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 B M 175 727 
 
 MOIES OP THE GEOLOGICAL SOYEY. 
 
 ENGLAND AND WALES. 
 
 THE 
 
 ERUPTIVE ROCKS 
 
 OF 
 
 BEEXT TOE AND ITS NEIGHBORHOOD, 
 
 INCLUDED IX 
 SHEET 25 OF THE ONE-INCH MAP OF THE GEOLOGICAL SURVEY; 
 
 WITH SOME 
 
 INTRODUCTORY REMARKS OX THE APPLICATION OF 
 THE MICROSCOPE TO PETROLOGICAL RESEARCH. 
 
 BY 
 
 FRANK RUTLEY, F.G.S. 
 
 PUBLISHED BY OEDEE OF THE LOEPS COMMISSIOXEB3 OF HEE MAJESTY'S TEEASUEY. 
 
 LONDON: 
 
 HINTED FOE, HER MAJESTY'S STATIONERY OFFICE, 
 
 AND SOLD BY 
 
 LONGMAN & Co., Paternoster Row; TRUBNER & Co., Ludgate Hill; 
 
 LETTS & SON, 33, King William Street; 
 EDWARD STANFORD, 5o, Charing Cross; and J. WYLD, 12, Charing Cross 
 
 ALSO BY 
 
 Messrs. JOHNSTON, 4, St. Andrew Square, Edinburgh ; 
 
 HODGES, FOSTER, & Co., 104, Grafton Street, and A. THOM, 
 
 Abbey Street, Dublin. 
 
 1878. 
 
 Price Fifteen Shillings and Sixpence. 
 
I BERKELEY 
 LIBRARY 
 
 UNIVERSITY OF 
 CALIFORNIA 
 
 EARTH 
 
 SCIENCES 
 
 LIBRARY 
 
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Geological Survey of England Sc Wales 
 
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MEMOIRS OF THE GEOLOGICAL SURYEY, 
 
 ENGLAND AND WALES. 
 
 THE 
 
 ERUPTIVE ROCKS 
 
 OP 
 
 BRENT TOR AND ITS NEIGHBOURHOOD, 
 
 INCLUDED IN 
 
 SHEET 25 OF THE ONE-INCH MAP OF THE GEOLOGICAL SURVEY 
 
 WITH SOME 
 
 INTRODUCTORY REMARKS ON THE APPLICATION OF 
 THE MICROSCOPE TO PETROLOGICAL RESEARCH. 
 
 BY 
 
 FRANK RUTLEY, F.G.S. 
 
 i___ 
 
 PUBLISHED BY OBDEB OP THE LOBM GOMMISSIOSBBS OP HBB JGAJESTY'S TRBASTTBY. 
 
 LONDON: 
 
 PRINTED FOE HER MAJESTY'S STATIONERY OFFICE, 
 
 AND SOLD BY 
 
 LONGMAN & Co., Paternoster Row; TRUBNER & Co., Ludgate Hill; 
 
 LETTS & SON, 33, King William Street; 
 EDWARD STANFORD, 55, Charing Cross; and J. WYLD, 12, Charing Cross : 
 
 ALSO BY 
 
 Messrs. JOHNSTON, 4, St. Andrew Square, Edinburgh ; 
 
 HODGES, FOSTER, & Co., 104. Grafton Street, and A. THOM, 
 
 Abbey Street, Dublin, 
 
 1878. 
 
 Price Fifteen Shillings and Sixpence. 
 
LIBRARY 
 
THE following Memoir by Mr. Rutley is an important 
 contribution to the history of Brent Tor, and the sur- 
 rounding igneous and metamorphosed rocks of that part 
 of Devonshire. His remarks on the microscopic analyses 
 of the minerals which compose the rocks, and the beauty 
 of the coloured plates prepared by the author, will be 
 appreciated by all who are interested in such subjects. 
 
 It is not uncommon in modern memoirs on petro- 
 graphical subjects, for authors to criticise the nomen- 
 clatures of old authors, as if they had been possessed of 
 all the knowledge that the microscope, polariscope, and 
 analytical chemistry bring to bear on petrology. 
 
 If on our national Geological Maps all the mine- 
 ralogical variations in the constitution of igneous and 
 metamorphic rocks were to be classified on such prin- 
 ciples, the number of colours and shades of colours avail- 
 able would probably be insufficient to compass all of the 
 stratigraphical and petrological distinctions which are 
 and will be made the number at present employed on 
 the Geological Survey being 103. It is therefore satis- 
 factory to those engaged in the broadly geological work 
 of mapping to find, as Mr. Rutley remarks, " that the 
 " views entertained by the late Sir Henry De la Beche 
 " are in the main correct, and represent a vast amount 
 " of truth derived simply from observation in the field.'* 
 This opinion is also entertained by Mr. J. A. Phillips, in 
 a late memoir in the Journal of the Geological Society, 
 in which he states that notwithstanding mineralogical 
 differences in many of the igneous rocks of Devon and 
 
 C 31. Wt. B 409. a 2 
 
 313038 
 
IV 
 
 Cornwall, the fact that their ultimate analyses is almost 
 the same, forms a new and additional justification for 
 Sir Henry De la Beche having often adopted one colour 
 and one general name for rocks, the mineral peculiarities 
 of which are, indeed, sometimes only due to subsequent 
 metamorphic action. 
 
 AND. C. RAMSAY, 
 
 Geological Survey Office, Director-General. 
 
 28, Jermyn Street, S.W. 
 27th June 1878. 
 
NOTICE. 
 
 This, the first special petrographical work issued by 
 the Geological Survey, has been written by Mr. E/utley 
 after a personal inspection of the rocks in the field, 
 followed by a microscopical examination of specimens 
 carefully selected by himself. 
 
 The district treated of was surveyed nearly half a 
 century ago, by Sir Henry De la Beche, who recorded 
 his observations and deductions in his report on the 
 geology of Cornwall, Devon, and West Somerset, pub- 
 lished in the year 1839, to which work Mr. Rutley's in- 
 vestigations may be regarded as a sequel. 
 
 At that time the microscope (except in the form of 
 pocket-lens), was little used in geological research, and 
 it is only comparatively lately that the importance of 
 determining the mineral composition of eruptive rocks, 
 and of accurately investigating their mineral structure, 
 has become fully recognized, as the light which such 
 investigations throw upon conditions under which those 
 rocks were formed has become more and more apparent. 
 
 In the present case the old geological boundary-lines, 
 as originally surveyed by Sir Henry De la Beche, and 
 now engraved upon the maps of the Geological Survey, 
 hold good as a rule, or, if defective, only require very 
 slight medication . 
 
 The present Memoir will, I believe, prove an important 
 and valuable addition to the other publications of the 
 Geological Survey, from the beauty and fidelity of Mr. 
 Rutley's microscopic drawings, as well as from the clear 
 and masterly way in which he has treated the subject. 
 
 The eruptive rocks, which are now collected during the 
 progress of the Survey in mapping new districts, are 
 subjected to microscopic examination, where such exami- 
 nation seems needful prior to the publication of the 
 maps and memoirs on the districts in which they occur 
 
 C.31. A 
 
VI 
 
 and the specimens thus examined will eventually be 
 placed in the Collection of Rocks when re-arranged in 
 the new room which will shortly be assigned to it in the 
 Museum of Practical Geology. The results of these 
 examinations will also be embodied in future editions of 
 the printed Catalogue of Rock Specimens, which will 
 henceforth have additional scientific as well as practical 
 interest in its bearings on a right interpretation of the 
 geology of the British Isles. 
 
 H. W. BRISTOW, 
 
 Geological Survey Office, Senior Director. 
 
 28, Jermyn Street, S.W. 
 19th June 1878. 
 
PREFACE. 
 
 SINCE the days of Sir Henry De la Becne, little has been done 
 to elucidate the geological relations and the mineral composition 
 of the Eruptive Kocks, treated of in this memoir ; and I have 
 now endeavoured by inspection of these rocks in the field, and by 
 microscopic examination of carefully selected specimens, to make 
 some addition to the knowledge so laboriously amassed, and so 
 well considered by the founder of the Geological Survey. The 
 addition is perhaps a small one, but, so far as it goes, I trust I 
 have dealt honestly with the subject. In many instances I have 
 expressed doubt as to the origin and mineral composition of these 
 rocks rather than shield my ignorance by definite statements upon 
 obscure and difficult questions. 
 
 To the Director General and the Director of the Survey I am 
 deeply indebted for the facilities which they have afforded me 
 for prosecuting this work ; and I would also here record my 
 obligations to Professor A. Rdnard, of Louvain, for the assistance 
 he has given me in determining several of the mineral components 
 of the rocks now described. 
 
 The landscapes are from sketches taken in the field, and I have 
 endeavoured to make the microscopic drawings truthful render- 
 ings of the actual sections, rather than diagrams. 
 
 FRANK RUTLEY. 
 
 A 2 
 
TABLE OF CONTENTS. 
 
 INTRODUCTORY REMARKS ON THE APPLICATION OF THE MICROSCOPE TO 
 PETROLOGICAL RESEARCH. 
 
 Advantages derived from the use of the pocket lens and the application of 
 simple tests before having recourse to high magnifying powers. Value of 
 high powers in the examination of fine-grained and compact rocks. Use of 
 polarized light. Microscopic characters of felspars. Alterations which rock- 
 forming minerals undergo. Use of reflected light in examination of opaque 
 minerals. Dichroism. Its use in distinguishing Hornblende, Augite, &c. 
 Microscopic characters of Olivine, Schorl, Nepheline, Apatite, &c. Use of 
 the term Viridite. Importance of mapping eruptive rocks on a large scale. 
 Practical application of petrological knowledge. Inferences with regard to 
 the origin of rocks. Instances of the use of the miscro scope in the deter- 
 mination of the mineral composition of rocks. Action of acids upon smooth 
 surfaces of rocks. Microscopic drawings. Importance of examining foreign 
 rocks, when already described - - Page 1 
 
 PART I. 
 
 EXAMINATION OP ROCKS IN THE FIELD. 
 
