate reefs by their contrast with their surround- 
 ings, while the change to dolomite has obliterated 
 their original features (see p. 30). It cannot be 
 denied, however, that coral-reefs and their associated 
 detrital deposits must exercise a very important 
 influence in the formation of solid limestone. 
 
 Even small knots and local groups of compound 
 corals are seen in ordinary limestones to serve as 
 a mesh in which other organic remains have become 
 entrapped. The ease with which the aragonite of 
 their skeletons becomes silicified causes them often 
 to stand out on weathered surfaces with all the 
 delicacy of structure displayed upon a modern reef. 
 
 Where limestones and shales are associated 
 together, a "knoll structure" may be found, the 
 limestone occurring in masses of a somewhat hemi- 
 spherical form, with the shales fitted against and 
 round them. In some cases this may be due to the 
 local distribution of patches of growing coral on the 
 old sea-floor; but in other cases the structure has 
 arisen from compression and brecciation of the strata, 
 the original beds of limestone becoming broken up 
 and the more yielding beds flowing round them. 
 This structure is well seen on a small scale in many 
 "crush-conglomerates," where the limestone appears 
 as knots and eyes, resembling pebbles. Yet near 
 at hand the true bedding may be traced, bands of 
 
ii] THE LIMESTONES 29 
 
 limestone alternating with shale, and a few cross- 
 joints indicating the possibility of a separation of 
 the limestone into blocks. These blocks become 
 rounded in the general rock-flow ; but Gardiner and 
 Reynolds (11) suggest solution by infiltering water as 
 an explanation of certain remarkable examples 
 studied by them. 
 
 ALTERED FORMS OF MASSIVE LIMESTONE 
 
 Magnesium carbonate is introduced by many 
 organisms as a primary constituent of limestone dibit). 
 A foraminifer, Nubecularia novorossica, has 26 per 
 cent, of magnesium carbonate in its shell, and alcyo- 
 narian skeletons may include 157 per cent., crinoid 
 ossicles 13 per cent., and the hard parts of calcareous 
 algse 12 per cent. (12). F. W. Clarke and W. C. Wheeler 
 (U.S. Geol. Surv., Prof. Paper 102, 1917) have gone 
 over the whole field of marine organisms, and indicate 
 an increase in magnesium carbonate under tropical 
 conditions. The matter is of interest in connexion 
 with the common occurrence of dolomitic lime- 
 stones. 
 
 Dolomite, the joint carbonate, CaMg(C0 3 )2, con- 
 tains 54*35 per cent of calcium carbonate and 45*65 
 per cent, of magnesium carbonate, or carbon' dioxide 
 47*8, lime 30*4, and magnesia 21*8. Its specific gravity 
 is 2'85. 
 
30 ROCKS AND THEIR ORIGINS [CH. 
 
 The occurrence of dolomite in intimate association 
 with calcite has been proved by E. W. Skeatsda) in 
 the case of modern coral-reefs, and the secondary 
 deposition of the mineral has been made clear. The 
 skeletons of the corals themselves may now consist 
 of dolomite, while calcite has crystallised in their 
 interstices, or remains as part of the original infilling 
 of mud. The presence of dolomite in reefs has, of 
 course, long been known, having been observed by 
 J. D. Dana in 1849, and it has been realised that, 
 by prolonged alteration, masses of Dolomite Rock 
 become built up (14). 
 
 Commonly, the process produces a Dolomitic 
 Limestone, in which calcium carbonate is still in 
 excess of the 54 per cent, which is present in the 
 mineral dolomite. 
 
 The alteration of the original limestone is, how- 
 ever, sufficiently profound. The ready crystallisation 
 of dolomite as rhombohedra destroys the organic 
 structure, and traces of corals or molluscan shells 
 disappear from great thicknesses of rock. It is 
 uncertain whether the process of dolomitisation 
 proceeds most rapidly in the evaporating waters of 
 the lagoons, or, as Pfaff believes, at considerable 
 depths, where the pressure may reach 100 atmo- 
 spheres. Magnesium carbonate, as we shall note 
 later, may be removed from dolomite in solution 
 under pressure at a greater rate than calcium 
 
[i] THE LIMESTONES 31 
 
 carbonate. If this occurs in sea-water, it would 
 seem to militate against the production of dolomite 
 in the lower levels of a reef. 
 
 The magnesium required for dolomitisation is 
 derived from the magnesium sulphate and chloride 
 of sea- water, calcium being removed during the change. 
 C. Element in particular urges that a concentrated 
 solution of sodium chloride at 60 C. assists the process 
 in the case of magnesium sulphate. Aragonite is more 
 susceptible than calcite. Element's temperature of 
 120F. can rarely be realised in lagoons or between tide- 
 marks ; but R C. Wallace(C. R Internat Geol. Congress, 
 1913, 879) suggests that dolomite is deposited after suf- 
 ficient calcite has come down and has thus caused greater 
 concentration of magnesium ions in the sea-water. 
 
 The intimate structure of modern dolomitic lime- 
 stone, as exhibited in coral-reefs, satisfies us that 
 many older or fossil dolomites were formed from 
 marine calcareous deposits while these were still 
 accumulating. In other cases we must admit that 
 the dolomite has developed in the neighbourhood of 
 joints after the consolidation of the rock. The view 
 that dolomitisation results from the mere removal of 
 calcium, the magnesium originally present in organic 
 skeletons becoming thus more concentrated, is not 
 borne out by recent observations. 
 
 Skeatsd5)has carefully compared the dolomite-rocks 
 of Tyrol with the materials of recent coral-reefs. In 
 
32 ROCKS AND THEIR ORIGINS [OH. 
 
 both there is a striking absence of detritus of inorganic 
 origin, and his work goes far to show that the much- 
 discussed Alpine dolomites were formed under condi- 
 tions which occur in the neighbourhood of existing 
 reefs. This, however, does not solve the question as 
 to whether we are dealing in Tyrol with fossil coral-reefs, 
 or with the calcareous type of ordinary marine sedi- 
 ments, which might undergo the same kind of 
 alteration. While Skeats finds in two dolomites from 
 recent reefs 43 per cent, of magnesium carbonate, 
 the substitution seems usually to terminate when 
 40 per cent, has been introduced. In Tyrol, however, 
 the process has gone so far as to give rise to true 
 dolomites, with 45*65 of magnesium carbonate. 
 
 The dolomites of the Jurassic series in north 
 Bavaria are massive rocks almost devoid of fossils, 
 traversed by shrinkage cracks, and associated with 
 richly fossiliferous stratified limestones. The relations 
 of these two types of rock are those of coral-reefs to 
 the bedded deposits on their flanks, and the dolomite 
 seems to merge horizontally into the stratified series. 
 As in Tyrol, fossils and corals are rare in the bosses 
 of dolomite, but the structural evidence is strongly 
 in favour of their having originated as steeply sided 
 reefs. 
 
 The dolomitic facies of the Carboniferous limestone 
 in our islands is an example of the second type of 
 origin, The dolomite here frequently occurs in 
 
n] THE LIMESTONES 33 
 
 irregular veins and patches. The introduction of 
 iron carbonate with the magnesium salt stains the 
 dolomite brown on exposure to oxidation, and its 
 limits are thus clearly seen in the general blue-grey 
 mass. The dolomitisation has evidently proceeded 
 from joint-surfaces inwards. It is often sufficiently 
 thorough to obliterate all traces of fossils, and the 
 shrinkage accompanying the chemical change has 
 produced numerous cavities, in which calcite has 
 subsequently crystallised. An expansion takes place 
 when aragonite is altered into dolomite, unless more 
 of the calcium carbonate is removed than is necessary 
 to give place to the magnesium carbonate introduced. 
 In the change from calcite, with a density of 272, to 
 dolomite, with a density of 2'85, there is, on the other 
 hand, a shrinkage of 4'54 per cent. Where the altera- 
 tion, then, takes place while the aragonite organisms 
 still remain as aragonite, and not as calcite, an expan- 
 sion rather than a contraction should occur in the 
 substance of a reef; but when an old limestone, in 
 which all the calcium carbonate is present as calcite, 
 becomes dolomitised, a considerable shrinkage will 
 occur, and rifts and hollows may remain obvious. 
 
 Very few dolomites, except those found in associa- 
 tion with rock-salt and other products of the evapora- 
 tion of lagoons, can now be attributed to direct 
 chemical deposition from the sea. 
 
 Daly (7) has argued that the first Palaeozoic and 
 
 a 
 
34 ROCKS AND THEIR ORIGINS [OH. 
 
 the pre-Cambrian dolomites were formed by precipita- 
 tion, since the calcium salts in those early days were 
 completely removed from the sea- water. Ammonium 
 carbonate, though effective in precipitating the 
 calcium salts, does not act on those of magnesium 
 until the calcium salts have been brought down. 
 But, under the conditions postulated for the river- 
 waters that reached the sea from the earliest con- 
 tinental lands, conditions involving the presence of 
 only small quantities of salts of calcium, the decay of 
 organisms on the sea-floor might lead to a deposition 
 of all the magnesium salts, following on those of 
 calcium, both coming down in the form of carbonates. 
 
 The experimental work of Pfaffde) should be 
 considered in connexion with Daly's suggestions, 
 since means are there indicated whereby basic 
 magnesium carbonate, precipitated from sea-water, 
 may associate itself with calcium carbonate to form 
 dolomite; shallow-water conditions, with concentra- 
 tion by evaporation, are required. 
 
 Daly compares analyses of river-waters now 
 running over pre-Cambrian rocks with analyses of 
 pre-Cambrian limestones, and the ratio of the carbon- 
 ates of magnesium and calcium is shown to be the 
 same in both series. 
 
 From what we have said, it now seems probable 
 that the great majority of dolomitic limestones owe 
 their magnesium to substitution from without. Direct 
 
II] THE LIMESTONES 35 
 
 precipitation of dolomite has, however, been invoked 
 to account for several cases of Permian age, such as 
 the Magnesian Limestone of the county of Durham. 
 Near Sunderland, this rock is greatly modified, 
 containing ball-like and other concretions, associated 
 with frequent cavities. Traces of the original bedding 
 remain, running through the concretions, and marine 
 fossils are abundant. Conybeare and Phillips, so far 
 back as 1822, stated that the nodules were devoid of 
 magnesia, though formed in a magnesian rock. In 
 spite of this, these objects long appeared as dolomite 
 in collections. E. J. Garwoodo?) showed conclusively 
 that they resulted from the concentration of calcium 
 carbonate in a concretionary form. Water containing 
 calcium sulphate after passing through a dolomite 
 is found to carry magnesium sulphate by a chemical 
 exchange. Skeatsds), moreover, points out that, under 
 a pressure of five atmospheres the magnesium car- 
 bonate of dolomite becomes more soluble than the 
 calcium carbonate in fresh water containing carbon 
 dioxide. 
 
 Dolomitic limestones may thus revert towards 
 more normal types ; but such cases are exceptional. 
 Occasionally the magnesium is worked up from the 
 carbonate into other forms, under the influence of 
 contact action from igneous masses. The term " de- 
 dolomitisation," used by Tealld9), is properly restricted 
 to such cases, where dolomite may separate into 
 
 32 
 
36 ROCKS AND THEIR ORIGINS [CH. 
 
 calcium carbonate, magnesium oxide, and carbon 
 dioxide. The magnesium oxide takes up water and 
 yields the flaky colourless mineral brucite. Where 
 silica is present, either as an impurity in the dolomite, 
 or introduced from an invading siliceous magma, 
 magnesium and calcium silicates may be built upd9). 
 Olivine thus arises, and, on becoming hydrated and 
 passing into serpentine, stains the rock in various 
 shades of green. The calcium carbonate crystallises 
 as a ground of granular calcite, and the whole mass 
 becomes a handsome Ophicaltite, or serpentinous 
 marble. The famous rock of Connemara, used in 
 polished slabs, has arisen in all probability in con- 
 nexion with igneous action. 
 
 Dolomitic limestones are liable to decay rapidly 
 in towns, owing to the formation of magnesium 
 sulphate, which, as shown above, is even more soluble 
 in water than is the accompanying calcium sulphate. 
 In the country, the crystals of dolomite resist ordinary 
 weathering by the carbon dioxide of the rain-water 
 better than those of calcite ; and the rock thus 
 becomes loosened through the loss of one constituent, 
 and crumbles into a dolomite sand (20). Compact 
 dolomites, however, have furnished some excellent 
 building-stones for country use, since here the more 
 resisting mineral forms the bulk of the rock. 
 
 The Phosphatic Limestones are commercially 
 even more important. Tricalcium orthophosphate, 
 
n] THE LIMESTONES 37 
 
 derived, perhaps, in the first instance from the decay 
 of bones of fishes and the excreta known as coprolites, 
 tends to become aggregated in certain limestones, as 
 in the chalk of Mons in Belgium and of Taplow in 
 Buckinghamshire. The phosphate replaces fora- 
 miniferal and other shells, and frequently forms 
 internal casts of fossils. In the latter case, it has 
 replaced the calcareous mud that first occupied the 
 shells. The observations of the "Challenger 7 ' 
 expedition show that concretionary calcium phosphate 
 is forming among the calcareous and glauconitic 
 oozes of existing oceans, nodular masses collecting, 
 in which foraminiferal shells are united and even 
 replaced by calcium phosphate. Where deposits of 
 guano are formed by sea-birds on surfaces of coral 
 limestone, as at Christmas Island to the south of Java 
 and at Sombrero in the Windward Islands, calcium 
 phosphate becomes washed downwards and replaces 
 part of the calcium carbonate of the rock. The 
 resulting phosphatic limestone is quarried on a 
 commercial scale, and the very existence of Christmas 
 Island is said to be threatened by the energy of 
 excavators. The "phosphorites du Quercy," well 
 known to agriculturists in France, are accumulations 
 in hollows and fissures of Jurassic limestone, and are 
 associated with the bones of fossil mammals. But in 
 this and in other cases there is much doubt as to 
 whether the phosphate is derived from the bones, or 
 
38 ROCKS AND THEIR ORIGINS [CH. 
 
 is locally concentrated, with other impurities, such 
 as sand and clay, through solution of the adjacent 
 limestone. 
 
 The most common substance that replaces calcium 
 carbonate in limestones is silica, in the form of Flint. 
 The nodules of this material, white on the outside 
 and richly black within, mark bands of stratification 
 in the Cretaceous chalk, and are among the best 
 known materials in south-east England. Their 
 fantastic forms have given rise to many speculations. 
 Sometimes, however, when fractured, they are clearly 
 seen to include the remains of fossil sponges. The 
 sponges may be represented merely as hollow casts ; 
 but there is abundant evidence in other cases that 
 they belong to genera which secreted skeletons of 
 amorphous (non-crystalline) silica during life. 
 
 The nodular flint has collected round the sponge, 
 while the sponge itself has often disappeared. 
 G. J. Hinde(2i) has shown how readily the spicules of 
 siliceous sponges go into solution. Even at the 
 bottom of existing seas they become rounded at the 
 ends, while their canals become enlarged. In some 
 fossil instances, they are replaced by calcite. 
 W. J. Sollas(22), emphasising this point, remarks that 
 "it may be taken as an almost invariable rule that 
 the replacement of organic silica by calcite is always 
 accompanied by a subsequent deposition of the 
 silica in some form or other." The occurrence of 
 
n] THE LIMESTONES 39 
 
 flints in chalk along parallel zones is no doubt a case 
 of the rhythmic deposits studied by R. Liesegang in 
 his "Geologische Difftisionen" (1916). 
 
 The pocket-lens will often show traces of sponge- 
 spicules, as dull little rods, in the translucent sub- 
 stance of a flint. But the microscope shows that the 
 mass of the flint has the structure of the limestone 
 in which it lies. The foraminifera and other small 
 structural features of the original rock are perfectly 
 preserved in chalcedonic (that is, minutely crystalline) 
 silica. Larger fossils, such as thick molluscan shells 
 and the tests of sea-urchins, may escape alteration, 
 while the chalk mud, the original ooze, with which 
 they are infilled has become completely silicified. 
 This explains the internal moulds of fossils in brown 
 oxidised flint that are found in gravel-pits on the 
 surface of the Chalk, and also the tubular hollows, 
 representing stems of crinoids, that often occur in 
 flint from the Carboniferous Limestone. In the latter 
 case, the fossil remained calcareous while the ground 
 became silicified, and the fossil was removed by 
 subsequent solution. 
 
 Where great thicknesses of strata, as may happen 
 in the Carboniferous Limestone, have become thus 
 silicified, it may be presumed that siliceous skeletons 
 were unusually abundant in the mass. But, as 
 L. Cayeux(23> observes, such skeletons may be in one 
 case entirely removed, and in another represented by 
 
40 ROCKS AND THEIR ORIGINS [CH. 
 
 massive flints ; in yet another case, the silica may 
 remain disseminated through the rock. The irregu- 
 larity of its segregation is shown by the growth of 
 flints in branching or hook-like forms, running from 
 one bed to another in a limestone. 
 
 Oolitic limestones and the skeletons of corals, 
 both having been originally made of aragonite, are 
 often replaced by flint, forming conclusive instances, 
 appreciable by the naked eye, of the secondary origin 
 of this form of silica. Traces of diatoms are com- 
 paratively rare, though they probably contributed to 
 the silicification of the freshwater Calcaire de la Brie 
 of the Paris basin. Radiolaria, however, have now 
 been well recognised as flint-formers, even in dark 
 "cherts" of Silurian age. Radiolarian cherts have 
 been taken as an indication that the beds in which 
 they occur were formed in oceanic depths. 
 
 It is difficult to determine the stage in the history 
 of a rock at which silicification has set in. As 
 A. Jukes-Browne (24) remarks, solution of the silica 
 skeletons may be accelerated by pressure, i.e. by the 
 depth of water in which the bed accumulated. Yet, 
 in comparison with the calcareous shells of foramini- 
 fera, radiolarian and diatomaceous remains are only 
 slowly soluble, and are found in the deepest spots 
 reached by soundings. H. B. Guppytes), on the 
 other hand, has observed silicification of modern 
 corals in reefs in the Fijis, and believes that the 
 
ii] THE LIMESTONES 41 
 
 process went on during the elevation of the area, 
 when waters containing silica became concentrated, 
 and parts of the mass were exposed to evaporation. 
 
 The instability of the non -crystalline siliceous 
 skeletons in geological time makes it probable that 
 a rock cannot long retain them when buried among 
 other strata in the earth. 
 
 It is clear that there is no support for the view, 
 current from the time of James Hutton onwards, that 
 nodular flints are formed by matter in hot solutions 
 entering pre-existing cavities in limestone rocks. 
 But there must be cases where the silicification of 
 limestone has arisen through its penetration by hot 
 springs. The presence of tabular flint in joints of 
 the Chalk shows that water has imported silica along 
 easy lines of passage from some other portion of the 
 rock. Just as stems of trees become replaced by 
 chalcedonic silica, so may beds of limestone be 
 converted into flint, especially in volcanic areas. 
 A. W. Rogers(26) records that recent limestones formed 
 in the Cape province by the evaporation of ascending 
 waters have already become silicified. These flinty 
 rocks have been found in the Kalahari Desert and 
 elsewhere, though not south of the Orange River; 
 the chemical change is probably due to the character 
 of local water rather than to temperature. Yet 
 it is remarkable how, in the vast majority of in- 
 stances, the partial or complete silicification of a 
 
42 ROCKS AND THEIR ORIGINS [CH. 
 
 limestone may be traced to an intermediate resting 
 stage of the silica in the form of skeletons of 
 the vegetable diatoms or the animal sponges or 
 radiolarians. 
 
 The decay of flint itself, by the removal of part of 
 its substance in solution, is the cause of the white 
 surface on specimens from the Chalk, and of the 
 crumbling white residues found in certain gravels. 
 This process has been fully discussed by J. W. Judd, 
 who believes that the material removed is silica in 
 the opaline condition (27). 
 
 LIMESTONE AND SCENERY 
 
 Limestones in the field are characterised by joints 
 which traverse considerable thicknesses of strata, 
 until some shaly bed is met with, in which earth- 
 stresses cannot set up such continuous planes of 
 fracture. Since the conditions of deposition may 
 remain constant for a long time in open seas, and 
 since stratification cannot be obvious until these 
 conditions change, limestones may have a massive 
 character that is exceptional among sedimentary 
 rocks. In some cases, however, where muddy rivers 
 in times of flood have brought in detritus from the 
 land, rapid and no doubt seasonal alternations of shale 
 and limestone may be observed. 
 
 The Chalk of north-western Europe remains 
 
ii] THE LIMESTONES 43 
 
 typically soft, lending itself to cliff-formation along 
 the coast, where landslides are frequent through 
 undercutting from below. Were it not for the 
 development of Hints along stratification-planes, it 
 would be impossible at a distance to detect any 
 bedded structure in the rock. Its representatives in 
 eastern France, in the north zone of the Alps, or in 
 the central Apennines, are compressed into far more 
 resisting masses, and rear themselves as terraced 
 crags and sheer rock-walls, in which the structure 
 due to vertical joints is paramount. The English 
 Chalk weathers into round-backed downs, clothed 
 with thin grass, and hollowed into combes by streams 
 that have long ago run dry. The soil owes hardly 
 anything but its abundant flints to the white lime- 
 stone rock on which it lies. Residual clays and 
 sands derived from the breaking up of later beds 
 allow of cultivation here and there, and beechwoods 
 flourish even on the crests of the high downs. But 
 water sinks freely into the ground, and may so far 
 saturate the mass as to appear again in wet seasons 
 in hollows of the surface as temporary springs or 
 " bournes." When deep wells are sunk and pumping 
 is begun, it is found that the supply varies greatly in 
 different spots under seemingly uniform conditions. 
 Even in so permeable a mass, there are waterways 
 where maximum flow occurs. Channels where water 
 soaks in from above, or weak places in the roofs of 
 
44 BOCKS AND THEIR ORIGINS [CH. 
 
 underground watercourses, become marked at the 
 surface by sinkings known as swallow-holes. These 
 increase in size with time, and are abandoned to the 
 growth of scrub and trees. 
 
 Among more consolidated limestones, as we have 
 hinted, the joints are effective in promoting bold rock- 
 scenery. The absorptive power of the rock, rather 
 than its hardness, prevents it from being washed 
 away. Water that might round the edges of escarp- 
 ments and send down taluses to modify the slopes 
 sinks into the ground and works out passages by 
 solution. On level surfaces, the solubility of lime- 
 stone in water charged with carbon dioxide from the 
 atmosphere is apparent by the formation of pitted 
 hollows, with edges between them that grow sharper 
 until they are worn through. Where a rain-drop first 
 secures a resting-place, its successors deepen the 
 little hollow. Water lies in this after every shower, 
 working its way gently downwards. In time the rock 
 may seem bored into as if prepared for blasting ; the 
 holes unite to form vertical grooves, and the surface 
 is cut deeply into fantastic forms. 
 
 The face of the rock, formed by weathering on a 
 valley-side or towards the sea, or occurring on any 
 mass that is being cut back and reduced by denuda- 
 tion, is likely to be vertical, or at any rate perpen- 
 dicular to the bedding. The form of the surfaces 
 of the beds is perpetuated by their fairly uniform 
 
n] 
 
 THE LIMESTONES 
 
 45 
 
 lowering through solution. The result is that strati- 
 fication-surfaces and planes perpendicular to them 
 
 Fig. 1. SURFACE OP LIMESTONE PLATEAU. Causse du Larzac, 
 Aveyron, France. 
 
 control in a very marked degree the scenery of lime- 
 stone lands (Fig. 1). 
 
46 ROCKS AND THEIR ORIGINS [CH. 
 
 Where the beds are level, with occasional partings 
 of a slightly different composition, the country will 
 develop terraces, like those of the Barren in northern 
 Clare. Where they are folded, as in the Juras, scarps 
 and dip-slopes follow one another picturesquely, the 
 weathered edge of the bed, the true escarpment, being 
 sometimes at an angle as steep as that of the dip. 
 Hence a false effect of sharp peaks is produced, when 
 these "edges" are seen end on at a distance. 
 
 The terrace-structure may be seen in miniature 
 forms upon a rocky shore, where the blocks loosened 
 from the escarpments of the successive beds are 
 carried away by the waves. Frost-action is powerful 
 in larger instances, and sends down huge blocks upon 
 the lower terraces. A combination of shale bands 
 and massive limestones, especially with a dip out- 
 ward from the highland, leads to destructive land- 
 slips, since the sloping surface of shale is lubricated 
 by water that passes through the limestone (see 
 Fig. 9). Outward slips of the coast are thus common 
 in Antrim, and have been extensive near Axmouth, 
 two regions where chalk rests upon Liassic clays. 
 