 Brief description of the area treated of in this memoir. Description of the 
 chief exposures of eruptive rocks shown on the one-inch map, with notes on 
 their mineral composition and mode of occurrence, and observations on the 
 boundary lines. Volcanic rocks in the immediate neighbourhood of Brent 
 Tor. Elvan Dykes. Margins of granitic masses - 14 
 
 PART II. 
 
 MICROSCOPIC EXAMINATION OF THE ROCKS. 
 
 Brent Tor series, Lower Grenofen, Shilla Mill, Gunnis Lake, Brazen Tor, 
 Greston Bridge, Churlhanger, W. Langstone, Indescombe, Cock's Tor 32 
 
 PART III. 
 
 DEDUCTIONS. 
 
 Boundary between the Devonian and Carboniferous Rocks. Division of the 
 rocks formerly coloured as Greenstone on the one-inch map. Consideration 
 of the stratigraphical relations of the rocks of Brent Tor. Why Brent Tor 
 has not been removed by denudation. Smear Ridge and White Tor probably 
 connected. Lava flows at Greston Bridge. Eruptive rocks around Exeter 
 not connected with those of Brent Tor. Geological changes which the 
 district has undergone. - 45 
 
 POSTSCRIPT - - 49 
 
 APPENDIX LIST OF MINERALS ----- 50 
 
LIST OF PLATES. 
 
 Page 
 I. MAP OF THE ERUPTIVE ROCKS IN THE NEIGHBOURHOOD OF BRENT 
 
 TOR (scale ^ inch to 1 mile) - - Front. 
 
 II. BRENT TOR. Seen from Hart's Hall near Morwellham (a distance of 
 
 6 miles to the south) - - - 19 
 
 III. BRENT TOR. View taken from about half a mile to the south-west, 
 
 showing the dip of the beds composing the tor - 20 
 
 IV. COCK'S TOR. Sketch taken from Great Staple Tor on the east, 
 
 showing the feature formed by the Amphibolite and the valley in 
 which the junction between this rock and the Granite exists. The 
 gently rising foreground is Granite - - 29 
 
 V. GREAT STAPLE TOR, on the western margin of Dartmoor, showing the 
 mural jointing in the Granite, which imparts a very distinctive 
 character to some of these Granite tors - 30 
 
 VI. Fig. 1. Elvan Quarry at Lower Grenofen, showing parallel master- 
 joints - - 31 
 Fig. 2. Brent Tor from the north, showing the craggy weathering of 
 
 the rocks on this side of the tor. 
 
 Fig. 3. Brent Tor from Smear Ridge, about 3 miles to the S.E. 
 Fig. 4. Greston Bridge. Section shown in the Quarry. Displaying two 
 flows of altered greenstone resting upon and parted by altered 
 shales. 
 
 CHROMOLITHOGRAPHS OF MICROSCOPIC 
 SECTIONS. 
 
 Plate VII. Fig. 1. Brent Tor (S. side near foot of tor) Rhyolitic Breccia. 
 
 Fig. 2. (N. side) Pumice-Breccia, x 25 
 
 Plate VIII. Brent Tor. N.W. side of foot of tor. Basalt with glassy 
 magma, mainly devitrified. x 55. Ordinary transmitted 
 light. 
 Plate IX. Fig. 1. Greston Bridge, altered Greenstone. 
 
 Fig. 2. ,, quartz vein in altered slate. 
 
 Fig. 3. ShillaMill. 
 
 Fig. 4. Top of Brazen Tor, Crystal of Biotite enclosed in Quartz 
 
 of Granite. 
 Fig. 5. Top of Brazen Tor, fluid cavities in Quartz of Granite, 
 
 containing bubbles and cubes of rock-salt, x 400. 
 Fig. 6. Churlhanger, Amygdaloidal Schist (Schalstein). 
 Plate X. Fig. 1. Brazen Tor, Amphibolite. x 55. 
 
 Fig. 2. Schorl, in Granulite near its contact with 
 
 Gabbro. x 25. 
 Fig. 3. Cottage Inn, near Tavistock, cleavage of pyroxenic mineral 
 
 in the Gabbro. 
 Fig. 4. Cock's Tor, altered titaniferous iron in Gabbro. 
 
 LIST OF WOODCUTS. 
 
 Fig. 1. Diagram to illustrate the augmentation of bands by overlap of 
 lamellae in obliquely cut sections of plagioclase when seen under the 
 microscope by polarized light - - 2 
 
 Fig. 2 and 3. Diagrams to illustrate the apparent graduation of imbedded 
 
 fragments into the surrounding matrix of rocks - 8 
 
 Fig. 4. Ideal section from the Dartmoor granite, to Ridge, near Petertavy, 
 showing the probable connexion of the outcrops of Gabbro and 
 Amphibolite - 15 
 
 Fig. 5. Vesicular bands in Ash at Langford Farm, near Tavistock - 22 
 
 Fig. 6. Jagged surface of Amygdaloidal Schalstein exposed in roadside at 
 
 Churlhanger - - 24 
 
 Fig. 7. Mural jointing in the granite of Brazen Tor - - 27 
 
 Fig. 8. Blocks of Gabbro at Horndon _ 27 
 
 Fig. 9. Imbedded vesicular fragments in the rock at Hardwick Quarry - 36 
 
 Fig. 10. Cavities in Schistose Ash, Hardwick Quarry - 47 
 
INTRODUCTION. 
 
 THE present Memoir being the first special petrographical 
 study hitherto published by the Geological Survey, it may be well 
 to make a few remarks upon the use of the microscope in this 
 branch of geological research. 
 
 Until within the last few years the minute structure of rocks 
 was comparatively unknown, and very great uncertainty existed 
 as to the mineral composition of many, while that of the fine- 
 grained varieties was a matter of mere speculation. 
 
 The pocket lens in the hands of the older geologists was the 
 sole means which they possessed of gaining an insight into the 
 mineral composition and structure of rocks which presented no 
 definite character to the unaided eye ; for although chemistry 
 afforded them a rough idea of the minerals which a rock might 
 contain, still it failed to indicate definite mineral species; and 
 structural peculiarities too minute to be recognised by the pocket 
 lens remained unknown. In spite, however, of the advance 
 which has been made of late years in this branch of petrological 
 knowledge by the help of the compound microscope, it must not 
 be supposed that the pocket lens is a comparatively useless instru- 
 ment. Much may be done by its aid, and often with better 
 results than would be attained by the use of higher magnifying 
 powers. This may be partly attributed to the greater ease with 
 which we recognise minerals when seen by reflected light, and 
 when showing actual crystalline faces and planes of cleavage, than 
 when seen by transmitted light and presenting usually only the 
 boundaries of crystals, and the little vestiges of cleavage which 
 remain between the abraded planes which limit a thin microscspic 
 section. 
 
 The compound microscope is undoubtedly a most valuable aid 
 in determining many of the minerals which compose rocks, but 
 it should always be borne in mind that when the component 
 minerals of a rock are sufficiently well developed to be re- 
 cognised under a pocket lens by their crystalline form, cleavage, 
 lustre, or by other tests, such as hardness, colour of streak, or 
 blowpipe characters, it is better to rely more upon such evidence 
 than upon the appearances seen under high magnifying power, 
 especially when the latter do not happen to be very characteristic. 
 It is, however, often impossible to arrive at any sound conclusions 
 respecting the mineral composition of some rocks by the simple 
 method just indicated. Thus, for example, in the Obsidians and 
 
ERUPTIVE ROCKS OF BRENT TOR, 
 
 Pitchstones many int'eresting structures occur, which in some 
 instances serve to indicate points connected with the solidification 
 of those rocks, while in other cases microliths of augite, &c., 
 crystals of felspars and micas are seen to constitute by no means 
 an unimportant part of some of them. In fine-grained aphanitic 
 rocks also, microscopic examination is the only method by which 
 we can form any definite notion of the different mineral species 
 of which they are composed, and even in rocks whose coarsely 
 crystalline structure permits a ready determination of many of 
 their components, the microscope often reveals the presence of 
 large numbers of minute crystals of other minerals, the existence 
 of which would otherwise have eluded notice, or have rendered 
 analytical results difficult of explanation. 
 
 The employment of polarized light in microscopic examination 
 is also of very great assistance in determining the components of 
 rocks. In many cases the crystalline systems to which felspars 
 may be referred may be ascertained with considerable certainty, 
 those belonging to the triclinic system exhibiting parallel bands 
 when viewed by polarized light, the bands being due to the con- 
 tiguous development of twin lamella. These bands are often very 
 numerous in plagioclase crystals, but unless the section examined 
 be cut at right angles to the planes of composition, the overlap of 
 adjacent lamellae may give rise to colourless stripes, as shown in 
 Fig. 1, where A and B represent twin lamellae polarizing in com- 
 plentary colours, the colourless bands produced by the overlaps of 
 the different lamellae being indicated by the unshaded stripes in 
 the lower figure, and an estimate of the number of lamellae by 
 the number of stripes would be liable to error. 
 
 Fig. 1. 
 
 A BABA8 AB 
 
 X 
 
 \ 
 
 
 s 
 
 \ 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 \ 
 
 
 A 
 
 
 R 
 
 
 A 
 
 \ 
 
 
 
 R 
 
 
 A 
 
 
 R 
 
 In the Orthoclastic felspars the twinning usually takes place 
 upon what is known as the " Carlsbad type," viz., in a plane 
 parallel with the clino-pinakoid, and I believe that no instances 
 are known in which an orthoclastic felspar exhibits more than 
 two such lamellae in an individual crystal. Orthoclase crystals are 
 also twinned at times upon another plan known as the " Baveno 
 type." In this case, the twinning plane extends between the 
 alternate edges formed by the basis and the clino-pinakoid, but 
 microscopic orthoclase crystals twinned upon this type are seldom 
 
INTRODUCTORY REMARKS. 3 
 
 or never seen. In some examples of massive Orthoclase, and 
 occasionally, but rarely in crystals, a very peculiar cross-hatched 
 structure is shown by polarized light. The true nature of this 
 marking is not yet satisfactorily ascertained, but it is very 
 characteristic of Orthoclase. 
 
 By means of the microscope many felspars are seen to be any- 
 thing but homogeneous ; patches, and, as in the case of Perthite, 
 lamellae of one species being intermixed with another. Not 
 merely does the microscope demonstrate the presence of impurities 
 in felspars, but in many, if not in most, other minerals. It is 
 also interesting to note, by its means, the way in which alteration 
 products or minerals of secondary origin are distributed in rocks ; 
 how augite and hornblende crystals have been replaced by 
 serpentinous matter ; how at times an augite crystal includes 
 little patches of hornblende, which shade away into the surrounding 
 mineral, and apparently represent an incipient stage of the 
 development of uralite. The alterations which ferruginous 
 minerals undergo are also well shown at times in microscopic 
 sections. Magnetite becomes altered into hydrous peroxide of 
 iron ; the titaniferous iron minerals also show curious alterations, 
 and pyrites and limonite are common as secondary products in 
 many rocks. The examination of these minerals can only be 
 effected, as a rnle, by means of reflected light, since they are, 
 in nearly all cases, too opaque to present any definite character 
 by transmitted illumination, other than the outlines of crystals. 
 These outlines enable us at times to discriminate between 
 magnetite and titaniferous iron; but to distinguish magnetite from 
 pyrites it is needful to employ reflected light, and even then a 
 satisfactory determination can often only be effected by blowpipe 
 or other analysis. 
 
 Felspars, both monoclinic and triclinic, frequently undergo a 
 considerable amount of change, a granular condition supervening, 
 which, if far advanced, obliterates all internal structure, so that 
 the felspars belonging to the one system cannot be distinguished 
 from those of the other. This change is generally attributed to 
 weathering ; but upon examining a section of basalt from 
 Debdon near Rothbury, Northumberland, cut so as to display 
 the pale grey weathered crust and the dark subjacent rock, I 
 was much surprised to find that the plagioclase crystals pre- 
 sented just as fresh an appearance as those in the umveathered 
 portion of the rock. If this be an exceptional instance I do not 
 know how to account for it ; certain it is, however, that in the 
 majority of cases where the weathering of a rock is far advanced 
 and minerals of secondary origin are plentifully developed, the 
 felspars very frequently exhibit the clouded and granular texture 
 just alluded' to. This is especially the case with the felspars in 
 diabase. 
 
 The method first pointed out by Tschermak of distinguishing 
 between the minerals hornblende and augite by means of the 
 dichroism of the former, and the absence of that character in the 
 latter mineral, appears up to a certain point to suffice ; still some 
 
4 ERUPTIVE ROCKS OP BRENT TOR. 
 
 augites, especially if the sections be thick, show slight dichroism 
 when a Nicol's prism is rotated beneath them, while in some 
 exceedingly thin sections of hornblende the dichroism is very 
 feeble. The test, as a rule, however, seems to hold good, while 
 when well-marked transverse sections of these minerals occur the 
 great discrepancy between the angles of the oblique rhomb'c prism 
 afford a ready means of distinguishing between them. The dichroism 
 shown by hypersthene and enstatite, and the absence or almost 
 total absence of that character in cliallage and bronzite, also serve 
 to some extent to facilitate the recognition of these minerals. 
 
 Considerable difficulty may, however, be experienced at times 
 in distinguishing between hornblende and magnesian mica when 
 these minerals are disseminated through a section in excessively 
 minute flecks, and when no definite crystalline form or cleavage is 
 discernible, the colours of the two minerals being often much 
 alike, and their dichroism when the mica is cut at right angles 
 or obliquely to the basal plane being of about equal intensity. 
 
 Under these circumstances it is well to search carefully over 
 hand specimens of the rock with a strong magnifier, in the hope 
 of finding a moderately well- developed crystal. 
 
 The characters by which some minerals are recognised under 
 the microscope are comparatively simple. Olivine, for example, 
 may generally be known by the roughened appearance which the 
 ground surfaces of its sections present, and by the rounded angles 
 usually shown when the crystals are definitely formed. Its want 
 of dichroism, its comparatively weak chromatic polarization, and 
 its freedom from inclosures of other minerals also serve to dis- 
 tinguish it. When unaltered, the crystals appear homogeneous in 
 composition and structure, but when they have undergone change 
 the result is usually serpentinous matter, which under polarized 
 light breaks up into feebly polarizing and irregular patches. 
 Schorl may usually be distinguished from hornblende by the 
 trigonal transverse sections which sometimes occur in a thin slice 
 of rock, and by the bluish tint which it frequently exhibits when 
 seen by ordinary transmitted light, especially when it occurs asso- 
 ciated with quartz. The hemihedral terminations of crystals are 
 not often seen in rock sections, and consequently this characteristic 
 is not, as a rule, to be observed. 
 
 Nepheline and apatite may be recognised by their hexagonal 
 transverse sections, and by the total extinction of light which 
 takes place in such sections between crossed nicols. The former 
 mineral usually occurs in larger crystals than the latter, the 
 apatite crystals frequently being developed upon a very small 
 scale, so that they appear as mere spicula? or microliths under 
 tolerably high magnifying power. Crystals of apatite often form 
 small colonies in certain spots in a rock, their distribution being 
 by no means regular. Apatite is a mineral of very common 
 occurrence in British eruptive rocks, but as yet the only known 
 instance of the occurrence of nepheline is in the Wolf Rock, off 
 the coast of Cornwall.* The longitudinal sections both of nephe- 
 
 * Vide paper by S. Allport, F.G.S., Geol. Mag., Vol. VIIL, page 247. 
 
INTRODUCTORY REMARKS. 5 
 
 line and apatite are rectangular in form, and polarize in more or 
 less vivid colours when the crystals are of moderate size. 
 
 Leucite is a mineral as yet unknown in British rocks. If, how- 
 ever, it should occur developed only in very minute crystals it 
 might easily be overlooked or mistaken for some other mineral, 
 since very small crystals of leucite, such as those which sometimes 
 occur in the leucitophyrs near Kome, do not display any of the 
 characteristics of the larger crystals, and their boundaries are 
 often so ill-defined that they look merely like rounded singly 
 refracting granules. 
 
 In the absence of definite crystals, or of characteristic crystal- 
 line aggregates, it is often difficult to distinguish between many 
 of the hydrous alkaline silicates, which occur so plentifully as 
 secondary products in many rocks which ,have reached different 
 stages of decomposition. These mostly present a green colour 
 when seen under the microscope by ordinary transmitted light, 
 and, owing to the difficulty in referring them to definite species, 
 Vogelsang proposed the use of the term viridite by which to 
 designate them, until further study should enable the observer to 
 assign them with precision to their respective species. The scaly 
 forms of viridite are usually considered to be some variety of 
 chlorite, while the fibrous kinds are regarded as serpentinous 
 matter. Opacite and ferrite are also provisional terms ; the former 
 applies to a black, opaque, amorphous substance, which often 
 occurs pseudomorphous after other minerals. It may represent 
 either earthy silicates or amorphous metallic oxides. The latter 
 term is given to earthy matter, probably oxides of iron, hydrous 
 and anhydrous. 
 
 From the foregoing remarks it will be seen that at the present 
 time there are several terms in use in mico-petrology which have 
 a very broad signification and are merely provisional, and it seems 
 desirable to retain these terms and to employ them in cases where 
 doubt exists, rather than run the risk of encumbering petrology with 
 descriptions apparently precise, really worthless. Unquestionably 
 the microscope is a most valuable assistant when judiciously used, 
 and its application to this branch of natural science promises to give 
 a rapidly increasing knowledge of the mineral composition and 
 minute structure of eruptive rocks. 
 
 Many interesting facts connected with the paragenesis of minerals 
 are disclosed by this method of research, and as our knowledge 
 becomes more extended we should look upon these points not 
 merely as questions of purely scientific interest, but we should 
 carefully amass and group our facts in the hope that hereafter 
 they may elucidate some general laws in nature, of which we are 
 not yet cognizant. It would for example be a work of highest 
 interest to map the eruptive rocks and those which have under- 
 gone well-marked metamorphism on a large scale, and to note 
 their most important variations in mineral composition. Sub- 
 sequently the metalliferous lodes might be accurately laid down, 
 and their characters compared with those of the adjacent rock 
 through which they pass. A map carefully constructed upon such 
 
6 ERUPTIVE ROCKS OF BRENT TOR. 
 
 a plan would possess considerable scientific interest and might 
 prove to be of great value in mining, since the strongholds of 
 many of the metalliferous lodes are the eruptive and metamorphosed 
 rocks, and if in time to come it should be found possible to 
 establish a definite classification of the association of certain 
 metalliferous lodes with rocks of a particular mineral com- 
 position, the microscope will have done as good work for the 
 cause of mining enterprise as for that of scientific truth. 
 
 It is well also to bear in mind the fact that the mineralogical 
 composition, the state of aggregation of the components, and other 
 physical characters of a rock, serve to indicate its utility or 
 worthlessness as a building stone ; and although it may be argued 
 that nature already demonstrates this in the weathered surfaces 
 of rocks, still we must remember that the agencies most de- 
 structive to building stones in large towns do not at all fairly 
 represent the purely natural work of decay which takes place 
 round about the quarry. A good instance of this is afforded by 
 the mineral apatite which is of common occurrence in many erup- 
 tive rocks. In these, when naturally weathered, the apatite scarcely 
 seems to undergo any change, yet we know that if acids were 
 present in any quantity in the atmosphere (as is always the case 
 in large towns), it would suffer considerably and in time become 
 decomposed. 
 
 So far as the origin of rocks is concerned, the microscopic 
 examination of their sections tends at times to throw some light 
 on questions of this nature. The deductions of Sorby, Renard, 
 and others upon the temperature and pressure under which certain 
 rocks have solidified, are the result of calculations based upon the 
 relative size of bubbles and crystals contained in the fluids which 
 occupy minute cavities in quartz, and other mineral components 
 of certain rocks. 
 