 Broken ground, then, occurs freely under limestone 
 scarps, and the falling blocks often prevent the growth 
 of trees. The freshness of the rock-face above and of 
 the talus below calls attention to spots where denuda- 
 tion is most active. Differences in the constitution 
 of the beds are indicated by differences of the slope 
 
II] 
 
 THE LIMESTONES 
 
 47 
 
 formed by denudation on the rocky walls. The huge 
 canons of Arizona afford effective illustrations. 
 
 Fig. 2. RAVINE IN LIMESTONE. Canon of the Dourbie, 
 Aveyron, France. 
 
 These canons owe much of their character to the 
 presence of vertically jointed limestone. The small 
 
48 ROCKS AND THEIR ORIGINS [CH. 
 
 rainfall of the region has allowed the rivers to deepen 
 their channels ahead of the wearing back of the walls. 
 Yet even where valleys are widened by rain and other 
 atmospheric agents, those formed in limestone will 
 maintain the character of ravines. In the valley-sides 
 of Derbyshire, or of the Franconian plateau, or of the 
 Arve near Sallanches, where the crags rise a mile or 
 more above the stream, we see how canon-cutting is 
 assisted by the joints in limestone. The ravine of the 
 Dourbie, east of Millau in Aveyron, in the romantic 
 region of the Gausses, is a winding gorge two thousand 
 feet in depth (Fig. 2). That of the Tarn, a little to 
 the north, has only recently been penetrated by a 
 road, cut out for the most part in a vertical rock- wall. 
 When we observe, especially from the stream it- 
 self, the details of these sheer valley-sides excavated 
 in limestone, we again and again detect evidences of 
 solution. High above the present water-level, the 
 rocks are rounded, and are often undercut, so that 
 they overhang (Fig. 3). In Millersdale in Derbyshire, 
 above grass-grown taluses, the surface is still smooth 
 to the hand, and we can picture the water swirling 
 against it, and washing it away, as it does now in 
 the bottom of the grim ravines of Carniola. A large 
 number of limestone canons clearly represent under- 
 ground waterways, the roofs of which have fallen in. 
 This may be true of the fine gorge of Gheddar, and 
 in many cases is proved by the existence of rock- 
 arches bridging across the hollow of the stream. 
 
II] 
 
 THE LIMESTONES 
 
 49 
 
 The characters of an unmitigated limestone region 
 are best seen when we travel east of the Adriatic. 
 
 Fig. 3. WATERWORN CLIFF OF LIMESTONE. 
 Millersdale, Derbyshire. 
 
 Bavine of 
 
 Here what have been styled the karst landscapes 
 become prominent, and may be followed through the 
 
 c. 
 
50 ROCKS AND THEIR ORIGINS [OH. 
 
 Greek isles to the Levant. Something of the kind is 
 realised in the terraced lands between the Rh6ne and 
 the upper reaches of the Durance ; lavender bushes 
 form dull-green spots on almost barren hills, and 
 the grey walls of old stone-built towns are barely 
 distinguishable against equally grey hillsides. But 
 towards Trieste the limestone lands are barer still. 
 The small amount of insoluble matter yielded by the 
 rock may accumulate in swallow-holes, which are 
 here called " dolinas," a Slavonic word really meaning 
 valleys. This residue appears in the dolinas as a red 
 clayey earth, the " terra rossa " of the Italian-speaking 
 Dalmatian coast. But on the surface of the plateaus 
 it is washed or blown away as soon as it is extracted 
 from the limestone. A. Grund(28) has suggested that 
 the frequency of frost-action in more northern areas 
 allows surfaces of limestone to be cumbered with 
 loose blocks among which soil-patches may gather ; 
 hence we do not find karst-features on the plateaus 
 of central Bavaria, Champagne, or the Cotteswold 
 Hills. Something approaching to a karst appears in 
 the wind-swept levels of southern Galway and of 
 Clare, and exposure to strong winds has probably 
 a good deal to do with the origin of the Causses and 
 the Illyrian karstlands. At the same time, the amount 
 of impurity in the limestone must strongly influence 
 the resulting landscape. The noble woods in the 
 limestone hollows of southern Ireland are rendered 
 
II] 
 
 THE LIMESTONES 
 
 51 
 
 possible by the clay soils derived from the limestone, 
 as much as by the sheltered nature of the ground. 
 
 Fig. 4. LIMESTONE COUNTRY DISSECTED BY RAVINES. 
 Karstland of Hercegovina, from the Maklen Pass. 
 
 In typical karstlands, water sinks in, and emerges 
 again on low ground, where the surface-forms cut the 
 
 42 
 
52 ROCKS AND THEIR ORIGINS [CH. 
 
 level of the subterranean water-table. Streams that 
 manage to hold their own for a time on the uplands 
 often disappear into the clefts. Marshes may occur 
 in hollows, but may have no outlet, except in vertical 
 directions, upwards by evaporation and downwards 
 through the dolinas. The dolinas correspond, as the 
 Slavonic shepherds so aptly perceived, to the river- 
 valleys of more normal areas. The landscape of 
 flowing streams has to be sought for in a mysterious 
 underworld, of which we can gain only a few glimpses. 
 What we know is largely due to explorers of singular 
 enterprise and resource, notably E. A. Martel and 
 the " speleologists " whom he has inspired. 
 
 A view over the plateau of Hercegovina shows us 
 how deep gorges, rather than ordinary river- valleys, 
 are prevalent where important streams run across 
 a karstland (Fig. 4). The roads are carried, where 
 possible, along the ravines, and the country possesses 
 a double life, that of the broad uplands, where tanks 
 have to be made to preserve the water, and that along 
 the commercial highways, four or five thousand feet 
 below. Even beside the rivers there is a sense of 
 desolation in the barren whiteness of the rocks. 
 The sunlight strikes on the wall of some theatre of 
 the limestone, carved out in old times by a side-swirl 
 of the stream, and the hollow glares like a white 
 furnace in the hills. The river in summer shrinks 
 among broad stony reaches, to which thin-flanked 
 
n] THE LIMESTONES 53 
 
 sheep are driven for a scanty pasture. Its clear 
 green water gives no promise of alluvium for its 
 banks. Limestone, even in temperate Europe, may 
 create the features of a desert land. 
 
 The most extraordinary rock-scenery in Europe 
 is due to limestone in the dolomitic state. It is not 
 clear if the crags and pinnacles of Tyrol are caused 
 by the change from calcium carbonate into dolomite, 
 whereby a granular mass has arisen, weathering freely 
 along its vertical joints. It may well be that these 
 compact limestones have developed an exceptionally 
 jointed structure under earth-stresses, and that 
 faulting has intensified their tendency to break up 
 into fort-like blocks. Stratified masses of more 
 normal Rhsetic limestones often provide a terraced 
 structure near the mountain-crests ; but in thousands 
 of feet of underlying dolomite vertical clefts prevail 
 entirely over planes of bedding. If, as is extremely 
 probable, these dolomite-rocks arose from the com- 
 posite masses that we style coral-reefs, stratification 
 was none the less a marked feature as their limestone 
 grew in thickness. This structure is still plainly 
 visible ; but the joints have been widened, and the 
 mass is cut up into stupendous pinnacles and domi- 
 nating towers. The Drei Zinnen near Landro, the 
 deeply notched wall of the Langkofel and the 
 Plattkofel, rising four thousand feet above a grassy 
 upland of normal Lower Triassic strata, and the 
 
54 ROCKS AND THEIR ORIGINS [OH. 
 
 overhanging crests of the Sett Sass above Buchen- 
 stein, are types of a country where dolomite is 
 pre-eminent, and where the zone of steep rock- 
 weathering is marked by the most fantastic forms. 
 
 ON MARBLES 
 
 Any limestone the markings or colour of which 
 render it suitable for ornamental purposes passes as 
 a Marble. "Fossil marbles" are often mere grey 
 limestones, in which the stems of crinoids, or the 
 curved sections of shells, or the radiating patterns 
 due to corals, please the eye with their variety 
 on a polished surface. The Purbeck Marble that 
 was so much used as a grey foil to the massive 
 white columns of cathedrals throughout England is 
 simply a freshwater limestone, of no great merit 
 as a building stone, crowded with the shells of 
 Paludina. The black marbles are limestones coloured 
 by one or two per cent, of carbon, derived from the 
 decay of organisms, and white shells may stand out 
 in them conspicuously, in contrast with the ground. 
 The red marbles of Plymouth and of Cork have 
 become iron-stained, and at the same time secondary 
 crystallisation has destroyed many of their original 
 features. In Little Island, near Cork city, earth- 
 movements have crushed the mass, which in con- 
 sequence shows signs of solid flow. The breaking of 
 a crystalline limestone under such stresses furnishes 
 
n] THE LIMESTONES 65 
 
 us with many handsome marble Breccias. The abrupt 
 juxtaposition of angular masses of various colours, 
 torn from beds originally distinct, renders some of 
 these rocks almost too startling for the decoration 
 of rooms of moderate size. 
 
 There seems no such thing in nature as amorphous 
 carbonate- of lime, and all limestones are therefore 
 formed of crystalline particles ; but the further 
 crystallisation of this material produces a true 
 marble, in which all traces of fossils may be lost. 
 Heat and pressure underground probably facilitate 
 this change, since even soft chalk is converted by 
 igneous dykes into granular marble. But where 
 the pressure is accompanied by the possibility of 
 movement, the shearing action breaks down the 
 grains, and a more delicate structure results. 
 
 We have already seen (p. 35) how dolomite may 
 undergo striking mineral changes through advanced 
 metamorphic action. Lime-garnets, wollastonite, di- 
 opside, and other silicates similarly develop in 
 ordinary limestones exposed to the intrusion of an 
 igneous magma. The extreme changes in such rocks 
 will be described when amphibolites are dealt with. 
 
56 ROCKS AND THEIR ORIGINS [CH. 
 
 CHAPTER III 
 
 THE SANDSTONES 
 THE ORIGIN OF SANDS 
 
 THE essential characteristic of Sandstone is that 
 it consists mainly of detrital grains of quartz, or 
 occasionally of grains of chalcedonic silica (flint); 
 these are found to scratch the steel blade of a knife, 
 and are not affected by boiling in ordinary acids. 
 The grains usually become cleaner in the boiling 
 process, since the cement that has bound them 
 together is liable to be destroyed. This cement may 
 cause effervescence, being often formed of chemically 
 deposited calcium carbonate. 
 
 When we consider the distribution of quartz in 
 nature, we look to igneous and metamorphic rocks 
 for the origin of the grains in sandstone. Quartz is 
 one of the commonest minerals ; but in granite and 
 quartz-diorite it rarely forms more than half the bulk 
 of the rock, felspar and mica and hornblende being 
 its associates. Veins of quartz (quartz-rock) traverse 
 many rocks, and become broken up into granular 
 forms on weathering ; but they are inconsiderable in 
 comparison with the bulk of the slates or schists in 
 which they lie. Mica-schists contribute a good deal 
 
HI] THE SANDSTONES 57 
 
 of quartz sand when they decay ; but this is mixed 
 with ferruginous clayey matter, and the soils produced 
 are yellow loams. 
 
 We are easily impressed, then, by the enormous 
 amount of denudation that was requisite to produce 
 our existing sandstones. Though nowadays sandstones 
 can be built up by the decay of older rocks of the 
 same kind, the quartz must have come originally 
 from igneous or metamorphic sources. Even in the 
 metamorphic rocks, a large part of the quartz is 
 probably detrital. 
 
 The microscopic characters of the quartz in 
 sandstone commonly attest its origin. The minute 
 liquid inclusions, with moving bubbles, that arise in 
 the quartz of igneous and metamorphosed rocks, are 
 easily seen in sections of sandstone. In some quartz- 
 ites, these inclusions run in continuous bands from 
 grain to grain, and have clearly arisen since the 
 detritus was cemented. But in ordinary sandstones 
 the inclusions in one grain have no relation to those 
 in its neighbours. The felspars, moreover, of igneous 
 rocks are commonly found, as rolled fragments, in 
 sandstone. Their grains are usually whiter and duller 
 than those of quartz, and may easily be distinguished 
 by the naked eye. 
 
 Small gleaming plates of mica from the parent 
 rock may accumulate with the quartz grains. The 
 dark micas of decaying rocks, rich in iron and 
 
58 ROCKS AND THEIR ORIGINS [CH. 
 
 magnesium, together with mineral silicates of calcium, 
 magnesium, and iron, such as the amphiboles and 
 pyroxenes, form on hydration soft green chlorite. 
 This mineral, in films and easily deformed flakes, at 
 times occurs as a sort of groundwork to the coarser 
 grains in sandstone, and colours the rock a delicate 
 grey-green. Fine-grained sandstones of this type are 
 difficult to distinguish from altered "greenstones," 
 such as basaltic andesites. When the quartz grains, 
 however, are large, as in the grits quaintly styled in 
 old days " grey wacke," they form a ready clue to the 
 origin of the rock. 
 
 Nature sifts the products of decay so thoroughly, 
 on any slope exposed to wind or rain, that the finest 
 materials are carried far away, and the undecompos- 
 able quartz remains predominant. The alluvium in 
 the upper reaches of streams is thus far more sandy 
 than the mixed material supplied at the outset from 
 the surrounding rocks. The more rapid flow of the 
 water on the steeper upland slopes naturally removes 
 the mud into the lowland. 
 
 When the detritus, still somewhat mixed, reaches 
 a sea-shore, wave-action is rapidly effective. Before 
 the continual wash and pounding of the water, any 
 residual clay, and the finely comminuted portion of 
 the quartz, are carried down the coastal slope. The 
 colour of the sea after storms is sufficient evidence of 
 the work that it performs. Beaches, then, arrive at 
 
Ill] 
 
 THE SANDSTONES 
 
 69 
 
 a great similarity of type. The inviting yellow sands, 
 formed of comparatively coarse material, occur alike 
 
 Fig. 5. SAND DEVELOPING FROM SANDSTONE, in semi-arid climate. 
 Near Laingsburg, Cape of Good Hope. 
 
 off shores formed of chalk, slate, granite, or boulder- 
 clay. 
 
60 ROCKS AND THEIR ORIGINS [CH. 
 
 From the beginning of sedimentation, sands have 
 thus tended to accumulate, and to become cemented 
 into sandstones. These rocks, in turn uplifted and 
 exposed, have yielded other sandstones. Since coarse 
 sand does not travel far from the region where it is 
 washed out of the parent rock, a thick mass of sand- 
 stone extending over many square miles may waste 
 away, and yet become perpetuated in the district. 
 Sandiness thus begets sandiness, and the physical 
 conditions due to the presence of sandstone may pre- 
 vail through long geological epochs (Fig. 5). 
 
 Of course, a submergence beneath the sea may 
 change all this in a brief time ; but wrinklings of the 
 crust, raising the sandstones into severer atmospheric 
 levels, may only accelerate their decay and render 
 the surrounding lands more sandy. 
 
 THE CEMENTING OF SANDS 
 
 The cement of sandstones is very varied. On our 
 modern coasts, springs draining from a limestone 
 land, or even running through banks of broken shells, 
 will deposit calcite in the interstices of the beach, 
 until slabs and shelves of conglomerate and sandstone 
 arise in 'defiance of the waves. On coasts where 
 calcium bicarbonate is abundant, it may be precipitated 
 by any cause that diminishes its solvent. Mere 
 evaporation, and the escape of carbon dioxide from 
 
in] THE SANDSTONES 61 
 
 the water as it is scattered into spray, lead to the 
 deposition of a cement between the grains of sand. 
 As Linck(6) shows, calcite is thus laid down in tem- 
 perate waters, while aragonite forms fibrous crystals 
 between the detrital fragments on the flanks of tropic 
 isles. Aragonite may also arise from the action of 
 ammonium carbonate or sodium carbonate on calcium 
 sulphate or calcium chloride in sea-water. Sands 
 thus become cemented by one or other form of calcium 
 carbonate. They include, moreover, calcareous algse, 
 foraminifera, and fragments of coral and sea-shells. 
 
 Fossil shells are usually represented in older 
 sandstones by mere external and internal moulds. 
 The texture of the rock allows of their being dis- 
 solved in percolating waters, while in clays belonging 
 to the same geological series they may be exquisitely 
 preserved. 
 
 In shallows, and especially in lakes, where soluble 
 salts of iron become readily oxidised, brown iron rust, 
 the mineral limonite, is continually forming at the 
 surface and sinking to the bottom, where it firmly 
 cements the sand. A group of bacterial) extracts 
 iron in this form from the water of freshwater lakes 
 and swamps, and greatly aids in its accumulation. 
 Though a red colour may appear also in marine 
 deposits, masses of red and purple conglomerates and 
 sandstones may reasonably be assigned a freshwater 
 or terrestrial origin. Such rocks are usually found 
 
62 ROCKS AND THEIR ORIGINS [CH. 
 
 to be devoid of marine fossils, and they often contain 
 traces of land plants. 
 
 Barytes (barium sulphate), which sometimes 
 occurs in veins simulating those of calcite, is an 
 occasional cement of sandstone, evidently arising 
 from subterranean waters. 
 
 Bands of flint (chert) occur in certain sandstones, 
 such as the Hythe Beds of the English Lower 
 Greensand Series. These are due to the cementing 
 of certain layers by chalcedonic silica, and the source 
 of this silica is seen in the hollow moulds of sponge- 
 spicules, and the glauconitic casts of their canals, 
 that commonly remain. G. J. Hindeteo) shows that 
 in the Cretaceous examples, as in so many other 
 flints, the majority of the spicules are of the 
 tetractinellid type. 
 
 Under arid conditions, as in parts of Africa, loose 
 superficial sands may become cemented by calcium 
 carbonate, or even by silica, brought up in water 
 rising by capillary action from below. 
 
 The sand-dunes of the coast of our own islands, 
 which cannot remain wet for long, become in places 
 toughened by a deposit of calcite derived from the 
 abundant shells of land-snails. In the Cape of Good 
 Hope (si) the dunes, as A. W. Rogers states, are con- 
 verted by invasions of calcium carbonate, " into hard 
 rock through a distance of many feet from the surface, 
 and where repeatedly wetted and dried, as happens 
 
in] THE SANDSTONES 63 
 
 where the sea has encroached upon old dunes, the 
 rock becomes intensely hard and weathers with a 
 peculiarly jagged surface." The General Post Office 
 and the South African Museum in Cape Town are 
 mainly constructed of this recently consolidated 
 rock. 
 
 The modern sandstones cemented by silica are 
 still more interesting. In the Cape of Good Hope, 
 and notably in the Kalahari desert, they form the 
 intensely hard rock known as Quartzite&i). The 
 cementing material is true quartz, which sometimes 
 deposits itself in bipyramidal crystals about the grains 
 of sand. The atoms in such crystals are arranged in 
 harmony with the grouping of those in the original 
 detrital grain, as is proved in thin sections under the 
 microscope by the optical continuity of the quartz 
 of the grain and of its coating. As silica continues 
 to be deposited, the coatings interlock, and the rock 
 passes into true quartzite. It is now often difficult 
 to detect the outline of the original grains. Such 
 superficial quartzites may be ten feet thick at most, 
 with uncemented sand below. Rogers suggests that 
 the cementing process may have originated in shallow 
 pools ; but it has obvious analogies with that which 
 forms iron-pans and superficial masses of calcium 
 carbonate in regions where capillary waters are 
 subject to prolonged evaporation. H. G. Lyonsoa) 
 has attributed the cementing of parts of the Nubian 
 
64 ROCKS AND THEIR ORIGINS [CH. 
 
 Sandstone in the desert of Lower Egypt to the silica 
 set free by the alteration of the felspars in the rock. 
 This change, he suggests, was accelerated by the 
 infiltration of sodium carbonate of local origin. 
 Fossil trees in these strata have been replaced by 
 silica. A further example is recorded by Armitage(34) 
 from Victoria, where friable ferruginous Cainozoic 
 sands have been converted into quartzite. This type 
 of rock, the hardest known, and associated in our 
 minds with high antiquity and metamorphic action, 
 proves, then, to be in process of construction at the 
 surface at the present day. 
 
 The observations of Rogers show that quartz and 
 not mere chalcedony is deposited on the grains of 
 sand. The " crystalline sandstones " of Permian and 
 Triassic age in England may, then, have acquired 
 their remarkable characters at the actual epoch of 
 their accumulation. This is rendered the more 
 probable by the recognised occurrence of arid 
 conditions, at any rate seasonally, when the strata 
 in question were being laid down. 
 
 These English "crystalline sandstones" were 
 described by H. C. Sorby(35), who showed that the 
 quartz deposited on the detrital grains was in optical 
 continuity with that of the grains themselves. J. A. 
 Phillips 06) regarded this quartz as crystallised out 
 during the kaoliriisation of felspars. The phenomena 
 of laterisation, however, give us a further suggestion 
 
in] THE SANDSTONES 65 
 
 as to the origin of the secondary silica. It is now 
 well known that tropical processes of weathering, 
 with alternations of wet and dry seasons, allow 
 alumina to be set free from combination with silica, 
 " lateritic " crusts thus arising on a great variety of 
 rocks. The felspars of a sandstone may, under such 
 conditions, become laterised rather than kaolinised, 
 aluminium hydroxide being left, and the silica pass- 
 ing into solution and appearing again in certain layers 
 as cementing quartz. The almost complete disap- 
 pearance of silica from the more advanced laterites 
 shows that it has been carried away elsewhere, and 
 the cement of quartzite may thus be derived from 
 rocks at a considerable distance. Just, however, as 
 the destruction of siliceous sponge-spicules implies 
 the formation of flint, so laterisation implies silici- 
 fication as a complementary process. 
 
 The fact that secondary quartz in quartzite often 
 arises in the rock itself is shown by the frequency of 
 quartz-veins in quartzites, while they are almost 
 absent from associated slates or schists. Hence it 
 appears that a removal of silica goes on at some 
 points, leading to an infilling of all the cracks and 
 interstices at another. 
 
 It is clear, then, that sandstones, according to the 
 mode in which they have been affected by percolating 
 waters, may vary from the crumbling uncemented 
 condition, known as Sand-rock, to that hardest and 
 
66 ROCKS AND THEIR ORIGINS [CH. 
 
 most resisting of rocks, quartzite. The permea- 
 bility of sandstone is responsible for a wide variety 
 of cemented types. 
 
 THE SAND-GRAINS OF SANDSTONE 
 
 Sandstones are originally permeable by water, 
 not because they possess a high percentage of pore- 
 space, or " porosity," but because the pores between 
 the grains are large. Water can thus move easily 
 by gravitation through the mass. The capillary rise 
 or spread of water is greatest in materials of very 
 fine grain, though in these it may be extremely slow. 
 For the most effective rise of water against gravity 
 by capillary pull, a large proportion of particles about 
 '02 mm. in diameter should be present. Sand-grains, 
 however, often measure *5 mm. in diameter, and the 
 fine mud or highly comminuted sand between the 
 coarser matter is the cause of the spread of water 
 through the mass when the supply comes from a 
 subterranean water-table. Rain, however, is of 
 course readily absorbed. It disappears so rapidly 
 on some barren sandstone areas, coated as they are 
 by loose sandy soils, that vegetation cannot make 
 a start, even where water is supplied. 
 
 Daubree, Sorby, and others have studied the 
 characters of sand-grains, and it has been pointed 
 out (37) that agitated water buoys apart and carries 
 forward by flotation grains with a diameter of *1 mm. 
 
in] THE SANDSTONES 67 
 
 or less. Hence coarser grains may become rounded 
 like pebbles, by friction on the bottom of a stream ; 
 but small ones remain angular throughout geological 
 periods, and even when transferred from one sand- 
 stone to another. When their surfaces have been 
 cleaned by boiling in hydrochloric acid, the sharpness 
 and irregularity of the quartz grains is strikingly 
 apparent. 
 
 Mingled with these grains, in addition to the 
 minerals previously mentioned, many interesting 
 crystals appear that have become concentrated in 
 the natural washing processes. Minute colourless 
 zircons and brown rutiles, derived from granite, have 
 collected, owing to their high specific gravity, in 
 certain sands. Magnetite and ilmenite may darken 
 the mass; monazite and thorite, which are sought 
 after for their constituents cerium and thorium, 
 become similarly selected in alluvial hollows, owing 
 to their density of 5. Whatever gathers thus in 
 sands may become preserved in sandstones, and the 
 study of thin sections of the latter under the micro- 
 scope is fruitful in suggestions as to their origin. 
 