 With regard to the numerous rocks which from time to time 
 have been described as volcanic ashes, tuffs, &c., great difficulty 
 exists in reconciling some of these descriptions with the micro- 
 scopic appearances which many of them present. In these 
 questions the microscope opens out a large field for scepticism. 
 The fragments of different minerals and rocks are of course to be 
 looked for in deposits of this class. The angular or rounded 
 surfaces of such fragments do not, however, afford us much help 
 in coming to any satisfactory conclusions, since, when the sur- 
 faces of the fragments are rounded, we may attribute that 
 rounding to attrition against other fragments, either in the throat 
 of a crater, or to the attrition which water-worn fragments have 
 undergone. Again, the angularity of the fragments will not 
 always serve as evidence of the history of the formation of the 
 rock which is composed of, or which contains them, since we 
 meet with angular fragments in unquestionable volcanic ejecta- 
 menta, and also in rocks whose origin has clearly been an aqueous 
 one. In the latter case it is true that the component fragments 
 of the rock are usually water- worn and rounded ; but in instances 
 where the materials composing the rock have on\y been transported 
 
INTRODUCTORY REMARKS. 7 
 
 a, short distance and deposited near the source from which they 
 were derived the rounding consequent upon severe and protracted 
 attrition is not to be expected. 
 
 Again, the fact, that the fragments are those of minerals which 
 compose eruptive rocks is no criterion that the rock containing 
 them has also had an eruptive origin. The felspars which occur 
 in the millstone grits and in other sedimentary rocks afford 
 good examples of this, and it needs no proof to show that in 
 many such cases the development of these crystals has not been 
 due to any metarnorphic action ; moreover the felspars and other 
 minerals of eruptive rocks when occurring in sedimentary deposits 
 are usually in a fragmentary condition, and this would not be the 
 case if they had been developed in situ. 
 
 In some instances volcanoes eject showers of fine dust, in which 
 scarcely any trace of definite minerals can be discerned, and the 
 mud thrown out from certain craters appears also to be devoid 
 of fragments or granules which can be referred to any of the 
 minerals which constitute crystalline eruptive rocks. Here again 
 a difficulty arises, for we have apparently no means of discrimi- 
 nating between these unquestionably volcanic products and 
 ordinary sedimentary matter. Lamination again is an equally 
 useless character, for we may have volcanic ashes deposited as 
 sediment in seas or lakes, and intermixed with or containing 
 intercalated strata of ordinary sediment, brought into those seas 
 or lakes by rivers. 
 
 These are facts well worthy of consideration, and although in 
 this Memoir I have had occasion to speak of certain rocks as 
 volcanic ash, and although evidence in many cases seems to 
 favour such a supposition, I have done so partly from deference 
 to the opinions of Sir Henry De la Beche, partly from a want of 
 evidence to disprove that they are composed of volcanic ejecta- 
 menta, and partly from my own belief, or bias. At the same 
 time I am also prepared to believe that these " ash beds" are often 
 in great part composed of ordinary sediment, while a similar 
 opinion is indicated in Sir Henry's own writings. Before quitting 
 the subject of clastic rocks, it may perhaps be well to point out 
 what may occasionally prove a source of difficulty to the micro- 
 scopic observer. In some instances, and notably in parts of 
 sections of some of the Brent Tor rocks, it appears that patches 
 of rock, different in character from that surrounding them, 
 shade off gradually into the adjacent matter of which the section 
 is composed, or, in the case of very finely vesicular fragments, 
 they seem to break up into little flecks and granules of 
 irregular form, suggestive of the partial disintegration of these 
 lapilli. This in many cases would I believe be an erroneous 
 interpretation of the phenomenon, which is probably due to the 
 planes of section including only portions of the outer crusts or 
 surfaces of irregularly shaped fragments, the separate patches 
 becoming so thin at their margins that they seem to shade off 
 gradually into the adjacent rock, while, when the lapilli are 
 vesicular and spongy, actual separation of the outlying intervesi- 
 
8 
 
 ERUPTIVE ROCKS OF BRENT TOR. 
 
 cular matter occurs. The accompanying diagrams, Figs. 2 and 3, 
 will, I think, sufficiently explain what is meant, the parallel lines 
 in the upper figures representing the planes of section and the 
 diagonally shaded portions indicating the possible forms of the 
 imbedded fragments before the sections are cut. 
 
 Fig. 2. 
 
 Fig. 3. 
 
 Speaking of some of the schistose ashes of Cornwall, Sir Henry 
 De la Beche observes : " They consist of schistose beds strongly 
 reminding us of the substance of greenstone, finely comminuted 
 and permitted to settle in water, in which calcareous matter 
 was occasionally present. The principal mineral substance has, 
 as Dr. Boase observes (Contributions towards a Knowledge of 
 the Geology of Cornwall ; Trans. Geol. Soc. of Cornwall, Vol. IV., 
 p. 175), an external character between hornblende and chlorite. 
 These rocks form part of a general series of variable appearance, 
 being sometimes slightly coherent, reminding us of the volcanic 
 ash of modern times ; at others vesicular, or even graduating into 
 greenstone and other trappean rocks, some of which seem to have 
 been in a condition of pumice, prior to the infiltration of carbonate 
 of lime and other substances into the cells which has now rendered 
 the whole a solid mass." Rep. Geol. Cornwall, Devon, and W. 
 Somerset, p. 57. 
 
IXTRODUCTOEY REMARKS. 9 
 
 And again : " There is in fact so intimate a mixture of compact and 
 schistose trappean rocks with the argillaceous slates, that the whole 
 may be regarded as one system, the two kinds of trappean rcoks 
 having probably been erupted, one in the state of igneous fusion, 
 and the other in that of ash during the time that the mud, now 
 forming slates, was deposited, the mixtures being irregular from the 
 irregular action of the respective causes which produced them ; 
 so 'that though the one may have been derived from igneous 
 action, and the other from the ordinary abrasion of pre-existing 
 solid rocks, they were geologically contemporaneous." Eep. Geol. 
 Cornwall, Devon, and "W. Somerset, p. 57. 
 
 I have, in this introduction, only dwelt briefly upon the micro- 
 scopic characters of a very few of the most common minerals 
 which help to form eruptive rocks, and of a few of the most 
 interesting questions relating to those rocks themselves. To give 
 anything beyond this very bare sketch of the characters of the 
 rock-forming minerals of frequent occurrence, would entail the 
 conversion of this introduction into a small text-book of micro- 
 petrology, while it would add greatly to the bulk of the Memoir, 
 and prove most wearisome reading to anyone not specially 
 interested in this branch of study. I have endeavoured very 
 briefly to point out how far the evidence to be derived from a 
 microscopic examination of rocks can be relied on, and how far the 
 older methods of determination formerly, and now, at the field 
 geologist's disposal, can assist and in some cases rival the work 
 performed by more elaborate appliances. 
 
 The microscopic examination of rocks not unfrequently subverts 
 preconceived notions regarding their mineral composition, in some 
 cases indicating differences where, to unassisted vision, no diffe- 
 rences are apparent, in others showing that rocks very dissimUar 
 in aspect have an identical or similar mineral composition. As 
 instances of the latter I may cite the quartz porphyry, or granitic 
 dyke occurring in Shilla Mill Quarry near Tavistock, the rock 
 although varying considerably in colour and structure in different 
 parts of the_ quarry being identical so far as the component 
 minerals are concerned. Again, the dark iron-grey and patently 
 micacaeous dykes of the Lake District are composed of the same 
 minerals as the brick-red dykes which are so common in the 
 Upper Silurian rocks of that country. In the latter, however, the 
 magnesian mica is but poorly represented, so that while the one 
 rock would be described as minette or mica-trap, the other would 
 almost be designated micaceous felstone. 
 
 Eocks intermediate in character between basalt and trachyte 
 occur in several parts of the Lake District and Wales, associated 
 in both instances with sedimentary deposits of silurian age. The 
 microscopic characters of those pertaining to the former localities 
 have been described by my colleague Mr, J. Clifton Ward, while 
 those of the latter 1 have only recently investigated by order of 
 Professor Ramsay. Mr. Ward's conclusions and my own coincide 
 very closely, and indicate a community in the mineral composition 
 of these approximately coeval but now widely separated lavas. 
 
10 ERUPTIVE ROCKS OF BRENT TOR. 
 
 Various forms of goniometers have from time to time been 
 devised for measuring the angles of crystals when seen under the 
 microscope. Of these that perhaps known as Schmidt's gonio- 
 meter is the simplest in construction ; but, when the section in 
 which the crystals occur is rather thick or dark, it is often difficult 
 to see the cobweb with sufficient distinctness to enable the 
 observer to adjust the instrument with precision. It is, moreover, 
 well to remember that we can seldom be sure that the sections 
 of crystals imbedded in thin slices of rock are precisely at right 
 angles to the faces whose included angle it is requisite to measure ; 
 so that even at the best we cannot obtain more than approximate 
 measurements as a rule, and these measurements are only available 
 for purposes of distinguishing between minerals when their res- 
 pective angles vary to a considerable extent. To effect crude 
 measurements of this kind it often suffices to draw two lines on 
 paper, by means of the camera, corresponding with the outlines 
 of the faces seen in the section, and then to measure off the angle 
 formed by the intersection of these lines with a protractor. This 
 simple method, however, will only give results to within 30 minutes, 
 or at best 15 minutes, when a protractor of ordinary size is used ; 
 and even if a more perfectly graduated instrument were em- 
 ployed it is doubtful whether better results would be obtained, 
 owing to the difficulty experienced in drawing the two lines with 
 any great amount of precision, as the boundaries of the crystals 
 are often rough and irregular Indeed when this method is 
 adopted it is usually desirable to draw the same lines several 
 times until they give similar readings with the protractor. 
 Where very precise measurements are requisite the observations 
 should be made with an ordinary reflecting goniometer upon 
 small isolated crystal?, i.e., if it be possible to isolate them from 
 the rock in which they occur. 
 
 In some cases it is desirable to etch the smoothly-ground sur- 
 faces of chips of rock with acids. By this means information may 
 occasionally be acquired, not merely with regard to the solubility 
 of certain mineral components, but at times evidence may be 
 procured of a somewhat different nature. For example, a rock 
 from Wrington Warren, near Bristol,* was found, after under- 
 going such treatment, to contain fragments of limestone in which 
 portions of minute crinoid stems were imbedded ; the limestone 
 fragments forming little angular patches in a basalt. 
 