 Some sandstones are remarkable for their highly 
 rounded and almost spherical grains. J. A. Phillips (38) 
 compared these with the wind- worn grains of deserts, 
 which assume similar forms and a considerable polish. 
 Large quantities of sand are carried from arid lands 
 into rivers, into lakes, or into the sea, and hence well 
 
 52 
 
68 ROCKS AND THEIR ORIGINS [OH. 
 
 rounded grains, in bedded rocks, and even in marine 
 sandstones, may have had a desert origin. J. W. Judd, 
 when examining the deposits of Lower Egypt for the 
 Royal Society, commented on the extreme freshness 
 of the felspathic particles in sands accumulating in 
 rainless areas, and recent observations on the soils 
 of semi-arid districts show their comparative poverty 
 in clay. Enough has been said to indicate the variety 
 of geographical considerations that may arise from 
 the examination of beds of sandstone. The grains 
 often prove, especially in the coarser types, to be 
 fragments of rocks rather than isolated minerals, and 
 thus furnish a picture of the materials that formed 
 the surface exposed to denudation. 
 
 The sandstones of finest grain may be found in 
 beds deposited almost on the limits of sedimentation 
 from the land, where they are interlocked with 
 material of truly pelagic origin. Marine muds often 
 contain a high percentage of comminuted quartz, 
 and the study of shales and slates of ancient days 
 shows how this almost indestructible mineral finds 
 its way into beds that might easily be classified as 
 clay s (41). 
 
 SOME CHARACTERS OF SANDSTONE 
 
 Earth-stresses and shrinkage give rise to joints 
 in sandstone, which may not be so clean and sheer 
 as those in limestone, but which affect even the softer 
 
in] THE SANDSTONES 69 
 
 forms. Cemented sand-dunes of modern date tend 
 to break away along vertical planes. Firmer sand- 
 stones give rise to stepped table-lands and " edges," 
 and the resistance of many types to atmospheric 
 decay renders their stratified structure strongly ap- 
 parent Small intervals in the process of deposition, 
 or slight changes in the coarseness of the sand 
 brought down by currents, give rise to laminated 
 and flaggy types. Where a broad shore has been 
 exposed between tide-marks, the drying and com- 
 pacting of the surface before the next layer is laid 
 down enables the latter to take a mould of the 
 inequalities of that below. Ripple-marks, sun-cracks, 
 rain-prints, and the footmarks of animals, are often 
 preserved in this manner. Where the shore is sub- 
 siding, they may persist through hundreds of feet of 
 strata. 
 
 Naturally, the best examples of these casts, and 
 of the original structure in the underlying bed, occur 
 where a little mud has been laid down over the sandy 
 flat. Clay by itself, if damp, does not retain the im- 
 pressions sufficiently long, and, when once thoroughly 
 dried, it crumbles when the next water overflows it. 
 But a foundation of firm sand with a thin mud-layer 
 on its surface, as may be recognised in some Triassic 
 deposits, furnishes excellent records of local weather 
 or of the movements of errant animals. On the flat 
 shores of lakes in a semi-arid climate, the water may 
 
70 ROCKS AND THEIR ORIGINS [OH. 
 
 retreat for miles, and return, perhaps months after- 
 wards, when rains in the hills have given it a new 
 burden of detritus. Under such conditions, broad 
 sun-cracked flats may be preserved, with perhaps 
 some plant-remains between successive layers (as w*x, 
 The castings and tracks of worms, and the tubes 
 of boring species, which are sometimes infilled by 
 sand of a different colour, are common in sandstones 
 of all ages. 
 
 SILICEOUS CONGLOMERATES 
 
 The deposits of wave-swept beaches leave us 
 Conglomerates formed of various types of pebbles, 
 among which quartz-rock and quartzite naturally 
 predominate. In some cases the pebbles are ready 
 formed when they reach their resting-place. They 
 come rolling out from lateral torrents into the quieter 
 waters of a main valley, as may be seen in summer 
 in the broad pebble-banks of the north Italian 
 streams. Thence they are washed by occasional 
 floods into the great confluent deltas that constitute 
 the upper part of an alluvial plain, or into lake-basins, 
 where they promptly settle along the shore. But 
 few such pebbles, except from pre-existing con- 
 glomerates or gravels on the shore-line, actually 
 reach the sea. The rolled stones upon sea-beaches 
 are mostly the products of marine action on the 
 spot. While the fine sand-grains go seaward almost 
 
in] THE SANDSTONES 71 
 
 unKarmed, the detrital stones, offering far less surface 
 in proportion to their mass, strike on their neighbours 
 as every wave shifts them on the beach, and soon 
 assume a rounded form. 
 
 The conglomerates ultimately consolidated may 
 reveal stratification only by the general arrangement 
 of their pebbles. These can rarely be spheres, since 
 they are not as a rule turned over, but are pushed 
 this way and that until they acquire a flat ellipsoidal 
 shape. They lie with their flatter sides in planes 
 parallel to one another. Generally, however, alterna- 
 tions of coarser and finer beds mark out the stratifica- 
 tion even in conglomerates. 
 
 The sands of deserts include abundant stones and 
 blocks of rock, and the loose material becomes, more- 
 over, sifted by the wind. True desert sands may 
 accumulate at one point, the very finest loamy 
 material may be carried away still farther to form 
 fields of fertile loss, and a rock-desert, formed of 
 stones resting on bare surfaces, may remain in large 
 areas of the arid region. The loose stones here 
 assume a characteristic shape, and have been known 
 under the German name of Dreikanter. They are 
 fairly flat below, and are cut away above by the 
 drifting sand into a form resembling a gable roof 
 dipping at both ends. Their surfaces are character- 
 istically etched. 
 
 Dreikanter have been found in beds that were 
 
72 ROCKS AND THEIR ORIGINS [CH. 
 
 formerly ascribed to deposition on the shores of 
 lakes, and it must now be borne in mind that 
 continued attrition by drifting sand affects mixed 
 detritus on a land surface much as the wash of waters 
 does upon a beach. Certain materials are cut away 
 more rapidly than others, and the residue assumes a 
 more and more quartzose type. In this way, sand- 
 stones, and conglomerates in which fragments of 
 quartzite and vein-quartz predominate over other 
 constituents, may arise as seolian beaches on dry 
 land. 
 
 SANDSTONE AND THE LAND-SURFACE 
 
 The permeability of sandstone has already been 
 referred to. The surface offered by it is typically dry, 
 and the soil, consisting mainly of grains of siliceous 
 sand, can neither retain the rain that falls nor draw up 
 water from below. The idea that trees can flourish 
 on sandstone soils because they require nothing from 
 the soil itself is of course erroneous. They depend 
 to a large extent upon the materials set free by the 
 decay of certain grains, or of the cement of the 
 underlying sandstone. In proportion as the sand- 
 stone is impure, that is, the more its constituents 
 deviate from pure quartz, the more chance there is 
 that it will provide a fertile soil. 
 
 On the whole, however, areas of siliceous con- 
 glomerate and sandstone are given over, even in 
 
in] THE SANDSTONES 73 
 
 temperate climates, to forest and heather. Where 
 the sandstone is still in the sand-rock state, bare 
 patches are likely to appear even in the heath that 
 has grown across it, and from these the wind carries 
 away shifting sands. 
 
 Everyone familiar with the Carboniferous areas 
 of the English midlands will realise the influence of 
 hard grit and sandstone in forming "edges" across 
 the country. The contrast between these escarpments 
 and the slopes of crumbling shale that often underlie 
 them gives diversity to the scenery of Yoredale and 
 the Peak. The more yielding sandstones of Cretaceous 
 age round about the Weald, or at the foot of the 
 Chiltern Hills near W^oburn, form rounded hills, 
 mostly clad with woods of coniferous trees. In 
 Surrey, unpaved cart-tracks, used for centuries, 
 have cut gullies in the unconsolidated Folkestone 
 Sands. 
 
 The underlying Hythe Beds, however, stand out 
 between Reigate and Guildford as a bold escarpment, 
 and it is interesting to reflect that this fine feature of 
 south-eastern England is probably due to the chert 
 which the beds contain (see p. 62). The local growth 
 of siliceous sponges in a Lower Cretaceous sea enables 
 Leith Hill in our days to dominate even the arch of 
 Ashdown Forest, where another untilled sandstone 
 area rises in the centre of the Weald. 
 
 The sands of Bagshot Heath, and numerous 
 
74 ROCKS AND THEIR ORIGINS [OH. 
 
 similar areas in the Paris Basin, show how impossible 
 it is to cultivate such strata, even near the best of 
 markets. The flint gravels that cover much of the 
 upland in the New Forest may also be borne in 
 mind, as presenting the worst features of highly 
 siliceous lands. 
 
 In a semi-arid climate, or one with only seasonal 
 rains, the processes by which sandstone begets sand- 
 stone tend to develop desert wastes. The soils 
 produced by weathering do not cake together, and 
 are carried away by wind during the drier months. 
 The bare rock appears over broad surfaces, just as 
 it does in storm-swept limestone areas, and any 
 hollow where shelter is afforded tends to become 
 filled with sand (see Fig. 5). 
 
 The hummocky and extremely irregular surface of 
 some of our Silurian areas, such as parts of the 
 Southern Uplands of Scotland and the hard- won 
 farmlands of Down and eastern Monaghan, is due 
 to the presence of resisting sandstones among the 
 shales. These sandstones, passing into true grits, are 
 repeatedly folded, and their upturned edges have re- 
 sisted even the passage of glacier-ice. They jut out 
 along the crests of ridges, and even the smaller beds 
 furnish angular fragments to the soils. 
 
 Far wilder scenery is formed by the more con- 
 tinuous sandstone masses of the Harlech Beds in 
 western Wales, which are grits so firmly cemented 
 
in] THE SANDSTONES 75 
 
 that the rock breaks across the quartz grains. Much 
 of the Old Red Sandstone is of equally hard quality 
 
 Fig. 6. SILICEOUS CONGLOMERATE. Characteristic weathering; 
 moraine-blocks at Coumshingaun, Co. Waterford. 
 
 (Fig. 6). Its purple or grey conglomerates, the pebbles 
 of which are quartzite in a quartz cement, form bare 
 
76 ROCKS AND THEIR ORIGINS [CH. 
 
 and rugged masses in the Great Glen south-west of 
 Inverness, and are responsible in Kerry for some 
 of the wildest rock-scenery in the British Isles. 
 Variations in coarseness allow of the development of 
 a marked stratification on the weathered mountain 
 sides, and differential erosion of the beds has taken 
 place where ice has pressed against them. Even on 
 precipices, grassy ledges may occur, marking bands 
 of sandstone or shale in the conglomeratic mass. 
 
 The red sandstones and conglomerates that form 
 huge outstanding bluffs from Applecross to the north 
 of Sutherland represent the denudation of a pre- 
 Cambrian mountain region. These Torridon Sand- 
 stones cover a very irregular surface of old gneiss, 
 with which their almost level strata are in striking 
 contrast. P. Lake (39) has compared them with the 
 deposits styled dasht in Baluchistan and Afghani- 
 stan, which similarly fill up valleys and cover hills, 
 as products of extensive and rapid denudation. 
 There is much, indeed, to suggest that the Torridon 
 Sandstone, some 10,000 feet in thickness, was ac- 
 cumulated in a dry country on a continental surface, 
 with the aid of floods during occasional rainy seasons. 
 
 Quartzite, which fractures into small angular 
 blocks under earth stresses, yields an intractable 
 surface of bare rock and taluses of shifting stones. 
 The latter sometimes crumble down into white sand, 
 which provides some basis for the growth of heather. 
 
Ill] 
 
 THE SANDSTONES 
 
 77 
 
 The numerous joints, independent of the bedding- 
 planes, cause the rock to break up almost equally on 
 
 Fig. 7. QTTARTZITE COXE. Croagh Patrick, Co. Majo. 
 
 any exposed slope, and the crests of quart zite hills 
 become typically converted into cones (Fig. 7). 
 Viewed from a distance, the white taluses, streaming 
 
78 ROCKS AND THEIR ORIGINS [OH. 
 
 down evenly from the crests, resemble caps of 
 snow. 
 
 The absence of soil and the smoothness of weathered 
 surfaces render quartzite mountains hard to climb. 
 The uniform cementing of the rock leaves the bed- 
 ding with little influence on the surface-features, and 
 rock-ledges and shelves are rare. The traveller 
 ascends over taluses of angular and obstinate blocks 
 towards slippery and inhospitable domes. But the 
 wildness of the scenery will be his sure reward. It 
 is of interest to reflect that the material of these bold 
 outstanding mountains may in certain cases have 
 originated, in all its hardness, in the levels of a 
 sun-parched plain. 
 
 CHAPTER IV 
 
 CLAYS, SHALES, AND SLATES 
 CHARACTERS OF CLAY AND SHALE 
 
 THE question of what is a true Clay has been 
 , much discussed, especially by agriculturists, in recent 
 years (39 w*). The material, as a rock, is commonly a 
 massive kaolin, and, if pure, should have the following 
 percentage composition : silica 46*3, alumina 39*8, 
 water 13*9. Some Pipe-clays, white and uncon- 
 taminated, closely approximate to this ideal. True 
 
iv] CLAYS, SHALES, AND SLATES 79 
 
 clays are very plastic when moistened, and shrink on 
 drying, forming a compact mass the particles of 
 which do not fall apart. When thoroughly dried, 
 however, and placed in water, lumps of clay break 
 up readily ; the water creeps in along their capillary 
 passages and expels trains of air-bubbles as it goes. 
 This fact has been utilised in the extraction of fossils 
 from a matrix of stiff clay. If the clay thus reduced 
 to powder is now "puddled" by the finger, it again 
 forms a closely adherent plastic mass. 
 
 The individual spaces between adjacent particles 
 in a clay are very minute, and this accounts for its 
 practical impermeability to water ; but the total 
 pore-space or " porosity " may amount to more than 
 fifty per cent, of the volume of the rock. Unless earth- 
 pressures have brought the mass into the condition 
 of shale or slate, the tiny mineral grains, which are 
 mostly flakes of kaolin, chlorite, or mica, have not 
 shaken themselves down into a closely aggregated 
 state. When moistened, however, and again dried, 
 the surface-tension of the film of water about any 
 group of grains, draws the particles nearer to one 
 another, and a considerable shrinkage of the mass 
 results. Alternate wetting and partial drying tend 
 to make a clay less obdurate and sticky, by increasing 
 the number of separate aggregates of grains. The 
 passages between these aggregates are no longer so 
 minutely capillary, and a clay soil becomes by this 
 
80 ROCKS AND THEIR ORIGINS [OH. 
 
 process distinctly " lighter " from the farming point 
 of view. 
 
 The larger cracks caused by shrinkage greatly 
 increase the evaporation of water, by exposing new 
 surfaces, which penetrate deeply into the clay. Hex- 
 agonal structure may develop by shrinkage on clay 
 flats, and is conspicuous in arctic tundra soils (Fig. 8). 
 
 The natural " flocculation " of clays, the process 
 by which compound grains are formed in place of 
 individual soil-particles, is assisted by the action of 
 water bearing certain salts in solution. Calcium 
 carbonate is an excellent flocculator, and this fact 
 has long led farmers to place burnt lime or powdered 
 limestone on their lands. Sodium carbonate, on the 
 other hand, is brought up in some dry regions by 
 capillary action, and exercises a reverse effect, keep- 
 ing the minute particles apart from one another, and 
 thus promoting thorough clayiness in the clay. 
 
 The particles that impart sticky and plastic 
 characters to clays are below 0*01 mm. in diameter, 
 and graduate down to sizes comparable with those 
 of chemical molecules. Particles with diameters less 
 than O'OOl mm. (1 micron) may be regarded as form- 
 ing a colloid with the surrounding water in the clay. 
 Colloidal matter arising from the decomposition of 
 silicates, especially under alkaline conditions, in- 
 cluding silica and aluminium and iron hydroxides, 
 is believed to gather on the smaller grains and to 
 
iv] CLAYS, SHALES, AND SLATES 
 
 81 
 
 affect their reactions with permeating solutions. 
 Clay soils are of course much more complex than 
 clay rocks, and their grains may be coated with col- 
 loids that are partly of organic origin. 
 
 "^ 
 
 %{ 
 
 Fig. 8. SHRINKAGE-CRACKS IN CLAY, with footprints of birds 
 in the foreground. Tundra of Mimer Bay, Spitsbergen. 
 
 Stickiness when wet seems to be largely a matter 
 of fineness of grain, while plasticity, the power of 
 retaining a form impressed on it by moulding, with- 
 out crumbling down when dry, is very probably due 
 to the platy and cleavable character of the minerals 
 
 c. 6 
 
82 ROCKS AND THEIR ORIGINS [CH. 
 
 that provide the material of the finest grades. Re- 
 actions on the grain-surfaces when wet may have 
 much to do with plasticity, and recent researches on 
 the flow of metals under polishing processes must be 
 taken into account in considering the characters of 
 the rocks described as clays. 
 
 A. Atterberg(4o) has shown how relative plasticity 
 may be determined in clays, and his classification is 
 useful from a geological as well as an agricultural 
 point of view. He determines what may be called 
 " stiffness " by noting the maximum amount of water 
 that can be taken up by 100 parts by weight of clay- 
 particles without two adjacent blocks of the wet mass 
 merging into one another on contact. 
 
 To the ordinary observer, a rock possesses the 
 properties of clay, and is a clay, if it contains more 
 than forty per cent, of particles less than "01 mm. in 
 diameter. 
 
 In most clays there is a large admixture of quartz 
 sand. The kaolin, derived originally from the decay 
 of other silicates, is rarely freed from a variety of 
 minerals and rock-fragments that were associated 
 with it in its place of origin. Grains of quartz and 
 unaltered felspar a tenth of a millimetre in diameter 
 distinctly " lighten " a clay soil, on account of their 
 relative coarseness. A sandy clay is styled a Loam, 
 and a fine-grained loam furnishes the ideal soil for 
 the general purposes of a farmer. It does not retain 
 
iv] CLAYS, SHALES, AND SLATES 83 
 
 water too long upon its surface, nor does it dry too 
 quickly after rain. Much of what we call boulder- 
 clay proves to be in reality a loam. 
 
 T.Mellard Reade and P. Hollands) have shown that 
 even in clays of marine origin there may be a consider- 
 able proportion of very fine quartz sand (see p. 87). 
 
 Calcium carbonate, usually occurring as fine rock- 
 dust derived from limestone, or as minute shell- 
 fragments, may be mingled with clay to form a Marl. 
 The term is not a quantitative one, and may be 
 applied to any clay that shows a brisk effervescence 
 with cold acids. Though unpleasantly sticky when 
 wet, marls flocculate themselves naturally by supply- 
 ing calcium carbonate in solution to waters that pass 
 through their crevices (see p. 80). 
 
 The stratification of clays may be invisible 
 throughout considerable masses, unless sandy beds 
 are intercalated among them. Yet, when a lump of 
 clay is dried and then placed in water, as previously 
 described, it will often break up along parallel planes, 
 which show that there is a regular arrangement of its 
 particles. The fact that so many of these particles 
 are platy becomes emphasised under the pressure of 
 subsequent sediments, whereby the platy surfaces of 
 the particles are brought into planes parallel with 
 one another. The clay then becomes a Shale, with 
 regular planes of fissility, which are parallel to those 
 of bedding. A certain amount of deformation of the 
 
 <-* 
 
84 KOCKS AND THEIR ORIGINS [CH. 
 
 rock accompanies this change, flow being set up 
 parallel with the bedding, and included fossils be- 
 coming sometimes flattened. This deformation is 
 especially noticeable in the case of plant-remains. 
 Shales may in time attain the density and fissile 
 structure of true slate. 
 
 The colours of clays and shales are of considerable 
 interest. Blackness is often due to organic matter, 
 and especially to fragments of plants, which retain 
 their woody structure and their carbonaceous cha- 
 racter when protected by clay from oxidation. 
 
 The bluish tint of clays is due to finely divided 
 iron pyrites (iron disulphide), which may occasionally 
 appear as distinct crystals or nodules of one or other 
 of its forms, pyrite or marcasite. On oxidation, 
 limonite arises, which colours the mass brown, as 
 is seen in the upper part of many clay-pits. The 
 occurrence of iron pyrites often dates back to the 
 time at which the clay accumulated. N. Andrussow(42) 
 points out that in the Black Sea there is an enormous 
 supply of decaying organic matter provided by the 
 floating organisms of the upper layers. This rains 
 continually down towards the floor. The portion 
 that reaches depths of over 100 fathoms escapes from 
 the voracity of free-swimming organisms and arrives 
 at the region where bacteria alone abound. These 
 bacteria act on dissolved sulphates, and also largely, 
 according to Andrussow, on the albumen of the 
 
iv] CLAYS, SHALES, AND SLATES 85 
 
 decaying matter. In both cases, sulphuretted hydro- 
 gen is produced. Andrussow treats the reduction of 
 the marine sulphates as a minor process, due to the 
 need that the bacteria have for oxygen in the deep 
 waters, which are insufficiently supplied. The sulphu- 
 retted hydrogen attacks the salts of iron, and iron 
 sulphide (FeS), and finally the disulphide, result. 
 
 Here we have an excellent illustration of how, in 
 deep basins, with imperfect vertical circulation, black 
 pyritous muds may arise, devoid of ordinary fossils. 
 The depths of the Black Sea are practically poisoned 
 by the abundance of sulphuretted hydrogen. But 
 numerous cases of shales are known to us where iron 
 pyrites replaces the shells of ammonites or forms 
 complete casts of bivalves, and has accumulated also 
 in concretions and crystalline groups. Such pyrites 
 is probably of secondary origin, or arose from the 
 reducing action of decaying organic matter on ferrous 
 sulphate in solution in the sea. 
 
 The oxidation of iron pyrites in shales gives rise 
 to aluminium sulphates, such as alums. Sometimes 
 sufficient heat is evolved during this oxidation to set 
 on fire carbonaceous matter present in the rock. 
 
 Pink-purple and green are common colours among 
 shales, and imply that the iron is in two different 
 states of oxidation. When the colour varies thus in 
 successive bands, we may believe that a climatic 
 change promoted the formation of feme salts on the 
 
86 ROCKS AND THEIR ORIGINS [CH. 
 
 land surface when the pink layers were being formed, 
 while ferrous (less oxidised) salts predominated when 
 the green particles were washed into the basin. 
 B. Smith (43) suggests that the organic matter and 
 humic acids which are swept down in times of flood 
 may temporarily prevent oxidation from occurring 
 in shallow lakes and pools. Dry seasons would thus 
 lead to the deposition of pink clays, while wet seasons 
 would furnish green ones. The green colour in shales 
 is mostly due to chlorite or to glauconite. 
 
 Subsequent deoxidation has been invoked to 
 account for the green colour of certain shales. 
 Organic matter may have been responsible, and the 
 green spots in purple slates have been attributed to 
 the decay of entombed organisms, the reaction having 
 spread outwards from a centre. 
 
 Clays, owing to their impermeability, preserve 
 fossils excellently, and the oldest shells and corals in 
 which the original aragonite has escaped conversion 
 into calcite occur in clays and shales of Mesozoic 
 age (see p. 22). 
 
 ORIGIN OF CLAYS 
 
 Something has been said on this matter in the 
 foregoing paragraphs. It is now recognised that a 
 pure china-clay or a pipe-clay, that is, a pure 
 kaolin-earth, does not arise from the sifting of the 
 products of surface-denudation. The alkali felspars 
 
iv] CLAYS, SHALES, AND SLATES 87 
 
 decompose as they lie in exposed layers of granite 
 and gneiss, but the kaolin thus formed under the 
 acid action of atmospheric waters is relatively small 
 in quantity, and cannot escape from its coarser as- 
 sociates, such as undecomposed felspar and quartz, 
 until it is carried away far from laud. Even then, as 
 the records of H.M.S. " Challenger " show(44), marine 
 muds may contain more than fifty per cent, of detrital 
 quartz grains, and quartz is always the most abundant 
 mineral among the larger particles of the mud. 
 
 Where, however, decomposition of the granitoid 
 rock has been exceptionally thorough, kaolin may be 
 present in sufficient quantity to predominate over 
 other materials. The product washed from the 
 surface then gathers as a white clay even in lakes, 
 and further artificial washing may extract from it an 
 actual kaolin-earth or china-clay. In such cases, the 
 rock has become rotted throughout in consequence 
 of subterranean action. Carbonic acid is the main 
 agent, and hydrofluoric acid is also indicated by the 
 secondary minerals associated with the kaolin; and 
 the appearance of white powdery kaolin in unusual 
 abundance on the surface is due to the local ex- 
 posure of a mass that was long ago made ready in 
 the depths. 
 
 The sifting action, however, of running waters, 
 and especially of the sea upon a shore, ultimately 
 causes clayey matter to be carried away into regions 
 
88 ROOKS AND THEIR ORIGINS [CH. 
 
 where it is slowly deposited. The flocculating action 
 of the salts dissolved in sea- water greatly assists the 
 precipitation of clay before it has reached some two 
 hundred miles from land. However, just as sandstone 
 begets sandstone, clays or shales exposed upon a 
 coast produce new clays close to shore. The estuary 
 of the Thames and many "slob-lands" serve as 
 examples. Off Brazil, red clays arise (45) from the 
 large quantity of "ochreous matter" carried from 
 the coast. Modern green marine muds are found to 
 contain glauconite, a silicate common in the English 
 Gault clays, and formed by interactions in the sea 
 itself. Modern blue muds (46) are recorded down to 
 2800 fathoms, and contain organic matter and iron 
 disulphide. 
 
 Much has been written by the observers on the 
 " Challenger " and by others on the red clay of truly 
 abyssal depths, which is attributed to the decay of 
 wind-borne volcanic dust, and of igneous matter 
 erupted on the sea-floor, rather than to any direct 
 transport by water from the land. 
 
 Clays may also accumulate on a land-surface from 
 fine volcanic ash, which decomposes through the 
 action of percolating waters. 
 
 SLATE 
 
 The relations between shale and Slate are so 
 obvious that slate may readily be regarded as a very 
 
iv] CLAYS, SHALES, AND SLATES 89 
 
 well compacted mud. The clayey material in it, like 
 that of muds, may be ordinary detritus or of volcanic 
 origin ; its colours repeat those of shales. Its 
 essential character, however, is the possession of 
 a "cleavage," that is, of well-developed planes of 
 fissility, which are often inclined to those of bedding. 
 The bedding may be indicated by bands of different 
 coarseness or constitution, and these may show 
 crumpling due to pressure that has been exerted on 
 the mass. The cleavage, however, may run right 
 across these bands, and the rock, as a rule, splits far 
 more cleanly along the cleavage-planes than a shale 
 does along its planes of bedding. 
 
 The early and historic observations on slaty cleav- 
 age have been excellently reviewed by A. Harker(47), 
 who also provides an independent investigation. 
 Reference may also be made to a later treatise by 
 C. K Leith(48), which contains numerous illustrations, 
 and to a discussion by G. W. Lamplugh(49). D.Sharpe 
 and H. C. Sorby, between 1847 and 1853, developed 
 the theory that rock-cleavage was due to compression 
 in a direction perpendicular to the planes of cleavage 
 and to expansion along them. As Harker points out, 
 it is unlikely that the expansion balances the com- 
 pression. The density of slate, about 27, is a good 
 indication that the "porosity," or percentage of pore- 
 space, has been reduced, while the mineral changes, 
 soon to be referred to, are also in favour of greater 
 
90 ROCKS AND THEIR ORIGINS [CH. 
 
 density. C. Darwin (so) laid stress on the connexion 
 between cleavage and the development of flaky 
 minerals, such as micas, along the cleavage-planes, 
 the structure ultimately passing into that known as 
 "foliation" (see p. 145). H. C. Sorby urged that com- 
 pression brings platy particles into parallel positions 
 throughout the mass, so that the plates, which may 
 consist of kaolin, mica, or chlorite, come to lie with 
 their broad surfaces perpendicular to the direction 
 of compression. At the same time, any constituents 
 capable of deformation become compressed in this 
 direction, become expanded in a direction perpen- 
 dicular to it, and are themselves converted into 
 lens-like forms or plates. T. Mellard Reade and 
 P. Holland (51) have emphasised the part played by 
 crystallisation at the close of the process of compres- 
 sion. They urge that the platy minerals, mica and 
 chlorite, are produced during the alteration of the 
 rock, and can spread with ease in directions perpen- 
 dicular to that of compression; they thus give rise to 
 slaty cleavage at a late stage in the deformation of 
 the rock. These authors, it will be seen, have 
 developed one of Darwin's principal propositions, as 
 to the close connexion between rock-cleavage and 
 foliation, and, in opposition to Sorby, consider the 
 platiness of the original constituents to be of less 
 importance. 
 
 In support of their view, in regard to the late 
 
iv] CLAYS, SHALES, AND SLATES 91 
 
 stage at which cleavage is induced, it may be noted 
 that the crystals of pyrite and magnetite that some- 
 times occur in slates and in the allied foliated schists 
 have developed at an earlier date as knots which 
 oppose the cleavage or the foliation (52). 
 
 Darwin observed that mineral differences some- 
 times occur along bands parallel with the cleavage- 
 planes. In such cases, the difference may be largely 
 one of grain, shearing having broken down the 
 minerals into a finer state along certain bands of 
 movement(53). Shearing of the rock may occur along 
 any of the cleavage-planes, which are superinduced 
 planes of weakness, and parts of the slate thus slide 
 over others, just as the mineral flakes slide over one 
 another in the directions in which expansion of the 
 rock is possible. Where traces of the original strati- 
 fication remain, it is easy to see if rock-shearing has 
 occurred. 
 
 Beds of different composition naturally take on 
 cleavage in very different degrees. Sandy layers 
 show the compression that has taken place by con- 
 torting ; but they cleave very poorly, and in proportion 
 to the amount of mud present in them. Where 
 clayey and sandy layers alternate, and the direction 
 of the cleavage is oblique to them, it is refracted, as 
 it were, on passing from one layer to the other ; it is 
 more highly inclined to the bedding in the sandy 
 layers and less so in the clayey layers. Hence a 
 
92 ROCKS AND THEIR ORIGINS [CH. 
 
 cleavage-surface forms a fold resembling the shape of 
 an italic S as it traverses each harder bed. Barker (54) 
 and Leith (55) discuss the cause of this from somewhat 
 different points of view. It is probable that such 
 cleavage-planes as develop within the hard bed are 
 approximately perpendicular to the direction in which 
 the compressive force acts, because there is in such 
 beds little possibility of lateral creep of the material 
 along the bedding-planes. In the softer layers, we 
 have to deal, not only with a tendency towards the 
 rotation of platy particles until their flat surfaces are 
 perpendicular to the direction of pressure, but also 
 with a tendency of the same particles to flow along 
 the bedding-planes. The resultant arrangement gives 
 rise to a cleavage nearer to the bedding-planes than 
 that in the more sandy layers. 
 
 Sometimes, after the cleavage is established, 
 compression folds it, just as strata may be folded. 
 Still greater compression may obliterate it and 
 establish a new cleavage, and all gradations towards 
 this result are traceable. The cleavage -layers, again, 
 may be wrinkled into a series of sharp folds, thrust 
 over in one direction, and parting may then take 
 place along the ridges of these folds, which furnish a 
 second series of planes of weakness in the rock. This 
 type of separation has been styled a strain-slip 
 cleavage, and by Leith a fracture-cleavage, ki distinc- 
 tion from ordinary or flow-cleavage. Shearing may 
 
iv] CLAYS, SHALES, AND SLATES 93 
 
 take place along it, and the true or flow cleavage- 
 planes become thus broken across and faulted. 
 
 Fig. 9. LANDSLIDE OF LIMESTONE OVEB SHALE. Near Luc-en-Diois, 
 Drdme, France. The scale is shown by the main road passing 
 among the blocks. 
 
94 ROCKS AND THEIR ORIGINS [CH. 
 
 Commercial slates should exhibit none of these 
 structures that interfere with genuine cleavage. An 
 argillaceous rock of uniform grain, compressed evenly 
 over a considerable district, is required for successful 
 slate-quarries: Yet all quarry men will admit that 
 the material varies from point to point, and that the 
 best slate runs in "veins." Some of the coarser slates, 
 with irregular surfaces, and with splashes of colour, 
 such as are provided by limonite, are sought after for 
 their picturesque effect; while slates which do not 
 split readily enough for roofing purposes may have 
 their use for flags, mantel-shelves, and billiard-tables. 
 
 ARGILLACEOUS ROCKS IN THE FIELD 
 
 Obviously, nothing can be more different than 
 the features of a country made of clay, when acted 
 on by denudation, and those of one where slate 
 prevails. In the former case, low rounded hills rise, 
 without any definite arrangement, above hollows 
 where rushes spring amid the grass. The streams 
 are muddy, and they readily cut their way down 
 to base-level, meandering thenceforward in a clay- 
 alluvium. Shales provide bolder features, but crumble 
 rapidly where the climate permits of frost and thawing. 
 They may be protected by more resisting rocks, 
 but provide oozy surfaces underground, over which 
 the higher masses may slide disastrously (Fig. 9). 
 Shale-beds, when uplifted and folded, slip away in 
 
iv] CLAYS, SHALES, AND SLATES 95 
 
 flakes from one another, supplying very ragged and 
 irregular material to the taluses, and exposing 
 
 Fig. 10. WEATHERING OP SHALE. Granite mountains behind. 
 Above La Grave, Lautaret Pass, Iseie, Fiance. 
 
 shimmering surfaces when damp with rain (Fig. 10). 
 Among hilly lands, the passes will often be found to 
 
96 ROCKS AND THEIR ORIGINS [CH. iv 
 
 be due to bands of shale, which are cut down by 
 weathering far sooner than the rocks on either hand. 
 In central England, the Lias shales, despite the 
 presence of some limestones, have been worn down 
 almost to a plain, wherever the overlying Middle 
 Jurassic limestone has been removed. 
 
 Slates, with their ragged edges and resistance to 
 rain, play their part in wilder mountain-scenery. 
 Frost-action destroys them, producing taluses that 
 slip frequently towards the valleys ; but the residual 
 crags assume more serrated forms, in contrast with 
 the smooth covering of the lower slopes. The 
 cleavage, when steeply inclined to the horizontal, 
 promotes the cutting of gullies down the mountain- 
 sides, and the intervening ribs of rock may easily 
 be mistaken for uptilted strata. The entrance to the 
 Pass of Llanberis at Dolbadarn is a fine picture of 
 slate-scenery. Eventually, mountains formed of slate 
 assume hog-backed and rounded forms, but they still, 
 where notched by streamlets, yield sheer cliffs and 
 picturesque ravines. 
 
 ON BOULDER-CLAY 
 
 The material known as Boulder-Clay presents such 
 distinctive features, and is so prevalent in our islands, 
 that it deserves a few separate remarks. From a 
 coating a foot or two in thickness, it swells in places 
 
98 ROCKS AND THEIR ORIGINS [CH. 
 
 to a hundred feet or more, and may form the impor- 
 tant round-backed hills to which Maxwell Close 
 reserved the name of drumlim. It consists essentially 
 of mixed materials, unsifted by water, huge boulders 
 of various rocks occurring side by side with angular 
 fragments and pebbles of all sizes, set in a ground- 
 work of loamy clay (Fig. 11). Sands and gravels are 
 often associated with the boulder-clay, and result from 
 the local washing of the mass in copious floods of water. 
 The blocks are here on the whole more rounded, and 
 the sandy part of the loam predominates. 
 
 Blocks of shale and limestone, and even of sand- 
 stone and quartzite, occurring in the boulder-clay, bear 
 the characteristic striations that we now recognise as 
 due to glacial action. The sand and small stones have, 
 in fact, been held against the larger ones by solid ice, 
 and have cut and grooved their surfaces. Shales and 
 schists have gone to pieces and, with impurities from 
 limestone, have provided the clayey groundwork. The 
 whole of the material has been at one time embedded 
 in and moved forward by glacier-ice. 
 
 Though Louis Agassiz developed his glacial theory 
 from studies in Switzerland, he possessed an imagina- 
 tion that ran before the knowledge of his time. Swiss 
 glaciers are now so limited that they are of very 
 little use to us when we seek to explain the origin 
 of boulder-clay. In arctic and antarctic lands, how- 
 ever, we meet with continental glaciers, many miles 
 
v] CLAYS, SHALES, AND SLATES 99 
 
 in width, moving across lowlands, in virtue of the 
 pressure from some great snow-dome, to which 
 additions are continually being made behind them. 
 
 Fig. 12. ARCTIC GLACIER charged with stones and clay. Side of the 
 Nordenskiold Glacier, Billen Bay, Spitsbergen. The top of the 
 glacier appears in the left-hand upper corner of the picture. 
 
 Even when fed by diminished snow-fields, like those 
 in Spitsbergen, these glaciers dominate the landscape 
 and form the principal rock-masses over hundreds of 
 
 72 
 
100 ROCKS AND THEIR ORIGINS [OH. 
 
 square miles. Such glaciers gather into their lower 
 portions all the loosened material on the hill-slopes 
 and valley-floors. With the tools thus supplied, 
 further material is plucked from jointed or fissile 
 rocks as the mass moves forward. Freezing and 
 thawing at the base of the great ice-sheet, as water 
 flows here and there beneath it, further disintegrate 
 the rocky floor. The broad ice-sheet sinks in a mass 
 of broken rock and sludge at one point, and at 
 another drags this mixed material forward as an 
 abrading agent. The lower half of such a glacier, or 
 the whole thickness of it near its front, where surface- 
 melting has removed the higher layers, is in reality 
 an agglomerate of stones and mud held together by 
 an ice-cement (Fig. 12). When an epoch of advance 
 is over, when the ice-sheet stagnates and its frozen 
 constituent melts away, it becomes more and more 
 like a boulder-clay as time goes on. True boulder- 
 clay then forms its surface, while ice remains plentiful 
 below. Since the stony matter is not evenly dis- 
 tributed, some parts of the surface sink more quickly 
 than others, through loss of a greater portion of 
 their former bulk. Roughly circular pits or " kettle- 
 holes " appear, in which water gathers. The water- 
 running from these washes across a part of the 
 boulder-clay, bears off the mud, and leaves bands 
 of sand and gravel. The clayey portion thus removed 
 may accumulate as a fine deposit in other outlying 
 
iv] CLAYS, SHALES, ANB v SLArS.< lt)i: 
 
 pools, and is interstratified, when the flow of water 
 is temporarily increased, with coarser and more sandy 
 layers. Ultimately, the frozen water of the ground- 
 
 Fig. 13. ARCTIC GLACIER AND BOULDER- CLAY. The Sefstrom Glacier, 
 Ekman Bay, Spitsbergen, in 1910, with boulder-clay in fore- 
 ground, marked by kettle-holes, and deposited by an advance 
 of the glacier over Cora Island in 1896. 
 
 work drains away, and only the stones and clay of 
 the ice-sheet remain upon the field. They form, 
 however, a very important residue, weathering in 
 
i02 BOOKS AND THEIR ORIGINS [CH. 
 
 steep cliffs and pinnacles in the dry air of the arctic 
 lands. The boulder-clay thus left shows a sharply 
 marked boundary where the edge of the stagnating 
 ice-sheet lay. It is, in fact, the surviving part of the 
 complex sheet, and now undergoes moulding, like 
 other rocks, by atmospheric agencies (Fig. 13). 
 
 Many interesting features of the hills called 
 drumlins cannot be discussed here. Their arrange- 
 ment with their longer axes in the direction of the 
 movement of the ice shows that they were moulded 
 in large measure within the ice itself, and came to 
 light as it melted away from above downwards. They 
 may be regarded as originating in tough and mixed 
 materials, ice and stones and clay, from the lower 
 layers of the ice-sheet, which became associated with 
 the purer upper ice in certain episodes of the flow. 
 Such mingling may occur at an ice-fall, or where 
 shearing over an obstacle takes place. In the former 
 case, the upper ice descends into the lower layers ; 
 in the latter, masses from below are pushed up into 
 higher levels. As the forward flow proceeds, the 
 masses representing the lower and stone-filled layers 
 are treated just as "eyes" of coarser material are 
 treated in a fluidal lava or in a rock deformed by 
 metamorphic pressures. The purer and more plastic 
 ice moves past and round them, and they assume an 
 elongated form (5&). When final stagnation and melting 
 have gone on, these masses are still separated from 
 
v] IGNEOUS ROCKS 103 
 
 one another as rounded hills. Their bases have 
 settled down upon the ice-worn surface, but their 
 flanks and crests retain traces of the moulding action 
 of the purer portions of the complex body styled an 
 ice-sheet 
 
 In recent years great interest has been aroused 
 by researches on boulder-clays of ancient date, es- 
 pecially those of Permo -Carboniferous age(57). These 
 compacted deposits contain abundant striated 
 boulders, and rest on glaciated rock-surfaces, which 
 have a surprisingly modern aspect when laid bare 
 by denudation. The grey-green Dwyka Conglomerate 
 that is so widely spread throughout South Africa 
 forms " kopjes " on the borders of the Great Karroo, 
 with spiky crests and irregularly weathered cliffs ; 
 but its original deposition as a boulder-clay has been 
 amply verified. It has now, moreover, been paralleled 
 by a very similar rock discovered by A. C. Coleman 
 in the Huroniau beds of Canada. 
 
 CHAPTER V 
 t 
 
 IGNEOUS ROCKS 
 INTRODUCTION (58) 
 
 IGNEOUS rocks, those varied masses that have 
 consolidated from a state of fusion, attracted atten- 
 tion in the eighteenth century through their active 
 
104 ROCKS AND THEIR ORIGINS [OH. 
 
 appearance in volcanoes. James Hutton in 1785 
 showed that the crystalline granite of the Scottish 
 highlands "had been made to invade that country in 
 a fluid state." More than a hundred years, however, 
 elapsed before geologists on the continent of Europe 
 were willing to connect superficial lavas with the 
 materials exposed by denudation in consolidated 
 cauldrons of the crust. 
 
 It is interesting therefore to note that G. P. Scrope 
 in 1825 treated of granite, without apology or hesita- 
 tion, in a work entitled " Considerations on Volcanoes." 
 So far from separating deep-seated from superficial 
 products, Scrope wrote of the molten magma in the 
 crust as "the general subterranean bed of lava." He 
 conceived this fundamental magma, "the original or 
 mother-rock," to be capable of consolidating as 
 ordinary granite. Successive meltings and physical 
 modifications of this granite gave rise, in his view, to 
 all the other igneous rocks. Scrope laid no stress, 
 however, on chemical variations within the magma, 
 but urged that the transitions observable between 
 different types of igneous material established a 
 community of origin. 
 
 The connexion between lavas and highly crystalline 
 deep-seated rocks, so simply accepted by Scrope, was 
 worked out some fifty years later by J. W. Judd 
 for areas in Hungary and in the Inner Hebrides. 
 The features displayed in thin sections under the 
 
v] IGNEOUS ROCKS 105 
 
 microscope were used by JudcJ, in a series of papers, 
 to substantiate his views ; but in France and Germany 
 these features became the source of subtle distinctions 
 between the igneous rocks of Cainozoic and pre- 
 Cainozoic days. The lavas, in which some glassy 
 matter could be traced, were said to be typically 
 post-Cretaceous, and essentially different from those 
 earlier types in which glass was replaced by finely 
 crystalline matter; while the coarsely crystalline 
 igneous rocks were uniformly regarded as pre- 
 Cainozoic. Glassy rocks, such as pitchstone, inter- 
 bedded contemporaneously in Permian or Devonian 
 strata, were described as "vitreous porphyries," while 
 those known to be of post-Cretaceous date might be 
 styled andesites, trachytes, or rhyolites. Luckily 
 common sense has recently triumphed in this matter, 
 and the relative scarcity of glassy types of igneous 
 rocks in early geological formations has been recog- 
 nised as due to the readiness with which glass under- 
 goes secondary crystallisation. The discussion has 
 ended by showing that we have no evidence of 
 world-wide changes in the types of material erupted 
 during geological time. 
 
 At the present day, attention has been focused on 
 the processes that go oil in subterranean cauldrons, 
 in the hope of explaining the differences between one 
 type of extruded rock and another. Doctrines of 
 descent have been elaborated, and one of the most 
 
106 ROCKS AND THEIR ORIGINS [OH. 
 
 subtle systems of classification (59) has been based 
 upon characters that the igneous rock might have 
 possessed, had circumstances not imparted others to 
 it during the process of consolidation. The principle 
 of this classification is, however, obviously correct, if 
 we wish to trace back a rock bearing certain characters 
 at the present day to the molten source from which 
 it came. 
 
 CHARACTERS OF IGNEOUS ROCKS 
 
 The characters of igneous rocks vary considerably 
 according as they have consolidated under atmos- 
 pheric pressure only, or under that of superincumbent 
 rocks. We must remember also that submarine lavas 
 have to sustain a pressure of an extra atmosphere for 
 every thirty feet of depth, or 400 atmospheres at 
 2000 fathoms, and that such rocks have a claim to be 
 regarded as deep-seated. The gases that igneous 
 rocks contain, probably as essential features of the 
 molten magma, and at a temperature above their 
 critical points, escape to a large extent near or at the 
 surface of the earth. The bubbles raised in lava, 
 whereby it is rendered scoriaceous, and the clouds 
 of vapour rising from cooling lava-flows and from the 
 throat of a volcano in eruption, are sufficient evidences 
 of this process. The extremely liquid lavas of Kilauea 
 in Hawaii, which emit very little vapour, are notable 
 as exceptions. In the case of masses that cool 
 
v] IGNEOUS ROCKS 107 
 
 underground, the retention of gases, and ultimately of 
 liquids, until a very late stage of consolidation retards 
 crystallisation until temperatures are reached lower 
 than those at which it starts in surface-flows. As 
 A. Harker points out(o), "the loss of these substances, 
 by raising the melting-points in the magma, may be 
 the immediate cause of crystallization, quite as much 
 as any actual cooling." 
 
 The formation of crystals in lavas is rapid, and 
 the average crystals are therefore small, and often 
 felted together in a mesh, the interstices of which are 
 filled by residual glass. 
 
 Long maintenance of the temperature appropriate 
 to the growth of the mineral species is the important 
 factor that affects the size of crystals. Pressure may 
 promote crystallisation, by raising the melting-points 
 of minerals; but, after a certain maximum effect in 
 this direction, it is quite possible that an increase 
 of pressure may actually lower the melting-points, 
 and cause one or other mineral to remain in solution 
 in the magma. It is not clear how pressure can affect 
 the size of any constituent, except by bringing about 
 conditions under which it can go on growing, while 
 other constituents remain in solution, or do not grow 
 so fast. 
 
 Such conditions may arise from the aid given by 
 pressure to the retention of what French geologists 
 have called agents min^ralisateurs. Several familiar 
 
108 ROCKS AND THEIR ORIGINS [CH. 
 
 minerals, for instance albite, orthoclase, and quartz, 
 require the presence of water for their formation. 
 Volatile substances, not utilised in the ultimate 
 product, no doubt similarly assist the formation of 
 many rock-forming minerals. Occasionally, moreover, 
 as in the development of the micas and certain of the 
 silicates known as zeolites, some proportion of hydro- 
 gen is retained by minerals thus crystallising from 
 the magma. Micas appear to require the presence of 
 fluorine for their development. J. P. Iddingsceo, how- 
 ever, lays stress in this case on the chemical activity 
 of hydrogen at high temperatures. 
 
 Igneous rocks, unless cooled with singular rapidity, 
 thus contain crystals of various kinds. In lavas, these 
 may form the globular aggregates known as spheru- 
 litesm, or may accumulate as a compact ground of 
 minute grains and needles, not quite resolvable with 
 the microscope. In many rocks of slightly coarser 
 grain, a compact lithoidal or stony texture is set up, 
 which the microscope resolves into an aggregate of 
 crystalline rods or granules. Such compact rocks are 
 often styled felsitic. In other types, as in ordinary 
 granite, the constituent minerals are easily dis- 
 tinguished with the naked eye. 
 
 The order in which these constituents have 
 developed is sometimes clear from the inclusion of 
 one mineral in another. When two substances are 
 dissolved in one another, there is a certain proportion 
 
v] IGNEOUS ROCKS 109 
 
 between them, varying with the substances, which 
 allows them to crystallise at the same time, instead 
 of in succession. This eutectic proportion, when 
 attained by two mineral substances in a magma, 
 brings about a complete interlocking of their crystals, 
 as is seen in the quartz and alkali-felspar of the rock 
 known as "graphic granite." The order of crystallisa- 
 tion of minerals from an ordinary non-eutectic magma 
 is profoundly affected by the proportions in which 
 their constituents are present in the mass. 
 