 The application of acids to thin sections under the microscope 
 is rather a difficult process, especially when the rock contains 
 carbonates, and it is needful to examine the deportment of other 
 adjacent minerals with acids. The difficulty in such cases consists 
 in restricting the drop of acid to any particular crystal or frag- 
 ment. Of course in the case of mounted sections covered with 
 glass it is necessary to remove the cover and clean off the super- 
 jacent balsam before any reagent can be applied, and this 
 
 * Described in the Appendix to the Geological Survey Memoir on the Bristol and 
 Somersetshire Coalfields 1876, p. 208. 
 
INTRODUCTORY REMARKS. 11 
 
 frequently entails the destruction of the section. The best way 
 is to apply the acid before the section is reduced to its ultimate 
 degree of thinness ; the acid can then be washed off, the process of 
 grinding continued, and the section finished in the usual way ; 
 and even in this case there is risk of affecting a greater thickness 
 of the section than might be anticipated. 
 
 A few words may here be said on the preparation of micro- 
 scopic drawings. In all cases it is desirable to make the outline 
 by means either of a neutral-tint glass reflector, or by an ordinary 
 camera (Wollaston's prism). The former entails either a reversal 
 of the object or a transfer of the outline, the latter involving 
 additional labour, while the former necessitates the filling in of 
 the detail of an object seen through a thick glass slide. The 
 Wollaston's prism is therefore best adapted for drawings of rock 
 sections. Some observers dispense with the use of any instru- 
 ment of this kind and make the outline by eye, placing the paper 
 on a level with the stage of the microscope. The detail should 
 always be filled in by eye alone, a moveable index placed in the 
 eye-piece of the microscope serving to record the precise spot 
 where the work is being carried on. Some draughtsmen are in 
 the habit of rendering their drawings diagrammatic, but this 
 practice, although it may have its advantages in forcibly depicting 
 the views of the observer, detracts from the value of the drawing 
 as a truthful delineation of the object. In this Memoir I have 
 tried to make faithful copies of the rocks as seen under the micro- 
 scope, so that, should my conclusions be doubted, my drawings 
 may at all events be beyond question. 
 
 In most cases the sections examined and described in this 
 Memoir have been prepared by Mr. Cuttell, of Ne\v Compton 
 Street, Soho ; a few I have cut for myself. Machines constructed 
 for this purpose may now be seen at the Loan Exhibition of 
 Scientific Apparatus at South Kensington, while descriptions of 
 how to prepare sections of rocks and minerals will be found in the 
 following works : (( How to Work with the Microscope," by 
 Dr. Lionel Beale ; " The Microscope in Geology " (an article in 
 the Popular Science Eeview, No. 25, Oct., 1867, p. 355), by the 
 late David Forbes ; and an article by J. B. Jordan, in the Journal 
 of the Quekett Microscopical Club. 
 
 It is greatly to be regretted that at present our literature upon 
 micro-petrology is very limited, and for this reason it is most 
 desirable to examine eruptive rocks from foreign localities, to 
 compare them with the descriptions given of them by different 
 continental petrologists, and to ascertain as far as possible in what 
 respects they may be correlated lithologically or otherwise with 
 the rocks which occur in our own country. 
 
 In investigations of this nature it does not suffice to rely implicity 
 upon microscopic research alone. The labours of the field geologist, 
 the crystallographer, the chemist, and the physicist should all be 
 impressed into the service, and it is for this reason that the work 
 of an individual observer is not likely to be based upon a thorough 
 knowledge of all these different branches of science ; while unless 
 C.31. 
 
12 
 
 EKUPTIVE ROCKS OF BKENT TOK. 
 
 the sum of such special investigations can be added up, and the 
 deductions which are based upon them duly weighed, we can only 
 regard our petrological conclusions as provisional, although in 
 many cases they may ultimately prove to be close approximations 
 to the truth. Petrology is but a branch of geology, yet petrology 
 is itself a composite science, while to thoroughly follow it out 
 in all its ramifications is a labour which of necessity exceeds the 
 capabilities of any single observer. It represents a large field 
 for inquiry. Its great aim an elucidation of the origin of rocks 
 by careful study of their mode of occurrence, their mineral 
 composition, and their structural characters, both gross and minute. 
 It is in fact the inanimate history of the globe based upon the 
 anatomy and histology of the earth's crust. Apart from its 
 scientific interest it has practical bearings of great industrial 
 importance, but the scientific knowledge must be accumulated 
 and arranged before it can be applied successfully to the interests 
 of the community at large. 
 
 The following is a rough explanatory table of the mineral com- 
 ponents, &c. of the rocks mentioned in this work : 
 
 BASALT (Dolerite, Aname- Plagioclase. 
 site, Melaphyre). 
 
 Augite. 
 
 Magnetite (Titaniferous iron), Apatite, Olivine. 
 
 Plagioclase (Oligoclase according to Dathe). 
 Augite, generally represented by pseudomorphs 
 
 of serpentine, &c. 
 
 Magnetite, Titaniferous iron, Apatite, Olivine. 
 Chlorite. 
 
 Plagioclase. 
 
 Hornblende. 
 
 Magnetite (Titaniferous iron), Apatite, Quartz. 
 
 Plagioclase. 
 
 Diallage. 
 
 Magnetite (Titaniferous iron). 
 
 A term used of late years as a synonym for 
 Diorite, but formerly embracing all varieties 
 of Basalt, Diorite, and Gabbro. In this work 
 it is used in the earlier sense of the term. 
 
 Any fine-grained varieties of the preceding rocks 
 which, owing to the imperfect or minute 
 development of the individual mineral com- 
 ponents, cannot be determined with precision. 
 
 HYPERSTHENE ROCK - The rocks formerly described under this name 
 
 are now, for the most part, regarded as 
 Gabbros, the Diallage or Enstatite which 
 they contain having been mistaken for Hypers - 
 thene. 
 
 AMPHIBOLITE (Horn- Hornblende, 
 blende Rock.) Quartz. 
 
 DIABASE - 
 
 DlORITE - 
 
 GABBRO - 
 
 GREENSTONE 
 
 APHANITE 
 
INTRODUCTORY REMARKS. 
 
 13 
 
 GRANITE - 
 
 GRANULTTE 
 
 QUARTZ 
 
 (Elvan). 
 
 PORPHYRY 
 
 Orthoclase (Plagioclase). 
 
 Mica (Potash or Magnesian, or both). 
 
 Quartz. 
 
 Orthoclase (Plagioclase). 
 Quartz. 
 
 Orthoclase. 
 Micas. 
 Quartz. 
 Felsitic magma. 
 
 PITCHSTONE (Rhyolite in Vitreous rock, variable in composition, but 
 
 part). 
 
 approximating to felspars and usually con- 
 taining water, the vitreous condition of rocks 
 of this class being due to rapid solidification. 
 The representatives of these rocks at Brent 
 Tor have undergone considerable physical if 
 not chemical change, and are now in great 
 part devitrified by the development of 
 Microliths. 
 
 A term which embraces Quartz-trachyte, Ob- 
 sidian, Pitchstone, &c. The Rhyolites are 
 classed as granitic, felsitic and hyaline. In 
 this work rocks pertaining to the two last- 
 mentioned classes are alone implied. 
 
 These include volcanic ashes and sedimentary 
 deposits composed wholly or in part of frag- 
 ments of rocks and minerals. 
 
 A rock which is possibly related to Diabase in 
 composition, has a schistose structure, and is 
 frequently amygdaloidal. 
 
 ALTERED GREENSTONE- This expression is used to denote eruptive rocks 
 
 so far decomposed that their normal mineral 
 constituents can only be partially recognised. 
 They were probably once Basalts or Diorites. 
 
 RHYOLITE 
 
 CLASTIC ROCKS - 
 
 SCHALSTEIN 
 
 -B 2 
 
14 EKUBTIVE ROCKS OF BEENT TOK. 
 
 PART I. 
 EXAMINATION OF THE ROCKS IN THE FIELD. 
 
 The area embraced in this Memoir has for its boundaries the 
 River Tamar on the west, and the western margin of the Dart- 
 moor granite on the east, while lines drawn east and west, about 
 a mile south of Lidford and a mile north of Calstock, represent 
 the northern and southern limits. 
 
 These purely arbitrary boundaries comprise about 60 square 
 miles, the area including only portion of a much larger district 
 in which beds of volcanic ash and bosses and dykes of other 
 eruptive rocks occur in great number, and often represent con- 
 tinuations of the same beds, or spurs from the same deep-seated 
 intrusive masses. 
 
 Within the small district of which this Memoir treats, there 
 occur many highly interesting and peculiar eruptive rocks, some 
 intruded through, and others interbedded with, the Carboniferous 
 and Devonian strata which lie on the west of Dartmoor. 
 
 The object now in view is an elucidation, to some extent, of 
 the mineral composition and structure of these rocks, not merely 
 by examination of them in the field, but also by more minute 
 investigation of carefully selected specimens ; thus supplementing 
 the work of the late Sir Henry De la Beche by methods of 
 research unknown to petrologists in the days when he surveyed 
 this district, and rendered it classic ground by the careful obser- 
 vations which he made and published in his Report on the Geology 
 of Cornwall, Devon, and West Somerset. The physical features 
 of this district need but a brief description here, since they are 
 admirably dealt with in the work just alluded to. 
 