 The minerals, when they have separated out, are 
 found to be mostly silicates. A few oxides, such as 
 rutile, magnetite, and ilmenite, may occur, the two 
 latter being especially common where iron is an im- 
 portant constituent of the rock. But almost all 
 igneous rocks consist largely of one or more species 
 of felspar, silica being here combined with alumina, 
 potash, soda, and lime. Free silica may remain, and 
 separates as quartz, or rarely as tridymite. Pale 
 mica occurs in many rocks of deep-seated origin. In 
 contrast with these light-coloured minerals, iron, mag- 
 nesium, and part of the calcium, appear in another 
 series of silicates, usually dark in colour, and this 
 series may be broadly styled "ferromagnesian." The 
 pyroxenes, of which augite is the type, the amphiboles, 
 of which hornblende is the type, dark mica (mostly 
 biotite), and olivine, are the ordinary ferromagnesian 
 minerals. 
 
110 ROCKS AND THEIR ORIGINS [OH. 
 
 Broadly, then, igneous rocks divide themselves 
 by texture into (i) those which are completely 
 crystalline, and in which the minerals are distinctly 
 visible ; (ii) those which are completely crystalline, 
 but in which the crystals are so small as to give rise 
 to a compact lithoidal ground-mass ; and (iii) those 
 in which some glass is present. The third group may 
 appear lithoidal, or in other cases actually glassy, to 
 the unaided eye. 
 
 This mode of division is justified from a natural 
 history point of view. The first group includes rocks 
 that have consolidated slowly underground. The 
 second includes rocks cooled more quickly, on the 
 margins of magma-basins, or as offshoots from them, 
 filling cracks in the surrounding rocks, and producing 
 wall-like masses known as dykes. The third group 
 appears mostly in dykes and lava-flows. 
 
 Where a dyke has intruded among heated rocks 
 and undergoes no sudden chilling, it may become 
 coarsely crystalline, even though comparatively small. 
 Some dykes exhibit a chilled margin .of glass along 
 their bounding surfaces, and are none the less com- 
 pletely crystalline at the centre, where cooling has 
 been slow. No structure is peculiar to dyke-rocks, 
 nor can a class be established for such rocks on 
 chemical or mineralogical grounds, even though a 
 few special types of igneous rock may at present 
 be known only among these minor intrusive bodies. 
 
v] IGNEOUS ROCKS 111 
 
 The fine-grained layers of volcanic dust, commonly 
 spoken of as ash, and the coarser tuffs, in which 
 
 Fig. 14. SIDE OF A VOLCANIC CONE. A.sh-layer of 1906 on the west flank 
 of Vesuvius. Cliffs of the exploded crater of Monte Somma behind. 
 
 lumps of scoriaceous lava are clearly visible, bridge 
 the gap between sedimentary and igneous rocks. 
 
112 ROCKS AND THEIR ORIGINS [CH. 
 
 The dust, during a great eruption, is distributed by 
 wind over hundreds of square miles of countiy. The 
 tuffs, deposited nearer the orifice of the volcano, vary 
 in coarseness from day to day, and exhibit marked 
 stratification. Ash-beds and tuffs may be laid out 
 in lakes or in the sea, and their layers may then 
 include organic remains. Waves may round their 
 particles on the shore, and may sift them till only a 
 coarse volcanic sand remains. 
 
 After an eruption, the newly deposited ash and 
 tuff usually form obvious layers on the surface of 
 the country. Landslips on the side of the volcanic 
 cone may reveal sections of the new coating and of 
 previously stratified material (Fig. 14). In certain 
 districts, sedimentary and other rocks torn off from 
 below form a large part of the fragmental deposits 
 of volcanic action. The characteristic volcanic cone 
 is itself due to the greater accumulation of tuffs and 
 ashes near the vent (Fig. 15). 
 
 The loose tuffs formed of scoriae allow water to 
 percolate easily through them, and a cone of fairly 
 coarse material resists the weather well. The re- 
 markable freshness of the extinct " cinder-cones " 
 of Auvergne was long ago thus explained by Lyell. 
 Surfaces of ash, on the other hand, are easily washed 
 down by rain in the form of dangerous mud-flows, 
 which spread across the lowlands, and give rise to 
 compact clays, shrinking as they dry. 
 
v] 
 
 IGNEOUS ROCKS 
 
 113 
 
 Lava- flows are masses of molten rock that have 
 welled out from the vent, without being torn to 
 
 Fig. 15. TUFF-COXE WITH TUFF-BEDS at the base. 
 Puy de la Vache, Puy-de-D6me, France. 
 
 pieces by the explosion of the gases that they con- 
 tained. The rapidity of their flow depends on their 
 
 c. 8 
 
114 ROCKS AND THEIR ORIGINS [CH. 
 
 chemical composition, on the amount of gases present, 
 and on the temperature at which they are extruded. 
 The more highly siliceous lavas, for a given tempera- 
 ture, are more viscous than those towards the basaltic 
 end of the series, which contain only about 48 per 
 cent, of silica. A lava of considerable fluidity will 
 consolidate in somewhat thin sheets with smooth and 
 ropy surfaces. A less fluid type will become markedly 
 scoriaceous, where the vapours endeavour to escape 
 from it ; the rugged crust formed on its upper cooling 
 surface will be broken up by the continued movement 
 of the more liquid mass below, and the blocks thus 
 formed may become rolled over the advancing front 
 of the flow and entombed in the portion that has 
 not yet consolidated. 
 
 The surface of ordinary lava-flows remains rough 
 for centuries, and only slowly crumbles down before 
 weathering to form a soil. While tuff-beds provide 
 light and fertile lands, the lava-streams remain 
 marked out among them, as sinuous bands of rock, 
 given over to an irregular growth of woodland. By 
 repeated outflows, lavas tend to fill up the interspaces 
 between the earlier streams, just as these have filled 
 up the hollows in the country over which they 
 spread. A uniform surface thus arises, and lava- 
 plains eventually bury a varied land of hill and dale. 
 Where a number of small vents have opened, perhaps 
 along parallel fissures in the earth, the flooding of 
 
v] IGNEOUS ROCKS 115 
 
 the country with igneous rock may lead to an ap- 
 pearance of stratification in masses extending over 
 hundreds of square miles. Sections in the igneous 
 series, however, show that the individual flows dove- 
 tail into and overlap one another, more rapidly than 
 is the case with the lenticular masses that constitute 
 an ordinary sedimentary series. 
 
 After the constituents of the lava have begun to 
 crystallise, and when the rock may be considered 
 solid, cracks due to contraction are set up. The 
 upper part of the flow, radiating its heat and parting 
 with its gases into the air above, solidifies com- 
 paratively rapidly, and cracks arise without much 
 regularity. Now and then, columnar structure, like 
 that of dried starch, appears on a small scale, the 
 columns starting from various oblique surfaces of 
 cooling, and lying in consequence in various directions 
 in the rock. 
 
 J. P. Iddings shows that curvature of the columns 
 will result if one portion of the surface loses heat 
 more rapidly than another. As the contraction- 
 cracks bounding the columns spread inwards, the 
 layer reached by them at any time in the lava will 
 be farther in from a part of the surface where cooling 
 is rapid than it will be from a part where it is slow. 
 Hence the layer in the lava where contractional 
 stresses are producing cracks, i.e. the layer reached at 
 any time by the inner ends of the contraction-columns, 
 
 82 
 
116 ROCKS AND THEIR ORIGINS [OH. 
 
 will be a curved one, and its curvature will increase 
 as it occupies positions more and more removed from 
 the surface of the lava-flow. The axes of the con- 
 traction-columns, as they spread, are perpendicular 
 to this layer, and the columns will thus curve as their 
 development proceeds. 
 
 The base of a massive lava- flow, however, cools 
 under much more uniform conditions, and the 
 columns, stretching upwards from the ground and 
 produced by slow contraction, give rise to the 
 regular prismatic structures long ago known as 
 " giants' causeways." The original Giant's Causeway 
 in the county of Antrim is the lower part of a 
 basaltic flow, exposed by denudation on the shore. 
 Fingal's Cave in Stafla owes its tough compact roof 
 to the preservation of that portion of the flow which 
 cooled downwards from the upper surface. G. P. 
 Scrope(63) long ago observed this dual structure in 
 columnar lavas. 
 
 The columns, or the more irregular joint-blocks 
 that sometimes represent them, are often subdivided 
 by further contraction into spheroids, the coats of 
 which peel off", as the rock weathers, like those of an 
 onion. The curved cross-joints of massive columns, 
 now convex upwards, noAV concave, represent the 
 same tendency towards globular contraction. 
 
 A lava-flow is sometimes divided into large rudely 
 spheroidal masses, which fit into one another, and 
 
v] IGNEOUS ROCKS 117 
 
 which show signs of more rapid cooling on their 
 surfaces. These were particularly observed on the 
 mountains near Mont Genevre by Cole and Gregory (64), 
 who compared the forms to " pillows or soft cushions 
 pressed upon and against one another." It was 
 suggested that these forms were produced by the 
 seething of viscid lavas, masses being heaved up 
 and falling over, and the outer layers having time to 
 cool in a glassy state before they were deformed by 
 contact with others. This pttloiv-structure has been 
 widely recognised, and J. J. H. Teall has remarked 
 how often " pillow-lavas " are associated with radio- 
 larian cherts. He regarded them, therefore, as of 
 submarine origin. Sir A. Geikieces), moreover, stated 
 that the spheroidal sack-like structure was produced 
 by the flow of such lavas into water or watery silt. 
 This acute suggestion has now been verified by 
 Tempest Anderson (66), who has observed in Samoa 
 the chilling of the lobes of lava, as they are thrust 
 off into the sea and washed over by the waves. 
 H. Devvey and J. S. Flett(67) have pointed out that 
 pillow-structure commonly occurs in lavas in which 
 there has been a conversion of lime-soda felspars into 
 albite, a change frequent in a series of rocks which 
 they call the "spilitic suite." The importation of 
 soda is attributed to vapours entering soon after the 
 consolidation of the rock, and it is urged that any 
 excess of sodium silicate must have escaped into the 
 
118 ROCKS AND THEIR ORIGINS [OH. 
 
 sea- water in which the pillow-lavas were produced. 
 Hence radiolaria will flourish in the neighbourhood 
 (presuming that a decomposition of the silicate can 
 be brought about), and their remains will in time 
 form flint in the hollows of the lavas. The paper 
 quoted contains numerous references to previous 
 work, and is a suggestive example of how petrographic 
 study may go hand in hand with the appreciation of 
 rocks from a natural history point of view. It is 
 only characteristic of the subject of petrology that 
 G. SteinmamKes) has with equal ingenuity explained 
 the relations between radiolaria and spilitic lavas 
 by reminding us that gravity-determinations show 
 an excess of basic material under the oceans and of 
 lighter material, rich in silica, under continental 
 land. Hence, when deep-sea deposits are crumpled 
 by earth-movements, basic types of rock, graduating 
 even into serpentine, become associated with radio- 
 larian chert, partly as extruded lavas, but usually as 
 intrusive sheets injected at the epoch of mountain- 
 building. 
 
 The characters of igneous rocks in cfyJces, that is, 
 of those types that have consolidated in fissures, 
 resemble in many respects the characters of lava- 
 flows. Chilling being usually equal on both surfaces, 
 glassy or compact types of rock occur on both sides, 
 and the dyke is, as previously observed, more crystal- 
 line in the centre. Columnar structures arise from 
 
v] IGNEOUS ROCKS 119 
 
 both surfaces, the dyke also shrinking parallel to its 
 margins. In the outer layers so formed, the columns 
 are small, and they increase in diameter nearer the 
 centre. In small dykes and veins, the columns may 
 run continuously from side to side; in larger ones, 
 they meet along a central surface, which forms, on 
 weathering, a plane of weakness in the rock. Dykes 
 may thus become worn away, decay spreading from 
 the central region, and leaving the more resisting 
 and more glassy portions clinging to the bounding 
 walls. 
 
 Where, however, the surrounding rocks are more 
 easily worn away than the igneous invader, as very 
 often happens, the dykes stand out on the surface as 
 great ribs and walls. 
 
 The rocks cooled in the deep-seated cauldrons, 
 under what are styled plutonic conditions, have parted 
 with their gases so slowly that they do not show 
 scoriaceous structure. They may become very coarsely 
 crystalline, like many of the Scandinavian granites ; 
 minerals, moreover, may be produced which are 
 unstable or difficult to form nearer the surface. 
 Crystals developed in plutonic surroundings become 
 carried forward when the partially consolidated mass 
 is pressed up to a volcanic orifice, and may undergo 
 resorption on the way. Many, however, escape, and 
 impart a porphyritic structure to lavas. The deep- 
 seated rock, from causes that promote the growth of 
 
120 ROCKS AND THEIR ORIGINS [CH. 
 
 one mineral and the retention of another in solution, 
 may also become " porphyritic " in situ, smaller 
 crystals, or even a eutectic intergrowth, finally filling 
 in the ground. 
 
 The viscidity of igneous rocks may cause any of 
 the types to show a fluidal structure. Constituents 
 already formed become dragged along in parallel 
 series as the mass moves forward. Sometimes a group 
 of spherulites, or a knot of "felsitic" matter caused 
 by the dense growth of embryo-crystals, is stretched 
 out into a sheet, and on fractured surfaces a landed 
 structure characterises the mass. These banded rocks 
 record, in their crumpled and obviously fluidal layers, 
 the formerly molten condition of the mass. Even 
 completely crystalline rocks may show parallel 
 arrangement of their minerals, owing to flow during 
 the last stages of consolidation, or to pressure from 
 the walls of the cauldron, influencing the positions 
 taken up by crystals that possess a rod-like or platy 
 form. 
 
 The conspicuously banded structures in some 
 crystalline rocks that are often grouped with the 
 metamorphic gneisses may, however, be best explained 
 by their composite .origin, and the history of the 
 structure is easily determinate in the field. A 
 common case arises where a granite magma, perhaps 
 already bearing crystals, is intruded, under pressure 
 operating from a distance, into a well-bedded series 
 
v] 
 
 IGNEOUS ROCKS 
 
 121 
 
 of sedimentary rocks. The sediments open up like 
 the leaves of a book and admit the invader along 
 
 Ik 
 
 - - v^5*^3y 
 
 I 
 
 Fig. 16. GRANITE INVADING MICA-SCHIST. Clifton, near Cape Town. 
 Adjacent sections were studied by Charles Darwin (see p. 156). 
 
 their planes of stratification. Even limestone may 
 thus become interlaminated with an igneous rock, 
 
122 ROCKS AND THEIR ORIGINS [CH. 
 
 and basalt has been known to separate the annual 
 rings of trees involved in it. This intimate ad- 
 mixture permits of extensive mineral changes, and 
 the two types of rock, probably very different in 
 geological age, become welded together into a com- 
 posite gneiss, both members of which have influenced 
 one another by contact-metamorphism, often across 
 a wide stretch of country (Fig. 16). 
 
 Intrusive igneous rocks in the field will, however, 
 ordinarily prove their character by cutting somewhere 
 across the prevalent structure of the district. When 
 the materials that elsewhere form dykes penetrate 
 between strata for considerable distances as intrusive, 
 sheets, they may yet be traced to some point where 
 they have made use of a crack across the bedding. 
 The necks or plugs of old volcanic centres sometimes 
 seem to occupy orifices drilled, or rather shattered, 
 by explosion right through the overlying obstacles. 
 The approximately circular necks in South Africa, 
 filled by brecciated masses of serpentinous rock, are 
 notable examples. The underground cauldrons them- 
 selves, when brought to light by denudation, are 
 represented by regions of crystalline rock, which may 
 have various relations to their surroundings. We 
 may trace, in every case, upon their margins the 
 ramifying veins that first proved to James Hutton 
 that granite was younger than the rocks among which 
 it lay. But the portion exposed may be merely the 
 
v] IGNEOUS ROCKS 123 
 
 top of a huge body or batliolite of igneous matter, 
 stretching far down into the crust ; or it may be part 
 of a localised knot, which filled up some cavity 
 provided for it by earth-movement, oozing in step by 
 step as room was made for its advance. In the latter 
 case, it was originally bounded above by some series 
 of strata which was arched up as a dome or as an 
 anticline. Or possibly strata have been moved apart 
 from one another, the upper ones sliding over the 
 lower ones and at the same time bulging upwards, so 
 as to leave a cavity of roughly hemispherical form. 
 Such a space, allowing relief from pressure, will be 
 occupied by igneous rock, which may or may not 
 have a direct root through the stratum underneath it. 
 The igneous mass may in such cases be merely an ex- 
 pansion of a large intrusive sheet. It sends off veins 
 into the roof above, and can only be distinguished from 
 a batholite by the presence of stratified rock beneath 
 it. Occurrences of this kind were first described in 
 the Henry Mountains of Utah by G. K. Gilbert, who 
 gave them the name of " stone-cisterns " or laccoliths, 
 a word now commonly written laccolites. It may be 
 questioned if the expansion of the gases in the 
 intruding igneous rock is sufficient in itself to form 
 the laccolitic dome. The igneous rock has probably 
 been pressed into position by the forces that produced 
 the earth-movements. 
 
 In many cases, batholites seem to have worked 
 
124 ROCKS AND THEIR ORIGINS [OH. 
 
 their way upwards without any relation to earth- 
 movements in the district. The processes by which 
 they come into place among other rocks are worthy 
 of separate consideration. 
 
 THE INTRUSION OF LARGE BODIES OF IGNEOUS ROCK 
 
 Attention has been already called to the composite 
 gneisses formed by the intrusion of an igneous magma 
 between the leaves, as it were, of sediments. Such 
 occurrences are often seen on the margins of batholites 
 or of any kind of igneous dome, and they no doubt 
 represent the picking oft' of layer after layer from the 
 walls surrounding the intrusive mass. If these layers 
 can become absorbed into the igneous rock, the crest 
 of the dome can advance, and the dome itself can 
 widen, so long as sufficient heat is supplied to it from 
 below. Space is found for the intrusive mass at the 
 expense of the marginal rocks ; but it is obvious that 
 the portions absorbed merely add to the bulk of the 
 igneous material. The composition of the latter must 
 also undergo modification. Its great size, reaching 
 as it does far down into the crust, in comparison with 
 the quantity of matter absorbed in the upper regions, 
 may render such modification very difficult to trace 
 beyond the latest zone of contact. 
 
 Petrologists differ very widely as to the extent to 
 which igneous masses assume their place in the upper 
 
v] IGNEOUS ROCKS 125 
 
 regions of the crust by processes of " stoping," absorp- 
 tion, and assimilation. The statement, however, in 
 a recent work that "the assimilation hypothesis" is 
 " still supported by some French geologists " is 
 calculated to surprise those who recognise the trend 
 of modern opinion both in America and on the con- 
 tinent of Europe. Far from the views of A. Michel 
 LeVy, C. Barrois, and A. Lacroix, surviving as an 
 expression of national perversity, they have been 
 supported to a remarkable degree by the observations 
 of Sederholm in Finland, of Lepsius and H. Credner 
 in Saxony, of A. Lawson and F. D. Adams in North 
 America, and by the careful reasoning of C. Doelter(69), 
 based largely on his own experimental work. A. 
 Harkertto) and J. P. Iddingsm) have argued that 
 assimilation is merely a local phenomenon, of little 
 importance in the theory of igneous intrusion. 
 W. C. Brbgger(72), however, who strongly supports the 
 laccolitic view for the Christiania district, expresses 
 himself with far more caution, and leaves the way 
 clear for conclusions as to absorption and mingling of 
 molten products in the lower regions of the crust. 
 
 Doelter lays stress on the influence of high 
 temperature, and especially of the highly heated 
 gases in the igneous rock, in promoting corrosion of 
 the cauldron-walls. He attributes greater power of 
 corrosion to the magmas rich in silica, and agrees 
 with R A. Daly that the rapidly moving basic magmas 
 
126 ROCKS AND THEIR ORIGINS [CH. 
 
 reach the upper layers of the crust in a condition of 
 comparative purity. Daly(73) may be looked on as an 
 extremist in this matter ; but it is hard for those who 
 have studied regions where the deep-seated cauldrons 
 have been cut across by denudation to avoid very 
 large views of igneous absorption. The contact-zones 
 between the igneous mass and the surrounding rocks 
 are often seen merely in cross-section on the flanks 
 of a batholite or laccolite. In the areas of ArchaBan 
 rocks, on the other hand, where prolonged denudation 
 has exposed the zones of repeated interaction over 
 hundreds of square miles on an approximately hori- 
 zontal surface, one may form some idea of the processes 
 that are still effective in the depths. 
 
 G. V. Hawes(74), in 1881, and A. Lawson(io7) at 
 Rainy Lake in 1887, recognised the importance of 
 the process known as " stoping," and J. G. Goodchild 
 (Geol. Mag., 1892, 447), dealt with it very clearly in 
 the Ross of Mull. Cracks in the overlying roof are 
 entered by the magma, blocks are wedged off, and 
 these are ultimately absorbed in the molten mass. As 
 the viscosity of the magma increases during cooling, 
 the blocks last detached may remain embedded in the 
 marginal zone. The remarkable purity of this zone, 
 however, in many cases has raised an obvious diffi- 
 culty; but it has been pointed out (75) that the modified 
 marginal and composite rock may continuously sink 
 down into the depths, aided by any of the causes that 
 
v] IGNEOUS ROCKS 127 
 
 promote magmatic differentiation, while a fairly pure 
 magma, almost of the original composition, is left on 
 the crest of the advancing dome. R. A. Dalyce) has 
 developed the stoping theory with considerable bold- 
 ness. The areas most likely to carry conviction to 
 those who doubt that igneous masses can be intruded 
 at the expense of their surroundings are those where 
 banded gneisses have arisen on a regional scale (see 
 p. 160). 
 
 THE RANGE OF COMPOSITION IN IGNEOUS ROCKS 
 
 The broad division of igneous rocks into those of 
 light colour and of low specific gravity on the one 
 hand and those that are dark and heavy on the other 
 is a very natural one, and Bunsen and Durocher 
 insisted that two magmas were fundamental in the 
 crust. In one of these, the "acid" magma, which 
 gives rise to granites and rhyolites, silica formed about 
 70 per cent, by weight of the ultimate rocks ; in the 
 other, it formed about 50 per cent, and the products 
 are basic diorites, gabbros, and basalts (77). The former 
 group of rocks is rich in alkalies, the latter, the 
 "basic" group, in calcium, magnesium, and iron. 
 The mixture of these more extreme types of magma 
 was held to give rise to what are now called "inter- 
 mediate" rocks. 
 
 Two other views are of course possible. If the 
 composition of the globe was originally uniform, the 
 
128 ROCKS AND THEIR ORIGINS [CH. 
 
 two magmas must have arisen by separation from one 
 of intermediate nature. Hence, in any cauldron in 
 the crust, in place of one of two magmas, an inter- 
 mediate magma may be presumed to exist, and to 
 split up, from various causes, into a number of parts 
 which are separately erupted at the surface. Charles 
 Darwin's (78) remarks as to the sinking of crystals in a 
 cooling magma, and the consequent production of a 
 trachytic and basaltic type in the same cauldron, led 
 the way to a general acceptance of the theory of 
 magmatic differentiation in laccolites and batholites. 
 W. C. Brogger's(79) brilliant explanation of the varia- 
 tion and succession of types of igneous rock in the 
 Christiania district has had a profound influence on 
 workers in other fields, and has perhaps directed 
 attention away from the parallel possibilities of 
 differentiation by assimilation. 
 
 The assimilation theory provides the second 
 possible view above referred to. A magma may 
 be modified by the rocks into which it intrudes, so 
 that a "basic" fluid may become charged with silica 
 from a sandstone, the product crystallising as a 
 granite ; while an "acid" fluid may become so charged 
 with limestone that diorite ultimately results. A. 
 Harker(so) has discussed both theories clearly, with a 
 strong leaning to the acceptance of magmatic differen- 
 tiation in the cauldron as the only important cause 
 of variation. R. A. Daly, on the other hand, goes at 
 
v] IGNEOUS ROCKS 129 
 
 least as far as Lacroix in France in supporting the 
 theory of assimilation. For him, the primitive igneous 
 magma is already basic, and basalts are therefore the 
 prevalent type of igneous rock. They reach us, 
 moreover, from considerable depths. The acid rocks 
 are formed by amalgamation of this magma with 
 siliceous material lying nearer the earth's surface. 
 Igneous rocks exceptionally rich in alkalies, the so- 
 called "alkaline " series, result from the absorption of 
 limestone in the magma ; denser lime-bearing silicates 
 are thus formed, which sink by gravitation, leaving 
 a lighter magma above in which soda has become 
 concentrated. Carbon dioxide liberated from the 
 limestone also plays a part in carrying up the alkalies 
 that might otherwise remain in a lower portion(si). 
 
 E. H. L. Schwarz(82) extends Daly's views with an 
 almost romantic fulness. He holds, with Chamberlin, 
 that the primitive globe resulted from the aggrega- 
 tion of basic meteoritic material. The more siliceous 
 crust arose from the withdrawal of magnesium and 
 iron into the depths by long-continued processes of 
 leaching and gravitation. The melting of this crust 
 produces the acid igneous rocks. Igneous cauldrons 
 originate in the heat due to faulting, or to crumpling, 
 or even to the impact of gigantic meteorites. When 
 a molten magma is locally established, variation occurs 
 in it by assimilation of different types of material 
 round it. 
 
130 ROCKS AND THEIR ORIGINS [CH. 
 
 The balance of judgment as to differentiation and 
 assimilation, which should be regarded as parallel 
 probabilities rather than as rival propositions, is 
 admirably held by C. Doeltercss), whose chapters on 
 this matter can be appreciated by all geologists. 
 
 It is of course possible that differentiation of type, 
 from various causes, has already proceeded so far in 
 the earth's crust as to produce noteworthy contrasts 
 in the rocks erupted in different areas. The interior 
 of our globe, on Chamberlin's planetesimal hypothesis, 
 need not have been uniform in constitution, either at 
 the outset or at any subsequent time. J. W. Judd(84) 
 has called attention to the existence of petrographical 
 provinces, a conception that has been very fruitful 
 in results. These provinces have been grouped by 
 Harkercss) in two branches, characterised respectively 
 by rocks rich in alkalies and by rocks rich in lime. 
 The former branch appears to be associated with the 
 movements of faulting and block-structure, rather 
 than of crumpling, that have produced E. Suess's 
 "Atlantic" type of coast. The rocks rich in lime, on 
 the other hand, are said to be characteristic of areas 
 that have been folded like the countries bordering 
 the Pacific. The names "Atlantic" and "Pacific" 
 have consequently been given to the two branches, 
 but these terms seem too geographical in their 
 suggestion. Dewey and FlettCse) have put forward 
 a third type of magma, giving rise especially to 
 
v] IGNEOUS ROCKS 131 
 
 albite as a primary or secondary constituent, and 
 characterised by the production of pillow-lavas. 
 This type is held to be associated with areas that 
 have steadily subsided, without much folding. 
 G. Steinmann(87), however, has connected the spilites 
 and "ophiolitic" rocks with regions of intense over- 
 folding (see also p. 118). 
 
 So far, there are many cases where it is difficult to 
 assign a petrographic province to one or other of 
 these branches, and the system seems to demand 
 more simplicity within the provinces than nature is 
 prepared to yield. 
 
 Whatever the causes of variation, it is necessary 
 to mark out by names certain kinds of igneous 
 material, and it is generally accepted that the types 
 thus set up are best based on chemical composition. 
 At the same time, the minerals present in the rock, 
 and also its structure, record certain phases of its 
 history, and deserve an important place in any system 
 of classification. The natural history of an igneous 
 rock is concerned with its mode of occurrence, and 
 no isolated specimen can satisfy the geological in- 
 vestigator. In the field, the porphyritic crystals, 
 which have an important influence on the total 
 chemical composition, may be found to be strangers 
 to the magma, and to have been derived from 
 some mass imperfectly absorbed'. The dark flecks 
 and patches in a granitoid rock, so often ascribed, 
 
 92 
 
132 ROCKS AND THEIR ORIGINS [CH. 
 
 somewhat mysteriously, to local " segregation " in the 
 magma, again and again prove to be metamorphosed 
 and minutely injected fragments of foreign rockstss). 
 
 None the less, a broad classification is possible on 
 chemical grounds, and the add, intermediate, basic, 
 and ultrabasic grouping adopted by Judd has been 
 found of great convenience. Among acid rocks we 
 have granite as the coarsely crystalline type, with 
 potassium felspars prevalent and the excess of silica 
 manifest as quartz. The finer grained and sometimes 
 compact types are the eurites, quartz-felsites, or 
 quartz-porphyries. When the rock contains more or 
 less residual glass, we have what are now known as 
 rhyolites, of which ordinary obsidian is the most 
 glassy representative. 
 
 The opposite types, those of the basic group, 
 include, at the coarsely crystalline end, gabbro and 
 basic diorite ; the finely crystalline forms are styled 
 dolerites, and those with a trace of glass, or at any 
 rate very fine-grained and compact, are basalts. 
 Glassy types are naturally rare in this group, owing 
 to the unsuitable chemical composition. 
 
 Between granite and gabbro lie various rocks of 
 intermediate composition, some of them rich in soda 
 rather than in potash. Syenite, granodiorite, and 
 the diorites with a prevalence of soda over lime, are 
 coarsely crystalline types. Compact types of these 
 of course occur. It will be sufficient, however, here 
 
v] IGNEOUS ROCKS 133 
 
 to name the forms with traces of residual glass, which 
 range from trachyte, the type rich in potash, to 
 cmdesite, which connects them with basalt, in a series 
 where lime ultimately predominates over soda. 
 
 In the ultrabasic group are a number of excep- 
 tional types. Olivine often becomes an important 
 constituent, and the rocks then decompose into the 
 soft green or reddish masses known as serpentine or, 
 more properly, serpentine-rock. 
 
 Igneous rocks, owing to their range of mineral 
 composition and of structure, combined with their 
 general hardness, lend themselves to various economic 
 purposes. While the granites, resisting atmospheric 
 attack admirably in a polished state, provide our 
 handsomest building-stones, dolerites and fine-grained 
 diorites, which owe their toughness largely to the 
 interlocked relations of their constituent minerals, 
 serve as our most satisfactory road-metals. 
 
 THE SCENERY OF IGNEOUS ROCKS 
 
 Volcanic landscapes, where activity is very recent 
 or still in progress, present a number of characteristic 
 surface-forms. The cones that have accumulated 
 round the vents surpass all other hills in regularity 
 of outline, and the crater in the summit is often 
 relatively large. Lava-cones may be steep-sided 
 bosses when formed of protrusions of viscid rocks 
 rich in silica, like the remarkable domes in the north 
 
134 ROCKS AND THEIR ORIGINS [CH. 
 
 of Bohemia, or they may present very gentle slopes 
 where fluid basic lavas have been extruded. 
 
 Tuff-cones are liable to be breached on one side, 
 owing to the outflow of lava which the crater-wall 
 could not sustain, and they then assume the form of 
 a mountain in which glacial influences have hollowed 
 out a cirque. 
 
 Rain washes down the loose materials from great 
 volcanic cones, and emphasises the concave curve of 
 the mountain sides, the form that is so beautiful in 
 Fuji-yama in Japan, and which Hokusai, with pardon- 
 able and affectionate exaggeration, reproduced in a 
 hundred illustrations. Ultimately, however, grooves 
 appear on the flanks of the cone, in which permanent 
 streams gather, and the slopes are dissected and worn 
 away. During this process, the tuffs yield steep and 
 fantastic forms, and wall-like dykes weather out 
 among them. The dykes are usually the last features 
 to decay. 
 
 Where the vent has been plugged with lava 
 at the close of its activity, the neck of rock often 
 remains standing above the surrounding country. 
 The site of cone after cone can be picked out in this 
 way in the Cainozoic volcanic areas of central 
 Germany. The jutting crag of trachyte or of basalt 
 has often been seized on as the site of a feudal castle, 
 under which the dependent agriculturists still gather 
 at nightfall in their red-roofed town. The group of 
 
v] IGNEOUS ROCKS 135 
 
 sheer-sided necks in the Hegau in southern Wiirttem- 
 berg, the Hohentwiel, Hohenkrahen, and the rest, 
 form a very striking landscape amid undulating 
 Cainozoic lands. 
 
 The lava-beds that cover wide areas are naturally 
 of basic composition. Basalts thus form enormous 
 plains with rugged surfaces, on which at last a red- 
 brown soil collects. When exposed to denudation 
 from the edge of the region inwards, they develop a 
 marked terrace-structure, through which the rivers 
 cut steep and grim ravines. Grass may grow on the 
 ledges and the tables ; but the scarps, controlled by 
 the well marked vertical jointing of the lavas, remain 
 sharp and prominent, and the rock falls away from 
 these walls in whole columns at a time. This struc- 
 ture is characteristically seen in northern Mull and 
 the adjacent smaller isles, and is still more impressive 
 from the centre to the north of Skye, where the rain- 
 swept terraces covered by grass and bog and scanty 
 oatfields, and the black steps of rock between them, 
 present a scene of strange monotony and desolation. 
 
 In regions less exposed to stormy weather, the 
 lava-plateaus may provide good soils. For instance, 
 after the great seaward scarp of the basalts has been 
 crossed in the counties of Antrim and of Londonderry, 
 the lava-fields, dropped by faults towards Lough 
 Neagh, are seen to be occupied by prosperous farms. 
 In arid countries, however, the savage surface of the 
 
136 BOCKS AND THEIR ORIGINS [cu. 
 
 flows merely becomes modified by red dust and 
 scoriaceous gravel, worn by wind and changes of 
 temperature from the upstanding portions of the land. 
 
 Where a stratified country has been freely invaded 
 by sheets of lava along its planes of bedding, the 
 stratification is emphasised 'in any part exposed to 
 weathering. The resisting igneous rock stands out in 
 scarps along the hills, and marks out any folds that 
 have been formed since the epoch of its intrusion. 
 
 When the beds remain fairly level, and are also 
 uplifted, flat-topped hills are formed by the intrusive 
 sheets, like those that may be carved out of a country 
 flooded over by lava-streams. The crystalline rock, 
 very probably a dolerite, protects what lies below it. 
 The kopjes north of the Great Karroo in the centre 
 of the Cape of Good Hope are thus level on the crest 
 and bounded by a steep wall or Jcrans of rock. 
 
 The edges of similar " sills " of igneous rock have 
 controlled much of the scenery between the Highland 
 border of Scotland and the Tyne. A fine example is 
 the indented scarp of the Great Whin Sill, a sheet of 
 dolerite intruded among the Carboniferous strata of 
 Northumberland. This mass forms a platform for 
 Bamburgh Castle against the wild North Sea, and is 
 traceable south-westward across the country towards 
 Carlisle. North of Hexham, its escarpment is occu- 
 pied by Hadrian's wall, and the town of Borcovicus 
 was planted on the edge, overlooking all Northumbria. 
 
v] IGNEOUS ROCKS 137 
 
 The farmers of North Britain and Ireland have 
 long known upstanding igneous dykes as unprofitable 
 " whinstones." The regularity of direction among 
 dykes over very wide areas points to their intrusion 
 along cracks produced by stretching of the crust 
 Radial grouping of dykes, such as one finds near 
 volcanic necks, or, on a gigantic scale, round Tycho 
 on the moon, may be due to explosive action; but 
 the majority of dykes seem to have followed upon 
 earth-movement. In the north of Ireland, from the 
 coast of Down to that of Donegal, a series of compact 
 rocks of Devonian age occurs in dykes lying almost 
 invariably north and south. The post-Cretaceous 
 dykes of the same region have a still more uniform 
 trend, from north-west to south-east. Such series 
 of dykes modify the scenery of coasts by forming 
 promontories and serviceable piers for boats. 
 
 The offshoots near the surface of a great intrusive 
 mass are far less regular. We are here close to the 
 zone of attack, the " shatter-zone," and the structures 
 or regular fracture-planes of the overlying rock only 
 partially control the position taken up by the in- 
 trusive magma. Irregular knots and bosses appear 
 in place of far-spreading sheets, and a network of 
 crossing veins occurs, instead of a system of co- 
 ordinated dykes. The resulting country is hummocky 
 and broken, and, where the cauldron itself has 
 become exposed, striking contrasts of surface are 
 
138 ROCKS AND THEIR ORIGINS [CH. v 
 
 seen as we pass from the igneous core to the older 
 and frequently stratified rocks upon its flanks. 
 
 Some large bodies of intrusive rock have, however, 
 been formed sheet by sheet, and a bedded sill-like 
 structure is then revealed in them on weathering. 
 Sir A. GeikieCsg) calls attention to this in his description 
 of the heart of the black gabbro mass in Skye. But, 
 as a rule, the continuity of structure in batholites, 
 and their characteristic joint-planes set at angles to 
 one another, cause them to appear as massive blocks 
 in the landscape, untraversed by any regular lines. 
 
 Granite, with its broad tabular jointing, which 
 is often developed parallel to a surface of cooling, 
 forms rounded slopes and domes after long-continued 
 weathering. When reared high into the zone of 
 frost-action, it develops spires and pinnacles, as in 
 the huge " aiguilles " of Mont Blanc. But, as decay 
 goes on, the uniform descent of boulders and sand 
 forms spreading taluses, banked against the lower 
 slopes, while the curving joints, not too closely set, 
 promote a smoothness on the higher lands. These 
 joints, moreover, divide the rock into boulders almost 
 ready-made. Tabular structure sometimes pre- 
 dominates ; but even in this case the exposed ends 
 of the layers soon become rounded, as the felspar 
 crystals pass into a powdery state. Commonly, a 
 rough spheroidal structure prevails, as may be 
 traced in many of the Dartmoor "tors,' ; and the 
 
Fig 17. WEATHERING GBANITE. Lundy Island. 
 
140 ROCKS AND THEIR ORIGINS [CH. 
 
 blocks that slip away through widening of the joints 
 become more and more rounded as their surfaces 
 crumble on the talus (Fig. 17). 
 
 In tropical lands, granite exfoliates under the 
 alternations of clear hot days and clear cold nights, 
 and the joint-structure allows of the formation of 
 great round-backed surfaces, on which spheroidal 
 boulders appear poised. These boulders are the 
 relics of an overlying layer of granite, most of which 
 has slipped away to the hill-foot. Their surfaces 
 crumble, owing to the unequal expansion of the 
 constituent minerals. When the rainy season sets 
 in, the decomposed crust is washed away ; during the 
 dry season it falls off in flakes and powder. In this 
 way the magnificent series of monoliths that surround 
 the grave of Cecil Rhodes in the Matopo Hills have 
 become separated out from a continuous sheet of 
 granite. They stand now like glacial boulders on 
 a surface almost as smooth as that of a roche mou- 
 tonnee (Fig. 18). The landscape for miles around is 
 fantastic with huge fallen masses, and Avith high- 
 perched blocks that seem about to fall. Similar 
 scenery is well known in central India, and exfoliation 
 controls the form of mountain-domes in California 
 and Brazil. J. C. Branneroo) lays most stress on 
 temperature-changes in the surface-zone, and little 
 on original spheroidal jointing, in promoting the 
 exfoliation of the rounded boulders. 
 
IGNEOUS ROCKS 
 
 141 
 
 The basic rocks present far more rugged outlines. 
 When a cauldron occupied by basic diorite or by 
 gabbro comes under denuding action, the numerous 
 
 Fig. 18. GRANITE WEATHERING UNDER TROPICAL CONDITIONS. Rhodes's 
 Grave, Matopo Hills, S. Rhodesia. The blocks like boulders are 
 residues of a sheet of granite that once overlay the hill. 
 
142 ROCKS AND THEIR ORIGINS [OH. 
 
 crossing joints oppose the formation of domes or 
 tables. The weather widens one groove here, another 
 there ; the rock breaks away in angular fragments 
 rather than as a powder over a broad surface, and 
 serrated edges and jagged pinnacles arise along the 
 crests. The diorites among our old metamorphic 
 rocks in Scotland or in Ireland can be recognised on 
 the skyline at considerable distances. Sir A. Geikie, 
 in his " Scenery of Scotland," has made the contrast 
 between granite and gabbro in the centre of the Isle 
 of Skye familiar to all geologists. Here the two 
 types of rock were erupted at no long interval, and 
 they have been exposed to denudation under the 
 same conditions. J. Macculloch dwelt in 18190D on 
 the relative resistance of the gabbro and the rapid 
 disintegration of the granite hills, quaintly remarking 
 of the latter that "the loose stones, by their constant 
 descent from the summits, obscure the rocky surface, 
 covering the sides with long torrents of red rubbish 
 even more unpleasing to the sight than their conoidal 
 forms/' Macculloch noted that the loose blocks in 
 the gabbro region lay much as they had fallen, 
 without the production of a sand. 
 
 In most mountain-chains produced by folding, 
 igneous matter has been forced up as an accompani- 
 ment of the earth-movements. The batholites formed 
 in anticlines, or consolidated masses thrust up from 
 the depths, stand out on weathering among schistose 
 
vi] METAMORPHIC ROCKS 143 
 
 or stratified hills. Their surfaces are marked by 
 accidents, and each peak as it comes into view offers 
 something of a new surprise. The wall of Mont Blanc 
 from the angle near Entreves, and the huge crag of 
 the Matterhorn above the valley of the Visp, have 
 illustrated to every traveller the dominance of igneous 
 masses in the landscape. In our own islands, the 
 granites of Ben Cruachan and Cairn Gorm have 
 resisted long ages of denudation ; an intrusive sheet 
 of finer grain forms the long sheer wall of Cader Idris ; 
 while obsidian lava-flows, now grey and dull and 
 crystalline, have furnished on Snowdon the finest 
 scenery of Wales. The fortress-town of Edinburgh 
 has arisen on the relics of a dead volcano ; and the 
 high moor of Leinster, so long the peril of the English, 
 records an igneous cauldron that was first exposed to 
 denudation at the opening of Devonian times. 
 
 CHAPTER VI 
 
 METAMORPHIC ROCKS 
 INTRODUCTION (92) 
 
 UNDER the term "metamorphism," considered 
 philologically, any change may be included that is 
 undergone by rocks after their original deposition. 
 Van Hise, in his monumental treatise, covers processes 
 
144 ROCKS AND THEIR ORIGINS [CH. 
 
 of cementation and alteration by percolating waters, 
 as well as those larger changes that accompany earth- 
 movement and the transference of rocks into regions 
 of igneous activity. It is, indeed, impossible to draw 
 any just line in this matter; but there is a general 
 agreement that "metamorphie rocks" are those that 
 have been altered by heat or pressure or both, either 
 on a local or a regional scale, with the result that new 
 structures, or new minerals, or both, have arisen in 
 the mass. The efficacy of heat alone or of pressure 
 alone, of contact-metamorphism or of dynamo-meta- 
 morphism, in producing considerable changes has 
 been much debated. Some of the thermal changes 
 have been already referred to in the chapter on 
 igneous rocks. While, moreover, the new structures 
 and the development of mica in ordinary slate bring 
 it into the metamorphic group, we have found it 
 convenient to describe the slates in connexion with 
 common clays. The rocks now to be dealt with give 
 evidence of more extreme changes, and the crystalline 
 character of their constituents is appreciable by the 
 unaided eye. For the most part, then, this chapter 
 treats of gneisses and schists. The wider use of the 
 terms schiste and schiefer on the continent of Europe 
 makes it necessary in most countries to style the 
 metamorphic forms "crystalline schists." 
 
 Over wide areas of certain countries, and some- 
 times when we approach the localised cores of 
 
vi] METAMORPHIC ROCKS 145 
 
 mountain-chains, the rocks show a parallel arrange- 
 ment of their constituents, reminding us of sediments; 
 but their constituents are all crystalline, and they are 
 more interlocked with one another than is the case 
 in ordinary strata. 
 
 Such rocks have long been said to be "foliated." 
 The term was used by G. P. Scrope as far back as 
 1825 ; but this author, in common with most geologists 
 of his day, regarded the mineral folia as resulting 
 from sedimentation. D'Aubuisson de Voisinsoa) had 
 already referred the parallelism of the feuillets of 
 mica in schists to some cause acting on them during 
 the consolidation of the rock from a plastic state ; but 
 it was left for Charles Darwin (94), in his remarkable 
 observations on metamorphic rocks in 1846, to separate 
 clearly foliation from stratification. 
 
 In all cases of metamorphism, we have to bear in 
 mind that the alteration may be both chemical and 
 physical. Substances may have been removed from 
 the rock, others may have been imported. The 
 crystalline constituents that are now present do not 
 necessarily result from the crystallisation of the 
 original materials of the rock. 
 
 MICA AND HORNBLENDE SCHISTS 
 
 Schists are the ordinary foliated rocks of fine or 
 medium grain. The folia are really flattened lenti- 
 cular mineral aggregates, often bent and waved, lying 
 
 c. 10 
 
146 ROCKS AND THEIR ORIGINS [CH. 
 
 on and against one another, with their platy surfaces 
 in parallel planes. They result (i) from the deforma- 
 tion under pressure of objects already present in the 
 rock, such as pebbles or crystals; or (ii) from the 
 development of minerals under pressure during the 
 process of metamorphism, such minerals being allowed 
 greater facilities for growth in directions perpendicular 
 to that from which the pressure is exerted; or (iii) from 
 the development of minerals, notably mica, along the 
 planes of weakness provided by stratification or by 
 cleavage. 
 
 The trend of foliation-planes across a country is 
 often, as Darwin pointed out, remarkably regular; in 
 some cases, it follows that of the stratification, in 
 others that of cleavage. The wrinkling of the folia- 
 tion must be ascribed to subsequent compression, and 
 all the features seen in the "strain-slip" structure of 
 slate (p. 92) are repeated on a somewhat coarser 
 scale in schists. 
 
 Many schists are undoubtedly produced by the 
 contact-metamorphism of shales. On the flanks of 
 mountain-chains, where argillaceous rocks have been 
 arched into domes, and where granite has intruded as 
 a core, the complete passage can be traced from sedi- 
 ment to schist. The clay-rocks lend themselves readily 
 to the production of mica, usually of the pale type. 
 Andalusite, and occasionally sillimanite and kyanite, 
 arise. Andalusite often forms grey prisms of irregular 
 
vi] MBTAMORPHIC ROCKS 147 
 
 outline, resembling slate-pencils, and standing out 
 above the mica on any weathered surface. Alman- 
 dine garnet is almost always present. Quartz occurs 
 in streaks and patches, which resolve themselves into 
 granular aggregates on microscopic examination. The 
 mica imparts a distinct foliation to the mass ; but the 
 original stratification is very often preserved, and the 
 minerals have developed along its planes. Small 
 differences in the constitution of the original strata 
 give rise to different types of schist, interbedded with 
 one another. Andalusite, for instance, may occur 
 only in certain argillaceous layers, while other layers 
 are quartzose, through the presence of original sand. 
 Mica-schist is the commonest type of metamorphic 
 rock. 
 
 Where mineralisation has taken place over a wide 
 area, it may be difficult to say if the foliation-planes 
 in a schist are those of bedding, or of superinduced 
 cleavage, or whether they indicate a sliding move- 
 ment in the mass under pressure, whereby all pre- 
 ceding structures have become obliterated. 
 
 Amphibole-schist, often styled epidiorite, consists 
 of foliated hornblende, or its greener ally actinolite, 
 associated with granular felspar and sometimes with 
 equally granular quartz. The amphibole being usually 
 prismatic, the crystals are found with their longer 
 axes arranged in parallel planes, and often streaked 
 out parallel to one another. Minute wrinklings, due 
 
 '10 2 
 
148 ROCKS AND THEIR ORIGINS [OH. 
 
 to subsequent yielding, are not so frequent as in 
 mica-schists. Amphibole-schists occur commonly as 
 knots and somewhat irregular masses among mica- 
 schists, and represent basic igneous rocks that were 
 interbedded or intrusive in the sedimentary series. 
 The pyroxene of the original rock has become re- 
 crystallised as hornblende, and the felspathic con- 
 stituent has rearranged itself in granular forms. 
 J. J. H. Teallos) has described in interesting detail 
 an example from the older rocks of Sutherland, and 
 his paper contains a useful discussion of problems of 
 pressure-metamorphism. 
 