 On reference to the Geological Survey Map, Sheet XXV. 
 (1-inch scale), it will be seen that the culm or carboniferous series 
 and the Devonian series of rocks abut abruptly against the 
 western flanks of the granite which constitutes Dartmoor, and 
 that from the line of contact a light, graduated wash of the 
 colour, used to denote granite, is carried for a little distance over 
 the adjacent culm and Devonian rocks. From this, however, it 
 should not be inferred that these sedimentary deposits approximate 
 either in mineral composition or in lithological character to the 
 granite itself; in other words, it must not be supposed that there 
 is a gradual passage from these rocks into granite, since there is 
 little evidence, so far as I have seen, at all events near the present 
 denuded surface on the east of Brent Tor, to show that such is 
 the case. In the absence of opportunity for more detailed investi- 
 gation it is, however, only right to quote the following state- 
 ment which occurs in Sir Henry De la Beche's Report, p. 267 
 " In numerous localities we find the coarser slates converted into 
 rocks resembling mica-slate and gneiss, a fact particularly well 
 exhibited in the neighbourhood of Meavy, on the south-east of 
 Tavistock." Where exposures of culm measures or Devonian 
 
EXAMINATION IN THE FIELD. 
 
 15 
 
 rocks occur near the margin of the granite, 
 appearances usually indicate a slight physical 
 change rather than a mineralogical differentia- 
 tion. Approximations to this latter phase are, 
 however, perceptible in places in the form of im- 
 perfectly developed crystals, which may in some 
 cases represent staurolite. About half a mile 
 to the S.W. of Cock's Tor the grits have under- 
 gone some alteration and hornblendic crystals 
 have been developed in them. It happens, unfor- 
 tunately, that where lines of actual contact exist, 
 they are generally masked by superficial accu- 
 mulations ; but it is often possible to procure 
 samples of the rocks within a few yards of the 
 contact, and if they then exhibit no alteration, 
 it is evident that it is either of very trifling extent 
 or that the demarcation between the two rocks is 
 a comparatively sharp one. 
 
 On referring to the Geological Survey Map it 
 will be seen that at two localities, Brazen Tor 
 and Waspworthy, there are small patches, mapped 
 in as greenstone, which are in actual contact 
 with the granite. These are Hornblende Rock 
 (Amphibolite), and the line of demarcation 
 between it and the granite is a very sharp one, 
 and corresponds exactly with the line drawn on 
 the map at the former locality ; but at the latter 
 the evidence is not so clear, as there is very little 
 rock exposed. A little further to the south-west 
 two large intrusive masses of somewhat similar 
 character constitute the features known as Smear 
 Ridge and White Tor. These, together with 
 the Brazen Tor and Waspworthy patches, are 
 probably connected at some little depth beneath 
 the present surface. The ideal section Fig. 4. 
 may serve to illustrate this. Cock's Tor situated 
 a little further to the south is composed of a rock 
 very similar in appearance to those just men- 
 tioned, but under the microscope it is seen to 
 contain little or no free silica, and to be com- 
 posed of diallage, plagioclase, titanic iron, and 
 apparently a little apatite. It must therefore be 
 regarded as a Gabbro. In support of this view, 
 I may add that Professor A. Renard of Louvain 
 has also examined this rock, and states that many 
 unquestionable Gabbros which occur in Belgium, 
 resemble it in every respect.* This rock extends 
 due west on the map, but is much obscured by 
 superficial accumulations on the west of the tor. 
 
 * Height of Cock's Tor, 1,472 feet; Brent Tor, cir. 1,100 
 feet ; Black Down, 1,160 feet above the sea level. 
 
16 ERUPTIVE ROCKS OF BRENT TOR. 
 
 At Sowtentown it is represented as bifurcating and spreading 
 westward in two broad and long strips. I ana inclined to think 
 that it does certainly cross the valley of the Tavy, but some 
 parts of the southern prolongation yield rocks of a totally 
 different character, and although Sir Henry de la Beche, when 
 describing them in his Report,* points out many of their 
 changes ; he appears to have entertained some doubt about the 
 exact relation which these rocks bear to one another, although 
 he has included them within the same boundary lines on the 
 Survey Map. To ascertain their precise relations to one 'another, 
 so as to map them separately, would certainly be a somewhat 
 difficult task, and the work would have to be executed upon a 
 larger scale than one inch to the mile. 
 
 At the Cottage Inn, on the main road north-east of Tavistock, 
 there is a good exposure of Gabbro, succeeded further along the 
 road by black gritty shale much veined and containing little cubes 
 of pyrites, but the contact is not visible. Beyond this, close to 
 a bend in the road, what appears to be a slaty breccia occurs, 
 in contact with a schistose rock which resembles some of the beds 
 described as ash. I believe that Gabbro, similar to that occur- 
 ring at the Cottage Inn, extends as far as Indescombe, where 
 another exposure is visible. Near this point the belt mapped as 
 greenstone is constricted, while further on it becomes schistose, 
 'and may in many cases represent consolidated ash beds. I am 
 even disposed to think that the constriction shown on the map 
 may amount to an actual separation of these rocks, which ap- 
 parently have nothing in common to justify their being mapped in 
 the same patch, the one being intrusive, whfie the other is inter- 
 bedded and schistose in character, nor can I conceive a passage 
 between rocks so dissimilar. That they may inosculate seems 
 possible as the two strips trend in the same east and west direc- 
 tion, and it may have been from some such inference that they 
 were" originally mapped together. The difficulty of ascertaining 
 the boundaries is considerable, the exposures not being very 
 numerous. 
 
 There are, however, one or two things to be said in defence of 
 this piece of mapping : 
 
 First. That the schistose rocks were called greenstone ash and 
 were mapped with the greenstones, while the broad signification 
 which the term greenstone possessed some years ago, embracing 
 as it did diorites, basalts, &c., serves to disarm criticism. 
 
 Second. The small scale of the map rendered the insertion of 
 much detail inadvisable. 
 
 Third. The details relating to lithological differences are given 
 in the "Report on Cornwall, Devon, and West Somerset," 
 
 * " Where greenstones approximate or come into contact with the granite, the 
 " crystallization has become large near it, and even a modification of the arrange- 
 " ment of the component particles seems to have been produced, so that the horn- 
 " blende resembles hypersthene." Geological Report on Cornwall, Devon, and 
 West Somerset, p. 268. 
 
EXAMINATION IN THE FIELD. 17 
 
 although such information is not embodied in the map on account 
 of the small scale upon which it is constructed. 
 
 Some of Sir Henry De la Beche's remarks upon these rocks 
 will be cited further on, together with a few additional obser- 
 vations. 
 
 The northern branch of the Cock's Tor fork extends westward 
 for a distance of over seven miles, and although in some parts the 
 exposures are few and unsatisfactory, still the evidence tends to 
 show that the belt is of a more or less schistose and ashy character 
 throughout a considerable portion of its length, and it therefore 
 becomes a question whether this strip should be represented as 
 actually joining the spur of gabbro which passes westward from 
 Cock's Tor. That the two strips proceeding from this spur are 
 denuded portions ol the same beds I have little doubt, as in both 
 cases they are in great part composed of schistose ash with some 
 local variations in structure, texture, and mineral composition, 
 while from the paucity of exposures in some places it is scarcely 
 possible to note the gradual passages which may take place 
 between them. North of these strips outliers of very similar 
 character occur, and on reference to the Geological Survey map 
 it will be noticed that the principal ones assume a somewhat 
 annular disposition around the mass which constitutes Brent Tor. 
 
 The strip which lies immediately south of Brent Tor, and which 
 passes west from Burnford Farm through Pillands and Churl- 
 hanger, is shown on the map to branch in a northerly direction a 
 little north of Foghanger, and about a mile further north it bends 
 again into a western course extending slightly beyond the Tamar 
 and ending abruptly a little south of Greston Bridge. From 
 the dips recorded on the Survey map, and from one or two 
 additional ones, it seems probable that the three strips of ash 
 which cross the main road from Tavistock to Brent Tor are but 
 repetitions of the same beds, while their tolerably persistent 
 character in no way tends to invalidate such a supposition. The 
 great doubt expressed by Sir Henry De la Beche about the 
 boundary between the culm and the Devonian series also serves 
 to render this question an open one. Towards Dunterton the 
 ash seems to give place to a compact rock resembling basalt, and 
 at Greston Bridge the isolated patch marked on the map is shown 
 to consist of two beds of a similar rock, parted by and resting 
 upon black shales approximating to Ljdian stone, PI. VI. fig. 4. 
 
 The strips of ash on the north and east of Brent Tor, although 
 showing local variations, may also be regarded, with a fair amount 
 of probability, as being continuations of the beds on the south and 
 west. Brent Tor itself differs considerably from these ash beds 
 which encircle it. On ascending the Tor one is struck with the 
 compactness of some of the rocks and with the slag-like vesicular 
 appearance of others, as compared with the fissile schistose character 
 of the neighbouring ashes. Imbedded fragments and lapilli, fre- 
 quently rounded, project from the weathered surfaces, and although 
 the rocks which compose the Tor appear at first sight to be 
 slightly varying conditions of the same kind of rock, yet they 
 
18 EEUPTIVE ROCKS OF BEENT TOE. 
 
 form as it were a group by themselves, and may be clearly dis- 
 tinguished from the ash beds which environ them. We shall 
 subsequently enter into some considerations connected with the 
 granite of Dartmoor and Kingston Down and the Elvan dykes, 
 which proceed from the granite masses, but at present it will be 
 better to restrict our attention to Brent Tor and the ash beds 
 which surround it. 
 
 Brent Tor is a small hill with a rather gradual slope upon the 
 eastern, and more abrupt ones on the northern and southern sides, 
 while on its western or north-western aspect it is craggy and 
 precipitous. (Plate VI. page 31, figs. 2 and 3.) It is a con- 
 spicuous object for many miles around, but this is due to its 
 occupying an elevated situation, the ground from which the tor 
 arises sloping gradually, but almost constantly, downwards in the 
 direction of Tavistock, while on the three remaining sides it also 
 declines, slightly on the north, but on the east and west into 
 valleys of considerable depth. Viewed from a distance, as from 
 some spots on the Morwell Downs, it appears to stand as an 
 isolated hill upon a plateau (Plate II.), and I am disposed to think 
 that this is the case, and that Brent Tor is a hard mass which has 
 offered considerable resistance to degrading agents, and that it is 
 surrounded by on old plane of marine denudation. Viewed from 
 the south-west a decided dip is apparent in the beds which 
 constitute the tor, as shown in Plate HI., but the dip varies, 
 appearing to curve upwards towards the northern side. This dip 
 is more perceptible at some distance from the tor than when the 
 observer is in its immediate vicinity. 
 