 AMPHIBOLITES 
 
 Hornblende-schists are often seen to pass into true 
 diorites; but they also have relationships with the 
 more puzzling rocks known as amphibolites. These, 
 again, graduate into pyroxenites, or rocks rich in 
 pyroxene, with granular quartz and triclinic felspar, 
 and into eclogites, which may be defined as pyroxenites 
 with garnet. 
 
 Pyroxene-eclogite, in South Africa, is associated 
 with diamond (96), and fragments of exploded eclogite 
 abound in the igneous vents from which the diamonds 
 are extracted. 
 
 What has been called " pyroxene-granulite " is 
 a dark granular eclogite, including rhombic pyroxene 
 side by side with garnet, and associated, in Saxony 
 
vi] METAMORPHIC ROCKS 149 
 
 and Skye, with igneous intrusions. In both localities 
 it has been shown to result from the inclusion of 
 basic rocks, such as dolerites and gabbros, in a bath 
 of some invading magma. The lens-like form of the 
 Saxon masses, and the occurrence also of sheets of 
 pyroxene-granulite interlaminated with fine-grained 
 granite, were till lately attributed to the rolling-out 
 action of pressure-metamorphism. By what H. Credner 
 calls a complete reversal of opinion, due mainly to 
 the opening of new railway-sections, the granular 
 eclogites of Saxony are now regarded as products 
 of extreme contact-alteration, combined with igneous 
 flow (97). A. Harkeros) similarly points out that ex- 
 amples in Skye are derived from basaltic lavas, into 
 which gabbro has intruded, producing a complete 
 reconstruction of the rock. 
 
 Where a series of igneous rocks and sediments, 
 in some cases already altered by pressure, has been 
 attacked and partly melted up by granite, amphi- 
 bolite-blocks are found as the common residue in the 
 mingled mass. The quartzites and mica-schists of the 
 mantle that overlies the granite dome may have 
 disappeared by st oping and absorption (see p. 126). 
 Rocks rich in amphibole remain, and they commonly 
 contain pyroxene as well as hornblende. In some 
 cases, as in Skye and Saxony, they may be traced 
 to basic igneous rocks; but in others they may be 
 referred with equal certainty to limestone. The 
 
150 ROCKS AND THEIR ORIGINS [CH. 
 
 interaction of the granite magma and the calcareous 
 sediment has produced a silicate rock completely 
 different from either. 
 
 Levy (99) and Lacroix have shown how the amphi- 
 bolites of France may sometimes represent dolerites, 
 sometimes limestones. Their work has recently 
 received striking support from the observations of 
 the Geological Survey of Canada doo). Streaky horn- 
 blende-gneisses over wide areas of Ontario are now 
 attributed to the partial absorption of overlying 
 limestone by what was once regarded as a "funda- 
 mental" granite. The amphibolite blocks have 
 become drawn out into bands that follow all the 
 flow-structure of the invading igneous mass. A small 
 area of the same kind was studied in 1900 in north- 
 west Ireland (101), where a remarkably pure granitoid 
 rock, consisting of quartz and alkali felspar, has 
 become enriched with dark mica at the expense of 
 blocks of amphibolite included in it. 
 
 METAMORPHIC MARBLES AND QUARTZITES 
 
 Some of the changes that convert limestone into 
 crystalline marble have already been referred to on 
 pp. 36 and 54. The presence of mica in limestones 
 may allow of foliation when pressure comes to be 
 applied to them, and calc-scldsts result. The mica 
 may be detrital, or may arise through the meta- 
 morphism of clayey bands ; but it forms weak layers, 
 
vi] METAMORPHIG ROCKS 151 
 
 along which the shearing movements take place 
 which lead to a schistose structure in the mass. 
 Pure granular marble may also occasionally become 
 converted into a calc-schist, by deformation of its 
 crystalline grains along gliding planes within each 
 crystal. 
 
 When we consider quartzites, the same question 
 rises as in the case of crystalline limestones, and it 
 is often difficult to state that a quartzite owes its 
 characters to metamorphism. Microscopic examina- 
 tion sometimes reveals the effects of earth-pressures 
 in the crushed and powdered condition of the larger 
 grains; and no rocks exhibit the power of such 
 pressures in producing structural modifications more 
 strikingly than the coarse quartz-grits that are 
 sometimes involved in regions of dynamic meta- 
 morphism. Pebbles and grains are alike deformed, 
 pressed out along planes of fracture, and finally 
 reduced to bands of powdered quartz. When fels- 
 pathic pebbles occur in these grits, the resulting 
 schistose mass has almost the appearance of a banded 
 igneous rock, and streaky white mica may arise from 
 the alteration of potassium felspar. 
 
 Some sandstones contain sufficient felspar or 
 calcium carbonate to form a flux when they are 
 subjected to thermal metamorphism. At times a 
 glass thus arises between the grains, and reacts upon 
 the original quartz. When the igneous magma has 
 
152 ROCKS AND THEIR ORIGINS [CH. 
 
 melted up a sandstone or a quartzite, blocks of the 
 sediment may remain surrounded by a mixed and 
 recrystallised product from both rocks. Wright and 
 Bailey (102) have studied an example in Colonsay, where 
 a hornblende rock has partly dissolved a quartzite, 
 the residual blocks being surrounded by "halos" 
 of interaction, composed of quartz and alkali felspar. 
 
 GNEISSES 
 
 Gneisses may be broadly defined as banded 
 crystalline rocks in which felspar is visible to the 
 unaided eye. Though this will include many igneous 
 masses, it is doubtful if a more rigid description can 
 be given. Numerous gneisses, in fact, owe their 
 parallel structures to flow while in a molten state. 
 Others are rocks that have been deformed by pressure, 
 and their constituents have become drawn out along 
 planes of solid flow. Where actual shearing has 
 taken place, the minerals in the close neighbourhood 
 of the planes of -movement may become especially 
 modified, ground down, and deformed. The foliated 
 structure may then be marked by the appearance of 
 differentiated bands. Such bands may also arise 
 from the spreading out under pressure of certain 
 large constituents, such as porphyritic crystals of 
 felspar, which produce white bands, or of pyroxene, 
 which will become modified into granular amphibole 
 and will produce dark streaks through the rock. 
 
VI] 
 
 METAMORPHIO ROOKS 
 
 Gneisses may also result from the intrusion of fels- 
 pathic igneous rocks, in sheets of varying thickness, 
 between the layers of a sediment or a schist (Fig. 19) ; 
 
 Fig. 19. COMPOSITE GNEISS. Gartan Lough, Co. Donegal. Frag- 
 ments of mica-schist project from a gneiss, the banding of which 
 follows the foliation planes of the schist. On the right the mass 
 retains less schist and is more granitic. 
 
154 ROCKS AND THEIR ORIGINS [OH. 
 
 or from the intrusion of one igneous rock into another, 
 with varying degrees of interaction and absorption. 
 It has often been presumed that the invaded igneous 
 rock must have been in such cases in a plastic state. 
 The supply of heat within the earth during such 
 processes, and the action of the gases, corroding, as 
 Doelter says, "like a blowpipe-flame/' are, however, 
 clearly sufficient to melt down large blocks, the 
 residue being then carried forward as wisps or bands 
 in the invader. 
 
 Many strikingly banded gneisses are thus of 
 composite origin. Their felspathic granitoid bands can 
 be traced in the field to an igneous source, while their 
 darker and usually micaceous layers can as surely 
 be attributed to the invasion and incorporation of 
 adjacent schists (Fig. 20). But it is quite possible 
 that in rarer cases the banded gneiss is a sedimentary 
 rock which has undergone what Judddoa) has styled 
 "statical metamorphism." The differences in suc- 
 cessive bands are then due to original differences in 
 successive strata; one has yielded a granitic layer, 
 one a layer of quartzite, one, which was more ar- 
 gillaceous, a layer of mica-schist. The bands in such 
 a gneiss record the stratification. 
 
 Gneisses are often described as if they consisted 
 of layers of various minerals, quartz, felspar, and 
 mica, alternating one with another. As a matter of 
 fact, a gneiss may exist in which there is no 
 
VI] 
 
 METAMORPHIC ROCKS 
 
 155 
 
 differentiation into layers; the whole of the con- 
 stituents have been drawn out and elongated, any 
 mica present becoming naturally conspicuous by its 
 
 Fig. 20. COMPOSITE GNEISS formed by intrusion of granite into 
 hornblende- schist. Angno, near Saltsjobaden, Sweden. 
 
156 ROCKS AND THEIR ORIGINS [CH. 
 
 flattened wisp-like forms. The banded gneisses, on 
 the other hand, where layer-structure is obvious, 
 consist in reality of bands of different rock-types. 
 Sometimes all the layers are granitoid, but one band 
 will contain only quartz and felspar, while another 
 will contain the same minerals with an admixture, 
 and perhaps a great predominance, of mica. 
 
 G. P. Scropedo4) made an immense step forward 
 when he realised in 1825 that such banded rocks, 
 " the inferior crystalline zones," might be pushed out 
 of position and "protruded " among others "in a solid 
 or nearly solid state." He goes on, " The protrusion 
 of the foliated rocks, gneiss, mica-schist, clay-slate, 
 etc. was chiefly occasioned by their peculiar structure ; 
 the parallel plane surfaces of their component crystals, 
 particularly the plates of mica, sliding with facility 
 over one another; while the laminar structure of 
 these rocks was in turn increased during this process, 
 the crystals being elongated in the direction of their 
 motion, as in the case of the clinkstones and pearl- 
 stones of the trachytic formation." After this, there 
 was little left for the later advocates of dynamo- 
 metamorphism to put forward. 
 
 While Darwin (105) recognised how the granite at 
 Cape Town had worked its way insidiously between 
 the layers of a schist, it was left for Michel Levy to 
 emphasise the part played by what is called lit-par- 
 lit injection in the making of banded gneiss (sec 
 
vi] METAMORPHIC ROCKS 157 
 
 p. 120). K. A. Lessen, Johann Lehmann, and other 
 distinguished workers in Germany made clear, on the 
 other hand, the effects of pressure in moulding and 
 reforming crystalline rocks, and even in bringing 
 about the crystallisation of certain minerals in a 
 previously sedimentary mass. 
 
 Thedynamo-metamorphic school assumed immense 
 importance from 1884 onwards, the date of the publi- 
 cation of Lehmann's work on "Die Entstehung der 
 altkrystallinischen Schiefergesteine," and for a time 
 the intrusion of igneous masses was held, both in 
 Germany and the British Isles, to have had a merely 
 local significance as a metamorphic agent. Where- 
 ever "regional metamorphism " was spoken of, 
 pressure-effects were held to be predominant. In- 
 deed, the profound modifications that may occur in 
 rocks when lowered into subterranean cauldrons is 
 only now becoming generally realised. The tendency 
 to regard the structures of large masses of gneiss as 
 of necessity due to deformation and shearing in a 
 solid state has, however, passed away doe). 
 
 Pressure-effects are of course clearly traceable in 
 most gneisses, and are of immense importance in 
 many metamorphic areas; but we find again and 
 again that gneissic structure has been injured rather 
 than developed by crushing subsequent to the con- 
 solidation of the rock. In some cases, where this 
 structure is due to igneous flow, which of course often 
 
158 ROCKS AND THEIR ORIGINS [OH. 
 
 took place under considerable pressure, even the 
 puckerings of the stratified or foliated rock which was 
 invaded by the igneous magma have been followed 
 by the invading sheets. In other cases, as in the 
 composite amphibolite gneiss of Canada, or the 
 similar rocks of the Ox Mountains in Ireland, the 
 contortions in the mingled mass are clearly due to 
 the viscid flow of the consolidating invader. 
 
 The growing appreciation of the views on re- 
 current thermal metamorphism that were originally 
 propounded by James Hutton in 1785 has led to the 
 assignment of far younger ages to many masses 
 previously regarded as "fundamental" and Archaean. 
 Some of these rocks are undoubtedly of high an- 
 tiquity, but are found to be intrusive in strata of a 
 late pre-Cambrian series. Others, such as the material 
 of the Saxon laccolite, and the gneisses on the north- 
 east Bohemian border, are now known to be of Upper 
 Paleozoic age. 
 
 THE QUESTION OF A FUNDAMENTAL GNEISS 
 
 Ever since A. C. LawsonuoT) showed in Canada 
 how the Laurentian gneiss had invaded and swallowed 
 up the overlying Keewatin rocks, suspicion began to 
 fall on the doctrine of a "fundamental" gneiss. We 
 may now well ask ourselves the following questions : 
 
 (i) Was there a time in the early history of our 
 globe when schists and gneisses were deposited as a 
 
vi] METAMORPHIC ROCKS 159 
 
 prevalent type of sediment, under conditions which 
 have not since recurred? 
 
 (ii) If so, which of the characters of these pre- 
 Cambrian rocks are original, and which have been 
 acquired through subsequent metamorphism ? 
 
 (iii) On the other hand, is the prevalence of 
 gneiss and schist in early pre-Cambrian groups of 
 rock due to the fact that, the older the rock, the 
 more metamorphism, by recurrent heat and pressure, 
 it is likely to have undergone? 
 
 (iv) We may prefer the theory of Laplace, that 
 the earth is cooling from a molten state; or the 
 planetesimal theory, according to which heat has 
 been developed during the consolidation and con- 
 traction of an agglomerate of solid particles ; yet in 
 either case we must admit that the earth's out^r 
 layers were once nearer to the heated parts of the 
 earth than they are now. Is it not likely, then, that 
 early sediments became frequently immersed in baths 
 of molten matter, and that contact-metamorphism 
 and admixture on a regional scale have produced in 
 them the characters that have been attributed to a 
 fundamental gneissdos)? 
 
 J. J. Sederholmdw) has traced in Finland four 
 groups of Archaean sedimentary material, which have 
 been successively invaded by granite from the depths. 
 The bare wave-swept isles of Spikarna, east of Hango, 
 serve as models of structures that are traceable 
 
160 ROCKS AND THEIR ORIGINS [CH. 
 
 throughout the Baltic lands. The more we regard 
 the oldest gneisses of one region after another, the 
 more we see in them igneous matter that has 
 attempted to assimilate sediments of still older 
 date. The banded structures that have been ap- 
 pealed to as indicating the power of earth-move- 
 ments to cjeform the solid crystalline crust prove, in 
 very many cases, to record the foliation of rocks that 
 were already metamorphosed before the igneous 
 matter spread among them. In some of these cases, 
 this foliation followed planes of original stratification, 
 and we are forced to conclude that true sedimentary 
 structure may after all control the features of a 
 gnarled and contorted fundamental gneiss. We are 
 still far from discovering the primitive crust formed 
 alrout a molten globe, and the brilliant proofs of 
 evolution in the organic world are unmatched by 
 any evidence of the evolution of rock-types during 
 geological time. 
 
 METAMORPHIC ROCKS AND SCENERY 
 
 Metamorphic rocks are usually associated with 
 the scenery of mountain, moor, and forest. The 
 highly altered siliceous masses furnish but indifferent 
 soils. The connexion between metamorphic rocks 
 and earth-crumpling, and their frequent penetration 
 by granite, lead to the production of rugged ridges 
 and high moorlands, among which denudation has 
 
vi] METAMORPHIC ROCKS 161 
 
 cut romantic glens. The schists weather out on the 
 valley-walls along their foliation-surfaces, and scarps 
 arise like those of stratified rocks. The face of such 
 a scarp is broken away in a zigzag and splintery 
 fashion, and the sharp edges of the foliated mass 
 stand out like teeth upon the sky-line. Gneisses 
 associated with the schists present a contrast of 
 smoother surfaces, wherever denudation has been 
 long continued. Foliated diorites and amphibolites, 
 however, may produce wild crags that even overhang ; 
 while recently exposed gneiss, at high altitudes, may 
 give rise to pinnacles and serrated forms. 
 
 Where alternations of quartzite and mica-schist 
 occur, irregularities of the surface are readily main- 
 tained. Heather climbs upon the yellow soils 
 furnished by the schist, and trees may gather in its 
 hollows; but the quartzite stands out bare and 
 dominant In some cases the upturned beds of the 
 latter weather out like dykes across the country. 
 
 AVorn-down plateaus of ancient gneiss, the mere 
 residues of mountain-land, may be seen in the storm- 
 swept levels of the Outer Hebrides, and in the 
 hummocky country, a swelling sea of bare grey rock 
 and peat-filled hollows, that borders all the west of 
 Sutherland. The irregular weathering of mica-schist, 
 and the readiness with which it can be carved by 
 streams, control the bold landscapes of the highlands 
 from the Trossachs to Lough Ness, and thence away 
 
 c. 11 
 
162 ROCKS AND THEIR ORIGINS 
 
 again to the northern sea. Here and there, great domes 
 of intrusive granite rise amid the broken moorlands ; 
 at times, a white cone of quartzite catches the eye 
 with a gleam like that of snow. We may traverse this 
 country as an introduction to the high glacial plateaus 
 and deeply notched seaward slopes of the metamorphic 
 lands of Norway ; or to the contrasts of jagged schists 
 and resisting gneisses that meets us as we near the 
 Alpine core. 
 
 REFERENCES 
 
 (The numbers of volumes are given throughout in thick type; 
 the dates are between brackets, and the page-references follow 
 in ordinary figures.} 
 
 1. Cordier, "Memoire sur les substances dites en masse, qui 
 
 entrent dans la composition des Roches Volcaniques," 
 Journ. de Physique, 83 (1816), 135, 285, and 352. 
 
 2. On specific gravity of mineral grains see especially W. J. 
 
 Sollas, Nature, 43 (1891), 404. 
 
 3. Sorby, Q. Journ. Geol. Soc. London, 14 (1858), 453. 
 
 4. Katzer, " Geologischer Fiihrer durch Bosnien," IX internat 
 
 Geologencongress (1903), 190. 
 
 5. A. W. Rogers, " Geology of Cape Colony," ed. 2 (1909), 401. 
 
 6. Linck, "Die Bildung der Oolithe u. Rogensteine," Neues 
 
 Jahrb. fur Min., 16 (1903), 495. 
 
 7. Daly, "The Limeless Ocean," Amer. Journ. Sci., Ser. 4, 23 
 
 (1907), 104, and "Evolution of the Limestones," Bull. 
 Geol. Soc. Amer 20 (1909), 153. 
 
REFERENCES 163 
 
 8. A. R. Horword, Geol. Mag. (1910), 173 ; and Cole and Little, 
 
 ibid. (1911), 49, with references to literature. 
 "The Atoll of Funafuti," Roy. Soc. London (1904). 
 M. Ogilvie (Gordon), " Coral in the Dolomites," Geol. Mag. 
 (1894), 1 and 49, and later papers. 
 
 11. Gardiner and Reynolds, "The Portraine Inlier (Co. Dublin)," 
 
 Q. Journ. Geol.- Soc., 53 (1897), 532. 
 
 11 bis. J. Walther, "Einleitung in die Geologie als historische 
 Wissenschaft " ; 3ter. Theil. " Lithogenesis der Gegen- 
 wart"( 1894), 707. 
 
 12. H. W. Nichols, Field Columbian Museum, Geology, 3 (1906), 
 
 48. 
 
 13. Skeats, " Limestones from upraised coral islands," Bull. Mus. 
 
 Comp. Zool. Harvard, 42 (1903), No. 2. 
 
 14. See generally W. Meigen, "Neuere Arbeiten iiber die Ent- 
 
 stehung des Dolomits," Geol. Rundschau, 1 (1910), 49. 
 
 1 5. Skeats, "Origin of the Dolomites of southern Tyrol," Q. Journ. 
 
 Geol. Soc., 61 (1905), 97. 
 
 16. Pfaff, "Beitrage iiber die Entstehung des Magnesits u. 
 
 Dolomits," Neues Jahrb. fiir Min., Beilage Bd. 9 (1894), 
 485. 
 
 17. Garwood, "On the origin of the concretions in the Magnesian 
 
 Limestone of Durham," Geol. Mag. (1891), 433. 
 
 18. Skeats, op. cit., ref. 15, p. 135. 
 
 19. J. J. H. Teall, "On dedolomitisation," Geol. Mag. (1891), 513, 
 
 and Rep. Brit. Assoc. ( 1 903). T. Crook, Geol. Mag. (1911), 
 339. 
 
 20. J. S. Howe, " Geology of Building Stones " (1910), 353. 
 
 21. Hinde, "On Beds of Sponge remains in the south of England," 
 
 Phil. Trans. (1885), Pt 2, 427. 
 
 22. Sollas, "On the structure of the genus Catagma," Ann. and 
 Mag. Nat. Hist., Ser. 5, 2 (1878), 361. Also ibid., 6 
 (1880), 447. 
 
 112 
 
1(54 ROCKS AND THEIR ORIGINS 
 
 23. Cayeux, "Etude micrographique des Terrains sedimentaires," 
 
 Mem. Soc. Geol. du Nord., 4 (1897), 443. 
 
 24. Jukes-Browne, "The amount of disseminated silica in the 
 
 Chalk in relation to flints," Geol. Mag. (1893), 545. 
 
 25. Guppy, " Observations of a Naturalist in the Pacific : Vanua 
 
 Levu" (1903), chap. xxv. 
 
 26. Rogers, op. cit., ref. 5, p. 403. 
 
 27. Judd, " On the unmaking of Flints," Proc. Geol. Assoc., 10 
 
 (1887), 217. Also Hintze, " Handbuch der Mineralogie," 
 1 (1906), 1473. 
 
 28. Grund, in Stille's "Geologische Charakterbilder," Heft 3(1910). 
 
 29. Rullmann, in Lafar, "Handbuch der technischen Mykologie," 
 
 3 (1904-6), and refs. in Centralblatt fiir Bakteriologie 
 (1904 and onwards). 
 
 30. Hinde, "Catalogue of Fossil Sponges," Brit. Mus. (1883), 28. 
 
 31. Rogers, "Geology of Cape Colony," ed. 1 (1905), 373. 
 
 32. Ibid., 357. 
 
 33. Lyons, "Libyan Desert," Q. Journ. Geol. Soc., 50 (1894), 534 
 
 and 545. 
 
 34. Victorian Naturalist, 27 (1910), 90. 
 
 35. Sorby, "Structure and origin of non-calcareous stratified 
 
 rocks," Q. Journ. Geol. Soc., 36 (1880), Proc., 63. 
 
 36. Phillips, " Constitution and history of Grits and Sandstones," 
 
 ibid., 37 (1881), 6. 
 
 37. A. Daubree, "Geologic experimentale " (1879), 256. 
 
 38. Phillips, op. cit., ref. 36, p. 26. 
 
 38 bis. J. Barrell shows how wind-borne sand may form a covering 
 to the dry and sun-cracked surface of a lake-deposit ; 
 "Relation between climate and terrestrial deposits," 
 Journ. Geol., 16 (1908), 280. 
 
 39. Lake and Rastall, "Text-book of Geology" (1910), 297. 
 
 Compare C. Lapworth, "Intermediate Text-book of 
 Geology" (1899), 176, and "Geological Structure of 
 N. W. Highlands," Geol. Surv. Scotland (1907). 
 
REFERENCES lii.i 
 
 39 bis. See A. B. Searle, "The Natural History of Clay " (1912). 
 
 40. A. Atterberg, " Die Plastizitat der Tone," Internat Mitt, fur 
 
 Bodenkunde, 1 (1911), 10. 
 
 41. Reade and Holland, "Sands and Sediments," Proc. Liv. 
 
 Geol. Soc. (1903-6). 
 
 4-2. Andrussow, "La Mer Noire," Guide des Excursions, vii me 
 Congres geol. internat. (1897). 
 
 43. B. Smith, " Upper Keuper Sandstone," Geol. Mag. (1910), 
 
 302. Compare F. Cresswell, Trans. Leicester Lit and 
 Phil. Soc. (1910). 
 
 44. J. Murray and A. Renard, " Deep Sea Deposits," Challenger 
 
 Rep/(1891), 231. 
 
 45. Ibid., 234. 
 
 46. Ibid., 229. 
 
 47. Barker, "Slaty Cleavage and allied rock-structures," Rep. 
 
 Brit. Assoc. (1885). 
 
 48. Leith, "Rock Cleavage," Bull. U. S. Geol. Surv., No. 239 
 
 (1905). 
 
 49. Lamplugh, " Geology of Isle of Man," Mem. Geol. Surv. Gt. 
 
 Brit. (1903), 72-86. 
 
 50. Darwin, "Geological Observations on S. America" (184C), 
 
 chap. vi. 
 
 51. Reade and Holland, "Green Slates of the Lake District, 
 
 with a Theory of Slaty Cleavage," Proc. Liv. Geol. Soc. 
 (1900-1), 124. 
 