 At the base of the tor, on the N.N.W., an aphanitic-looking 
 rock occurs, which is probably the " greenstone " mentioned by 
 Sir Henry De la Beche (Report on Devon and Cornwall, p. 121). 
 From microscopic examination I am inclined to regard this rock 
 as a much decomposed basalt, with an originally glassy magma 
 which is now devitritied. On ascending the tor this is succeeded 
 by another rock very similar in appearance, from the weathered 
 surface of which angular and sub-angular fragments project. The 
 latter are mostly fine-grained and resemble the matrix, which is 
 here and there vesicular. Still higher on the north side the rock 
 seems persistently fine-grained and aphanitic in appearance, the 
 weathered surfaces showing in places a brecciated structure, but 
 with the fragments resembling the matrix. At one spot at the base 
 of the tor, on the north-western side, small slaty-looking patches 
 are imbedded in the fine-grained rock. About half way down the 
 south side of the tor, one or two of the beds appear to be 
 scoriaceous on the weathered surfaces. When this weathering has 
 more completely decomposed the contents of the vesicles the rock 
 resembles a cinder, but where less weathered the fresh fracture 
 merely shows a light spotted aphanitic-looking rock, somewhat 
 similar in matrix to those already observed. 
 
 In one exposure of rock on the lower part of the south side of 
 the tor, strong vertical joints are visible running about north and 
 south, and 15 south of west. 
 
PLATE II. 
 
 19 
 
 BRENT TOR FROM HART'S HALL, NEAR MORWELLHAM. 
 
20 
 
 ERUPTIVE ROCKS OF BRENT TOR 
 
 PLATE III. 
 
 
 
EXAMINATION IN THE FIELD. 21 
 
 In the field on the south side of the tor slightly schistose rock 
 occurs, with projecting nodules on the weathered surface. These 
 nodules are sometimes fine-grained and compact like the matrix, 
 but they are usually scoriaceous, the vesicles often being filled with 
 a white zeolitic mineral. The sandy-looking nodules frequently 
 contain irregular scoriaceous patches, and lower down in the field 
 a rock occurs with included fragments resembling slate and red 
 jasper. The appearance, however, is deceptive, for by microscopic 
 examination the rock is seen to be a partially devitrified pitchstone, 
 while the imbedded fragments are finely vesicular lapilli resembling 
 pumice. Still further down the hill, close to South Brentor 
 village, there are blocks which contain slaty-looking fragments 
 and scoria. ' . 
 
 Some of the rocks which constitute the tor present, under 
 the microscope, a decidedly rhyolitic aspect and approximate in 
 character either to the felsitic or the hyaline rhyolites. They are 
 for the most part micro- felsitic ; but it is possible that they were 
 originally hyaline and that their felsitic character is due to devi- 
 trification. 
 
 Upon first examining Brent Tor it appeared to me probable 
 that the rocks of which it consists might merely represent part of 
 an old bed or series of beds of sediment into Avhich scoria and 
 lapilli had been showered, but further inspection, coupled with a 
 microscopic examination of the rocks, seems conclusively to show 
 that the statement of Sir Henry De la Beche was a perfectly 
 correct one, viz., " That in the vicinity of Brentor a volcano 
 " had been in action, producing effects similar to those produced 
 <f by active volcanoes from a similarity of causes, forcibly presents 
 (( itself/'* Had there been a wide spread of rock like that which 
 forms Brent Tor, it would from its thickness and superior hard- 
 ness have offered great resistance to denuding agents in other 
 parts of the neighbourhood, for as there is no violent contortion 
 or folding of the beds in this area, but simply gentle curvature, it 
 seems highly improbable that, in the absence of faulting, Brent 
 Tor merely represents the last trace of a wide-spread deposit. 
 Conclusions upon this subject might be more easily arrived at if 
 exposures of rock in the vicinity were more numerous and dips 
 could be determined with greater precision ; but unfortunately the 
 lie of the rocks is very obscure in some places. 
 
 Having thus far described what is to be seen upon the ground 
 at Brent Tor itself, it is now advisable to give some account of 
 field observations made upon the various beds of volcanic origin 
 which occur around, and which possibly emanated from it. 
 These beds are mainly schistose in structure ; but whether this is 
 to be attributed to original deposition, or whether it may in part 
 be due to the pressure of an enormous mass of superincumbent 
 rock which once undoubtedly overlaid them, appears to me to be 
 an open question. In other words, is this schistose structure one 
 which has been superinduced by vertical pressure, and actually a 
 
 * Rep. Geol. Cornw., Dev.,aDd W. Somerset, p. 122. 
 
22 ERUPTIVE ROCKS OF BRENT TOR. 
 
 cleavage which is more or less coincident with the bedding ; 
 cleavage transverse to the bedding planes being absent simply 
 because the rocks have, so far as dips can be noted, undergone 
 but little pressure laterally ? The persistence of this structure 
 over a considerable area would fit in equally well with either 
 hypothesis ; but the almost equally persistent vesicular character 
 of these beds, and their strong resemblance in many instances to 
 doleritic rocks, seems to lend some plausibility to the assumption 
 that the schistose structure may be due to a kind of cleavage. 
 That these beds are in places of an ashy character seems probable, 
 since fragments of felspars, lapilli, and scoria are frequently met 
 with in them, although they are often only sparsely distributed. 
 We must, however, always bear in mind the possibility of such 
 beds resulting partially or wholly from the degradation of pre- 
 existing eruptive rocks. 
 
 In the immediate vicinity of Tavistock, and to the east of the 
 town, two strips of schistose rock occur which appear in no way 
 to differ from much of the ash which lies between Tavistock and 
 Brent Tor, and it is quite possible that these two strips are 
 merely repetitions of the same beds. The strip which stretches 
 up to Mount Tavy, on the eastern side of the river, dips in a 
 N.W. direction ; the dip of the other strip, which occurs nearer 
 the town, is not ?.o evident. On the road leading out of Tavistock 
 in a westerly direction, just before reaching Downhouse Farm, 
 killas is exposed in the roadside ; it is somewhat contorted and 
 affords no trustworthy dip. At the cross roads about a quarter of 
 a mile north of Stiles Wick a rock occurs containing white flecks 
 of kaolinised felspar and hornblende : it is apparently a decomposing 
 diorite. The exposures of rock about this spot are scarcely worth 
 consideration, and I am not sure that they represent rock in place. 
 At Downhouse farm I picked up a piece of vesicular ash in the road, 
 but could not see any in place. In the side of a lane leading into 
 the main road exposures of scoriaceous ash and killas may be seen, 
 but it is very doubtful whether they are in place. Probably they 
 are merely loose pieces imbedded in the bank. The ash contains 
 cubes of decomposing pyrites, some of them half an inch in 
 diameter. In the main road from Tavistock to Milton Abbot 
 there is a good exposure of schistose ash, which at Langford 
 Farm shows interbedded vesicular bands (Fig. 5) about three to 
 
 Fig. 5. 
 
 four inches in thickness, lying in a somewhat horizontal position, 
 but the beds appear to dip to the S.S.W. at a low angle. I am 
 
EXAMINATION IN THE FIELD. 23 
 
 inclined to think that the Hardwick and Langford Farm strip of 
 ash terminates at the constriction shown on the map, in the 
 neighbourhood of Indescombe. 
 
 We will next consider the belt of rock which runs in an east and 
 west direction by Ridge, Kilworthy, Lamerton, and Shortaburn. 
 At Ridge, on the road from Tavistock to Petertavy, it seems 
 closely to resemble that exposed at the Cottage Inn on the 
 main road on the western bank of the Tavy. It therefore 
 approximates more nearly to a gabbro than to an ash, and con- 
 sequently I think that there ought to be a separation of this rock 
 from that which occurs further to the west, although a continuous 
 band is coloured on the Survey map. It is possible that a passage 
 may occur between these crystalline rocks and the schistose beds, 
 but it is more probable that this is not the case. Near 
 Kilworthy, i.e., between Hardwick and Kilworthy, there is a 
 fine quarry on the east side of the road leading from Tavistock to 
 Brent Tor. It is a vesicular ash, manganese oxides occurring in 
 the vesicles and in small druses and cavities. Balls of a harder 
 and more compact stone are met with in these beds. According to 
 one of the quarrymen these balls lie in definite layers, but, although 
 I saw several in place, I could not find evidence enough to 
 corroborate this statement. By the same authority I was told 
 that a band of hone-stone occurred to the south of the quarry, but 
 I was unable to find any exposure of it. The quarry shows no 
 killas, either at top or bottom, and represents a thickness of 
 over 20 feet of ash. It is being worked as a building stone, and 
 is almost exclusively used for the construction of Kelly College. 
 A specimen picked up in the quarry shows rounded vesicles as 
 large as a walnut. The rock seems to vary but little in 
 lithological character throughout the quarry, although in some 
 places it appears more like a decomposing vesicular basalt than 
 an ash. It is, however, more or less schistose in character 
 throughout. It is traversed by numerous joints and is a soft stone 
 to work. By the roadside just opposite Lamerton Church, there 
 is a small exposure of rubbly, decomposed ash, which when much 
 decomposed becomes a yellowish, ochreous earth. About 
 Shortaburn there are no exposures of ash visible, but some of the 
 loose stones and road metal in the neighbourhood contain 
 scoriaceous fragments. Near Two well the loose stones built into 
 the banks and hedges, and lying in the roads, are vesicular ash 
 much decomposed, the cavities containing manganese oxides. No 
 exposures of rock in place are visible either here or in the fields 
 about Shortaburn, but there is a tolerably good feature in places 
 which no doubt served to determine the boundary lines drawn on 
 the map. Some exposures of ash are mentioned by Sir Henry 
 De la Beche as occurring near Combe. 
 
 In the belt of ash lying north of that last mentioned, the best 
 outcrops appear to be in the neighbourhood of Churlhanger. In 
 the roadside, just S.W. of the farm house, there is an exposure 
 of five or six feet of a vesicular schistose rock : the latter character 
 gives a peculiarly jagged appearance to the exposed surface. 
 