 52. A. Barker, "On 'eyes' of Pyrites &c.," Geol. Mag. (1889), 
 
 396. 
 
 .~>3. T. N. Dale illustrates an extreme case, "Slate Deposits of 
 U.S.," Bull. U.S. Geol. Surv., No. 275 (1906), 31. 
 
 54. Harker, op. cit., ref. 47, p. 19. 
 
 55. Leith, op. tit., ref. 48, p. 152. 
 
 56. I. Russell, " Glaciers of N. America " (1897), 25. 
 
 57. See, for instance, T. W. Bdgeworth David, "Evidences of 
 
166 ROCKS AND THEIR ORIGINS 
 
 glacial action in Australia," Q. Journ. Geol. Soc., 52 (1896), 
 289. 
 
 58. For general discussions of Igneous Rocks, see J. J. H. Teall. 
 
 "British Petrography" (1888) ; H. Rosenbusch, "Mikro- 
 skopische Physiographic," ed. 4 (1905-7); F. Zirkel, 
 "Lehrbuch der Petrographie," ed. 2 (1894); A. Harker, 
 "Natural History of Igneous Rocks" (1909); J. P. 
 Iddings, "Igneous Rocks," 1 (1909). 
 
 59. Cross, Iddings, Pirsson, and Washington, "Quantitative 
 
 Classification of Igneous Rocks" (1903). 
 
 60. Harker, op. tit, ref. 58, p. 186. 
 
 61. Iddings, op. cit., ref. 58, p. 130 &c. 
 
 62. Ibid., pp. 228-241. 
 
 63. Scrope, "Considerations on Volcanos" (1825), 141. 
 
 64. G. A. J. Cole and J. W. Gregory, "Variolitic Rocks of Mt 
 
 Genevre," Q. Journ. Geol. Soc., 46 (1890), 311. 
 
 65. A. Geikie, "Ancient Volcanoes of Gt Britain," 1 (1897), 25. 
 
 Also C. Reid and H. Dewey, " Pillow lava of Cornwall," 
 Q. Journ. Geol. Soc., 64 (1908), 264. 
 
 66. Anderson, " Volcano of Matavanu," ibid., 66 (1910), 632. 
 
 67. Dewey and Flett, "British Pillow lavas," Geol. Mag. (1911), 
 
 202 and 241. 
 
 68. Steinmann, "Die Schardtsche Ueberfaltungstheorie &c.," 
 
 Ber. nat. Gesell. Freiburg i. B., 16 (1905), 44. 
 
 69. Doelter, " Petrogenesis " (1906), 33 and 109-123. 
 
 70. Harker, op. cit., ref. 58, p. 82. 
 
 71. Iddings, op. cit., ref. 58, p. 280. 
 
 72. Brogger, "Die Eruptionsfolge bei Predazzo," Vidensskab. 
 
 Skrifter (1895), No. 7, p. 152. 
 
 73. Daly, " Secondary origin of certain Granites," Am. Journ. 
 
 Sci., Ser. 4, 20 (1905), 185, with useful references to 
 Bayley and others. 
 
 74. Hawes, " The Albany granite and its contact phenomena, 
 
 ibid., Ser. 3, 21 (1881), 31. 
 
REFERENCES 167 
 
 75. G. A. J. Cole, "Geology of Slieve Gallion," Sci. Trans. K. 
 
 Dublin Soc., 6 (1897), 242. 
 
 76. Daly, " Mechanism of igneous intrusion," Am. Joum. Sci., 
 
 Ser. 4, 15 (1903), 2G9, and later. 
 
 77. For a recent review in favour of this theory, see Loe winson 
 
 Lessing, "The fundamental problems of Petrogenesis," 
 Geol. Mag. (1911), 248 and 289. 
 
 78. Darwin, "Geological Observations on volcanic islands " (1844), 
 
 chap. vi. 
 
 79. Brogger, "Die Eruptivgesteine des Kristianiagebietes " 
 
 (1894 &c.). 
 
 80. Harker, op. cit., ref. 58, chaps, xni and xiv. 
 
 81. Daly, "Origin of the alkaline rocks," Bull. Geol. Soc. Am., 
 
 21 (1910), 108, and Journ. Geol., 19 (1911), 309. Also 
 E. H. Smyth, Am. Journ. Sci., 33 (1913), 33. See, how- 
 ever, H. I. Jensen, as to primitive accumulation of alkalies 
 in the upper layers; "The distribution of Alkaline Rocks," 
 Proc. Linn. Soc. N. S. W., 33 (1908), 521. 
 
 82. Schwarz, " Causal Geology " (1910). 
 
 83. Doelter, op. cit., ref. 69, pp. 71-213. 
 
 84. Judd, " On Tertiary gabbros &c.," Q. Journ. Geol. Soc., 42 
 
 (1886), 54. 
 
 85. Harker, op. cit., ref. 58, p. 90, and Nature (Sept 1911), 319. 
 
 See also Jensen, ref. 81, p. 522. 
 
 86. Dewey and Flett, op. cit., ref. 67, p. 245. 
 
 87. Steinmann, op. cit., ref. 68, p. 64. 
 
 88. See especially W. J. Sollas, " The volcanic district of Car- 
 
 lingford," Trans. R. I. Acad., 30 (1894), 502. 
 
 89. A. Geikie, op. cit., ref. 65, 2, 344 and fig. 348. 
 
 90. Branner, "Decomposition of rocks in Brazil," Bull. Geol. Soc. 
 
 Am., 7 (1896), 255. 
 
 91. Macculloch, " Description of the Western Islands of Scot- 
 
 land," 1 (1819), 267. 
 
168 ROCKS AND THEIR ORIGINS 
 
 92. For general discussions of Metamorphic Rocks, see A. 
 
 Delesse, "Etudes sur le Metamorphisme des Roches" 
 (1858); Lehmann, " Untersuchungen iiber die Entste- 
 hung der altkrystallinischen Schiefergesteine JJ (1884); 
 A. Geikie, "Text-book of Geology" (1903), 764-807 and 
 728 ; Van Hise, " A Treatise on Metamorphism," U. S. 
 Geol. Survey, Mon. 47 (1904); U. Grubenmami, "Die 
 krystallinen Schiefer," ed. 2 (1909) ; A. Geikie and others, 
 " The Geological Structure of the N. W. Highlands of 
 Scotland," Mem. Geol. Surv. Scotland (1907). 
 
 93. D'Aubuisson de Voisins, "Traite de Geognosie" (1819), 1, 
 
 298. 
 
 94. Darwin, ref. 50. 
 
 95. Teall, " Metamorphosis of Dolerite into Hornblende-Schist," 
 
 Q. Journ. Geol. Soc., 41 (1885), 133. 
 
 96. T. G. Bonney, " The parent rock of the diamond in S. Africa," 
 
 Geol. Mag. (1899), 309. 
 
 97. R. Lepsius, " Geologic von Deutschland," 2ter. Teil (1903), 146 
 
 and 169; H. Credner, "Die Genesis des sachsischen 
 Granulitgebirges," Renuntiations-programm (1906). 
 
 98. Harker, "Igneous Rocks of Skye," Mem. Geol. Surv. Scotland 
 
 (1904), 115. 
 
 99. Levy, "Excursion a Aydat," Bull. Soc. geol. France (1883), 
 
 916; "Granite de Flamanville," Bull. Carte geol. France 
 5 (1893), 337. 
 
 100. F. D. Adams, "Haliburton and Bancroft areas," Mem. Geol. 
 
 Surv. Canada, No. 6 (1910), 120. 
 
 101. G. A. J. Cole, "Metamorphic rocks in E. Tyrone and 
 
 S. Donegal," Trans. R. I. Acad., 31 (1900), 453. 
 
 102. W. B. Wright and E. B. Bailey, "Geology of Colonsay,' 
 
 Mem. Geol. Surv. Scotland (1911), 28. 
 
 103. Judd, " Statical and dynamical metamorphism," Geol. Mag. 
 
 (1889), 246. 
 
REFERENCES 169 
 
 104. Scrope, op. cit., ref. 63, p. 234. 
 in."). Danviii, op. cit., ref. 78, chap. vii. 
 
 106. See especially J. Home and B. Greenly, "Foliated Granites 
 
 &c. in E. Sutherland," Q. Jourii. Geol. Soc., 52 (1896), 
 633. 
 
 107. Lawson, " Geology of Rainy Lake Region," Ann. Rep. Geol. 
 
 Surv. Canada for 1887 (1888). 
 
 108. Compare Chamberlin and Salisbury, " College Text-book of 
 
 Geology" (1909), 428, and other works by these authors. 
 109t Sederholm, "Om granit och gneis i Fennoskandia " (with 
 English summary), Bull Comm. geol. Finlande, Xo. -23 
 (1907), and elsewhere. 
 
 TABLE OF STBATIGBAPHICAL SYSTEMS 
 
 QUATERNARY GROUP 
 Post-Pliocene and Recent 
 
 CAJNOZOIC GROUP 
 Pliocene 
 Miocene 
 Oligocene 
 Eocene 
 
 MESOZOIC GROUP 
 Cretaceous 
 Jurassic 
 Triassic 
 
 PALEOZOIC GROUP 
 Permian 
 Carboniferous 
 Devonian 
 
 Gotland! an ( = Silurian or Upper Silurian) 
 Ordovician (or Lower Silurian) 
 Cambrian 
 
 PRE-CAMBRIAN GROUP 
 
INDEX 
 
 indicates that the name is quoted in the list of 
 references, pp. 162-169.) 
 
 Acid igneous rocks, 127, 132 
 Adams, F. D., 125, ref. 100 
 Africa , S. , 148. See Cape of Good 
 
 Hope and Rhodesia. 
 Agassiz, A., 25 ; L., 98 
 Agents mingralisateurs, 107 
 Algae, calcareous, 20, 25 
 Alkaline igneous rocks, 129 
 Alps, 14, 16, 23, 138, 143, 162 
 Ammonites, 23 
 Amphibole-Schist, 147 
 Amphibolite, 148 
 Anderson, T., 117 
 Andesite, 133 
 Andrussow, N., 84 
 Antrim, Co., 46, 135 
 Aragonite, deposition of, 17 ; in 
 
 shells, 22, 86 
 Armitage, 64 
 Ash, 88, 111 
 
 Assimilation in igneous rocks, 128 
 Atlantic and Pacific types of 
 
 igneous rocks, 130 
 Atterberg, A., 82 
 Auvergne, 112 
 Axmouth, 46 
 
 Bacteria, extraction of iron by, 
 
 61 
 
 Bagshot Heath, 73 
 Bailey, E. B., ref. 102 
 Banded structure, 120 
 Barrell, J., ref. 38 bis 
 
 Barrois, C., 125 
 
 Barytes in sandstone, 62 
 
 Basalt, 132, 135 
 
 Basic igneous rocks, 127, 132 
 
 Batholites, 123 
 
 Bavaria, dolomites of, 32 
 
 Belemnites. 23 
 
 Black Sea, 17, 84 
 
 Bohemia, 134, 158 
 
 Bonney, T. G., ref. 96 
 
 Boulder-clay, 96 
 
 Bournes, 43 
 
 Brachiopods, 24 
 
 Branner, J. C., 140 
 
 Brazil, 88, 140 
 
 Breccia, 55 
 
 Brogger, W. C., 125, 128 
 
 Brongniart, A., 2 
 
 Bunsen, E. W., 127 
 
 Cader Idris, 143 
 
 Calcareous Tufa, 14, 16 
 
 Canada, 103, 150, 158 
 
 Canons of Arizona, 47 
 
 Cape of Good Hope, 16, 41, 59, 
 
 63, 103, 121, 136, 156 
 Gausses, 45, 48, 50 
 Cayeux, L., 39 
 Cephalopods, 23 
 Chalk, 20, 42 
 Chamberlin, T. C., 129 
 Chara-limestone, 19 
 Cheddar, 48 
 
INDEX 
 
 171 
 
 Chert, 40, 62 
 
 China-clay, 86 
 
 Christiania district, 125, 128 
 
 Christmas Island, 37 
 
 Clare, Co., 46 
 
 Clay, 78 
 
 Cleavage, 89 
 
 Close, Maxwell H., 98 
 
 Cole, G. A. J., 117, refs. 8, 75, 101 
 
 Coleman, A. C., 103 
 
 Colonsay, 152 
 
 Columnar structure, 115 
 
 Composite gneiss, 122, 153 
 
 Cones, volcanic, 112, 133 
 
 Conglomerates, 70 
 
 Connemara marble, 36 
 
 Contact-metamorphism, 144 
 
 Conybeare, W. D., 35 
 
 Coral-reefs, 25 ; silification in, 40 
 
 Cordier, P. L. A., 3 
 
 Cork marble, 54 
 
 Credner, H., 125, 149 
 
 Crinoidal limestone, 2i 
 
 Crook, T., ref. 19 
 
 Cross, W., ref. 59 
 
 Crush-conglomerates, 28 
 
 Cryptozoon, 20 
 
 Crystallisation from fusion, 107 
 
 Dale, T. N., ref. 53 
 
 Daly, R. A., 18, 33, 125, 127, 128 
 
 Dana, J. D., 30 
 
 Darwin, C., 25, 90, 128, 145, 156 
 
 Daubree, A., 66 
 
 D'Aubuisson de Voisins, 145 
 
 David, T. W. E., ref. 57 
 
 Dedolomitisation, 35 
 
 De la Beche, H., 18 
 
 Delesse, A. , ref. 92 
 
 Derbyshire, 48, 73, 97 
 
 Desert sands 68 71 
 
 Dewey, H., 117, 130 
 
 Diatoms, 40 
 
 Differentiation in igneons rocks, 
 
 128 
 
 Dinaric Alps, 16, 23, 52 
 Diorite, 132 
 
 Doelter, C., 18, 31, 125, 130, 154 
 Dolerite, 132 
 Dolinas, 50 
 
 Dolomite, 12, 26, 29, 30 
 Donegal, Co., 137, 150, 153 
 Down, Co., 74, 137 
 Dreikanter, 71 
 Drumlins, 98, 102 
 Durham, dolomite of, 35 
 Durocher, J., 127 
 Dwyka Conglomerate, 103 
 Dykes, 110, 118, 137 
 Dynamo-metamorphism, 144 
 
 Eclogite, 148 
 Edinburgh, 143 
 Egypt, 22, 64, 68 
 Ehrenberg, C. G., 5, 20 
 Epidiorite, 147 
 Eurite, 132 
 
 Eutectic proportion, 109 
 Exfoliation of granite, 140 
 
 Felsitic structure, 108 
 Ferromagnesian minerals, 109 
 Fiji Is., 40 
 Fingal's Cave, 116 
 Finland, 159 
 Flagstones, 69 
 Flett, J. S., 117, 130 
 Flint, 38, 62 ; gravels, 74 
 Flocculation of clay, 80 
 Flow-cleavage, 92 
 Fluidal structure, 120 
 Foliation, 90, 145 
 Foraminifera, 20 
 
L72 
 
 ROCKS AND THEIR ORIGIN 
 
 Fracture-cleavage, 92 
 Freshwater molluscs, 23 
 Fuji-yama, 134 
 Funafuti atoll, 19, 26 
 Fundamental gneiss, 158 
 Fusulina limestone, 21 
 
 Gabbro, 132, 142 
 Gardiner, C., 29 
 Garwood, E. J., 35 
 Geikie, A., 117, 138, 142 
 Giant's Causeway, 116 
 Gilbert, G. K., 123 
 Glacial gravels, 98 
 Glaciers, arctic, 98 
 Glassy igneous rocks, 110 
 Glauconite, 20, 88 
 Globigerina-ooze, 20 
 Gneiss, 122, 152, 158, 161 
 Gordon, M. Ogilvie, 27 
 Granite, 132, 138 
 Granodiorite, 132 
 Great Salt Lake, Utah, 15 
 Great Whin Sill, 136 
 Greenly, E., ref. 106 
 Gregory, J. W., 117 
 Greywacke, 58 
 Grund, A., 50 
 Guppy, H. B., 40 
 
 Halimeda, 19, 29 
 
 Harker, A., 89, 107, 125, 128. 
 
 130, 149 
 
 Harlech Beds, 74 
 Hawaii, 106 
 Hawes, G. V., 126 
 Hebrides, 116, 135, 152, 161 
 Hegau, the, 135 
 Henry Mountains, Utah, 123 
 Hercegovina, karstland, 14, 52 
 Highlands of Scotland, 76, 143, 
 
 Hinde, G. J., 38, 62 
 Holland, P., 83, 90 
 Hornblende- Schist, 147 
 Home, J., ref. 106 
 Horwood, A. E., ref. 8 
 Howe, J. A., 13 
 Button, J., 41, 104, 122, 158 
 Hydrozoa, 25 
 
 Iddings, J.P., 108, 115, 125, ref. 59 
 
 Igneous Eocks, 103 
 
 India, 140 
 
 Intermediate igneous rocks, 127, 
 
 132 
 
 Intrusion of igneous rocks, 124 
 Intrusive sheets, 122, 136 
 Irish Channel, limestone in, 17 
 Iron-bacteria, 61 
 Iron Pyrites in muds, 85 
 
 Jajce, 16 
 
 Jensen, H. T., ref. 81 
 
 Judd, J. W., 6, 42, 68, 104, 130, 
 
 154 
 
 Jukes-Browne, A., 40 
 Jura Mts., 46 
 
 Kalahari desert, 41, 63 
 Kaolin, 87 
 Karlsbad, 14 
 Karst, 49 
 Katzer, F., 16 
 Kerry, 76 
 Klement, C., 31 
 Knoll structure, 28 
 
 Laccolites, 123 
 Lacroix, A., 125, 150 
 Lake, P., 76 
 Lamellibranchs, 22 
 Lamplugh, G. W., 89 
 Landslips, 46, 94 
 
INDEX 
 
 173 
 
 Lapwprth, C., ref. 39 
 Laterisation, 64 
 Laurentian gneiss, 158 
 Lautaret Pass, 95 
 Lava-flows, 113 
 Lava-plains, 114 
 Lawson, A. C., 125, 126, 158 
 Lehmann, J., 157 
 Leinster granite, 143 
 Leith, C. K.,89 
 Leith Hill, 73 
 Leonhard, K. von, 3 
 Lepsius, R. 125, ref. 97 
 Lessing, L., ref. 77 
 Levy, M., 6, 125, 150, 156 
 Limestones, 12, 150 ; deposited 
 from solution, 14 ; organic. 19 
 Linck, G., 16, 18, 61 
 Lit-par-lit injection, 157 
 Lithoidal structure, 108 
 Lithothamnium, 20, 29 
 Little, O. H., ref. 8 
 Llanberis, 96 
 Loam, 82 
 
 Londonderry, Co., 135 
 Lessen, K. A., 157 
 Lower Greensand, 62, 73 
 Lundy Id., 139 
 Lyons, H. G., 63 
 
 Macculloch, J., 142 
 Magmas, igneous, 127 
 Magmatic differentiation, 128 
 Magnesian limestone, 35 
 Magnesium in organic skeletons, 
 
 29 
 
 Marble, 36, 54, 150 
 Marl, 83 
 
 Martel, E. A., 52 
 Matopo Hills, 140 
 Matterhorn, 143 
 Metamorphic Rocks, 143 
 
 Mica-Schist, 147, 161 
 Millepora, 25 
 Millersdale, 48 
 Minerals, 6, 8 
 Mojsisovics, E., 27 
 Monaghan, Co., 74 
 Mont Blanc, 138, 143 
 Mont Genevre, 117 
 Mull, 135 
 Murray, J., 25 
 
 Nagelfluh, 14 
 
 New Forest, 74 
 
 Northumberland, 136 
 
 Norway, 162 
 
 Nubian Sandstone, 63 
 
 Nummulitic limestone, 21 
 
 Obsidian, 132 
 Old Bed Sandstone, 75 
 Oolitic grains, 15, 17 
 Oolitic Limestone, 18, 40 
 Ophicalcite, 36 
 
 Order of crystallisation of min- 
 erals, 108 
 Ox Mountains, 158 
 
 Paris basin, 40, 74 
 Petrographical provinces, 130 
 Pfaff, 30, 34 
 Phillips, J. A., 64, 67 
 Phillips, W., 35 
 Phosphatic limestone, 36 
 Phosphorites du Quercy, 37 
 Pillow-structure, 117 
 Pipe-clay, 78 
 Pisolite, 15, 18 
 
 Planetesimal theory, 129, 130, 159 
 Plutonic conditions, 119 
 Porosity of sandstone, 66; of 
 
 clay, 79 
 Porphyritic structure, 119 
 
174 
 
 ROCKS AND THEIR ORIGIN 
 
 Portland stone, 18 
 Portrane, ref. 11 
 Purbeck Marble, 54 
 Pyroxenite, 148 
 
 Quartz veins, 56, 65 
 Quartz-felsite, 132 
 Quartzite, 63, 76, 151, 161 
 Quartz-porphyry, 132 
 
 Kadiolaria, 40, 118 
 Ravines in limestone, 48 
 Eeade, T. M., 83,90 
 Red Clay of deep seas, 88 
 Regional metamorphism, 157 
 Reynolds, S. H., 29 
 Rhodesia, 140 
 Rhyolite, 132 
 Richthofen, F. von, 25, 27 
 Ripple-marks, 69 
 Rock, definition of, 7 
 Roestone, 15 
 
 Rogers, A. W., 41, 62, 63 
 Rosenbusch, H., 6, ref. 58 
 Rothpletz, A., 27 
 Russell, I., ref. 56 
 
 Samoa, 117 
 
 Sand-dunes, 62, 69 
 
 Sand-rock, 65 
 
 Sands, origin, 56 ; cementing of, 
 
 60 ; grains, 66 
 
 Sandstones, 56 ; "crystalline," 64 
 Saxony, 148, 149, 158 
 Sea, action of on shore, 58, 87 ; 
 
 calcium carbonate in, 16 
 Searle, A. B., ref. 39 bis 
 Sederholm, J. J., 125, 159 
 Semper, K., 25 
 Serpentine, 133 
 Schists, 145, 161 
 Schwarz, E. H. L., 129 
 
 Scoriae, 112 
 
 Scoriaceous structure, 106 
 
 Scrope, G. P., 104, 116, 145, 156 
 
 Shale, 83, 96 ; colours of, 85 
 
 Sharpe, D., 89 
 
 Shell-marl, 23 
 
 Silicates in igneous rocks, 109 
 
 Silicified wood, 64 
 
 Sills, igneous, 136 
 
 Skeats, E. W., SO, 31, 35 
 
 Skye, 135, 138, 142, 149 
 
 Slate, 88, 96 
 
 Smith, B., 86 
 
 Snowdon, 143 
 
 Sollas, W. J., 38, refs. 2 and 88 
 Sorby, H. C., 5, 64, 66, 89, 90 
 Southern Uplands, 74 
 Spherulites, 108 
 Spilitic lavas, 117, 131 
 Spitsbergen, 81, 99, 101 
 Sponges, siliceous, 38, 62 
 Steinmann, G., 118, 131 
 Stoping process, 126 
 Strain-slip cleavage, 92 
 Stratigraphical systems, 169 
 Sun-cracks, 69 
 Surrey Hills, 43, 73 
 Swallow-holes, 44 
 Sweden, gneiss of, 155 
 Syenite, 132 
 
 Teall, J. J. H., 117, 148 
 Teira rossa, 50 
 Terrace-structure in limestone, 
 
 46 ; in basalt, 135 
 Thames, material in solution, 17 
 Torridon Sandstone, 76 
 Tors, 138 
 Trachyte, 133 
 Travertine, 15 
 Tridacna, 23 
 Trieste, 50 
 
INDEX 
 
 175 
 
 
 Tuff, 111 
 
 Tyrol, dolomites, 26, 31, 53 
 
 Ultrabasic igneous rocks, 132 
 
 Van Hise, C. B., 143 
 Vesuvius, 111 
 Victoria, Australia, 64 
 Volcanic ash, 88, 111; cones, 
 
 112, 133; dost, 111; necks, 
 
 122, 134; tuff, 111 
 
 Walther, J., ref. 11 bis 
 
 Weald, 73 
 
 Weathering in tropics, 64, 140 
 
 West Indies, 18, 37 
 
 Whinstone, 137 
 
 Wright, W. B., 152 
 
 Yellowstone Park, 15 
 Yoredale, 73 
 
 Zirkel, F. von, 6, ref. 58 
 
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