24 ERUPTIVE BOCKS OP BRENT TOIL 
 
 Fig. 6. It is full of small specks and granules of calcspar, 
 and where these have been weathered out the rock is seen to be 
 
 Fig. 6. 
 
 finely vesicular. In places larger amygdaloids of calcspar occur, 
 and also narrow calcspar veins. The schistose structure appears 
 to coincide with the bedding, which in one place is nearly hori- 
 zontal. In a field close to the road, but at a considerably higher 
 level, there is also a good exposure of the rock. It is much the 
 same in character as that in the road beneath. Nodular-looking 
 patches weather out in relief on the surface, and show a finely 
 spongy, vesicular structure, which upon fracture is seen to extend 
 half an inch or more from the surface, but further inwards the 
 vesicles are filled with calcspar. This rock appears to be identical 
 with some of the schalstein which occurs in Nassau. 
 
 ELVAN DYKES. 
 
 To the south and south-east of Tavistock several elvan dykes 
 are marked on the map. I was only able to visit the two northern- 
 most, which are mapped as trending due east and west. On 
 attempting to follow out the northern one from Morwell Rocks, 
 I was unable to find any exposure of the dyke until reaching 
 Shilla Mill, where there is a good quarry, now being worked for 
 road metal, not far from the banks of the Tavy, and about three 
 quarters of a mile from Ramsland. There is nothing but elvan 
 exposed in this quarry except a little rubble which lies on the 
 top, and which very likely represents the disintegration of the 
 underlying rock. On the south side of the quarry the rock is 
 of a dark grey colour, with irregular dark greenish-black patches 
 imbedded in the grey, granular, felsitic matrix, but further to 
 the north of the quarry it becomes a quartzose felspar porphyry 
 of the usual elvanitic type. In places the rock contains a little 
 pyrites or mispickel, and also specks of copper pyrites and some 
 chlorite. From Morwellham to Ramsland there seems to be no 
 definite exposure of the elvan, and certainly there is none in any 
 of the paths or roads in the neighbourhood. The house and 
 
EXAMINATION IN THE FIELD. 25 
 
 walls at Roman's Lee were built out of this elvan about nine years 
 ago, and the stones lying in the vicinity resemble the darker 
 variety of rock from the northern elvan at Shilla Mill quarry. 
 
 The best exposure that I could find of the long southern elvan 
 course was at the Lower Grenofen quarry. At this spot the 
 rock has been worked for kerbstones, &c., the consignments having 
 mainly been to London. The quarry is now abandoned. There 
 is an exposure of over thirty feet of elvan. The felsitic matrix 
 of the rock has a pinkish tinge, but loses colour on exposure. 
 Porphyritically imbedded crystals of orthoclase occur plentifully 
 in this matrix, the other components are seemingly mica, horn- 
 blende, and chlorite. The rock is irregularly jointed, but there 
 are some well-developed parallel master joints running several 
 feet apart, Plate VI., fig. 1. 
 
 The annexed extract relating to the Morwell Down tunnel' 
 was copied for me by Mr. G. Chowen of Whitchurch, to whom 
 I was much indebted for information and assistance when in that 
 neighbourhood. 
 
 " The following are the strata which have been passed through, commencing 
 at the north end of the tunnel ; the provincial names of the different rocks 
 being given as they are generally used by the Cornish miners. 
 
 Fathoms. 
 
 Killas, metalliferous slate - - - - 311 
 
 Elvan, chlorite and quartz - - 11 
 
 23 
 6 
 12 
 26 
 ) 
 
 with veins of quartz J 
 
 Elvan ... - 15 
 
 Killas 3 
 
 Grouan - ..... 7 
 
 Elvan, quartz granular and crystalline - 12 
 
 Killas 408 
 
 Entire length of tunnel - 1,2/0 
 
 " The directions of all these beds seem to be parallel to each other and to 
 range nearly east and west. 
 
 " Two facts have been ascertained by its progress. 1st. Relative to the 
 rocks, that the killas of which the hill is mainly formed is traversed by beds 
 of other rocks, the direction of which is inclined to that of the metalliferous 
 veins, and which have a pretty uniform dip or underlay to the north. 2nd. 
 Relative to the metallic veins or lodes that they traverse all the strata, and 
 that they have a remarkable difference in their dip or underlay on the two sides 
 of the hill; those on the north side dipping to the north and those on the 
 south side to the south. 
 
 " The lodes near the centre of the Morwell Down hill are intersected by two 
 cross lodes or cross courses. The killas on each side of one of these occurs in 
 a remarkably altered state as a soft clayey substance, so incoherent as to have 
 rendered the preservation of a passage through it until it was securely arched 
 very difficult. Nineteen metalliferous veins and two cross courses were inter- 
 sected by the tunnel in the Morwell Down hill." Paper by J. Taylor, Trans 
 Geol. Soc. Lond., vol. iv. 1817. 
 
 MARGINS OP GRANITIC MASSES. 
 
 The only spot where I have been able to see any definite 
 alteration of the Devonian slates near their contact with granite, 
 is at the Morwell Rocks. Here the slates resemble in places 
 
 Killas 
 
 Grouan, clay-porphyry 
 
 Killas 
 
 Grouan 
 
26 ERUPTIVE HOCKS OF BKENT TOR. 
 
 * 
 
 finely speckled fruchtschiefer,* the speckles probably representing 
 an incipient development of chiastolite or staurolite, but no 
 definitely formed crystals are to be seen. These speckled slates 
 lie on the opposite side of the Tamar to that on which the granitic 
 mass of Hingston Down occurs, and are apparently about a 
 quarter of a mile from the contact. It is probable, however., that 
 alteration of this kind occurs in many other localities in the 
 district, but exposures of contact, or in the immediate neigh- 
 bourhoods of contact of the sedimentary with the eruptive rocks, 
 are few, and the time placed at my disposal for the prosecution of 
 this work precluded the possibility of persistently following the 
 boundary lines in search of such exposures. 
 
 Opposite the Morwell Hocks, upon the other side of the Tamar, 
 there is a good quarry of granite worked by the Gunnis Lake 
 Company. It was formerly called the " Tamar Granite quarry " 
 and has been worked for over 70 years with little inter- 
 mission. The granite is fine-grained, and the orthoclase in it is 
 mostly white, but there is a course which contains pinkish ortho- 
 clase, no sharp demarcation, however, existing between this and 
 the other variety. A very fine-grained rock, with a few large 
 quartz blebs or crystals occurs in one part of the quarry (it is 
 called elvan by the quarrymen). The granite gets out in good- 
 sized blocks, and is used for building purposes in London, Exeter, 
 and other places. It seems very free from pyrites, and is appa- 
 rently a stone well adapted for architectural purposes. 
 
 At Brazen Tort (pronounced " Brauzen Tor "), which is situated 
 on the western margin of Dartmoor, and about six miles north- 
 east of Tavistock, the granite consists of large orthoclase crystals, 
 biotite and quartz, the crystals of orthoclase often exceeding two 
 inches in diameter. On reference to the published geological map 
 it will be seen that there is a patch of rock coloured as greenstone, 
 which is represented as being in , contact with the granite. The 
 junction is not actually visible, but the line may be drawn within a, 
 few yards. The granite close to the contact is very fine-grained 
 and felstone-like, loose blocks showing black nests and segregations 
 of schorl. It becomes more and more porphyritic towards the tor 
 itself. Here and there quartz veins an inch or two in thickness 
 are seen to cut through the fine-grained granite. Towards the 
 top of the tor the black schorlaceous segregations form projecting 
 knobs on the weathered surface of the rock. The nearest approach 
 to contact shows a rapid change from gabbro into schorl-spotted 
 elvanitic rock or fine-grained granite, and then passes on into 
 porphyritic granite. The line on the map is very correctly drawn. 
 The mineral components of this rock are coarsely crystallized and 
 loosely aggregated. It would not make even a moderately good 
 
 * The fruehtschiefer of continental petrologists embraces spotted and speckled 
 shales or mica- schists ; the spots consisting of talcose or micaceous, scaly aggregates, 
 or segregations of dark, opaque granular matter, which is probably graphite or some 
 manganese oxide. 
 
 j- See also pages 38 and 42. 
 
EXAMINATION IN THE FIELD. 
 
 27 
 
 building stone. At the top of Brazen Tor the granite shows very 
 regular mural jointing. Fig. 7. 
 
 Fig. 7. 
 
 Of the junction between greenstone and granite indicated on 
 the map, about half a mile to the west of this, I could get no good 
 evidence. The rock mapped as greenstone occurs close to the 
 little bridge near Waspworthy, but the actual junction is not 
 visible. In a field north of the brook, close to Waspworthy, a fine- 
 grained or compact amphibolite occurs ; granite blocks are also 
 scattered about on the surface. It is possible that the Brazen Tor 
 patch of amphibolite joins this, but no trustworthy exposures are 
 to be seen near the road, nor in any of the adjoining fields. 
 
 That these patches are also connected at some depth'Jbeneath the 
 surface with the mass lying between Crayston and White Tor and 
 that which constitutes Smear Ridge, there can be little doubt. In 
 a field on the east side of the road, close to Horndon, blocks of 
 gabbro occur in large numbers, some of them of considerable size, 
 up to 9 feet in height, such as the block locally known as the 
 " Master Rock," Fig. 8, and some of them apparently in place. 
 
 fig. 8. 
 
 These are outside the boundary of the greenstone marked on the 
 Survey map, and one would at first sight be inclined to think this 
 boundary line to be incorrectly drawn ; but, on examining an old 
 C. 31. C 
 
28 ERUPTIVE ROCKS OP BRENT TOR. 
 
 grubbing at Kingsford Corner, killas will be found at a depth of 
 about 12 feet, clearly showing that, -if the line can be extended at 
 all, the alteration .will only be a slight one, embracing perhaps a 
 quarter of a square mile, and that it was drawn carefully, and 
 within what Sir Henry De la Beche considered safe limits. In 
 his Report on the Geology of Cornwall, Devon, and West Somerset, 
 he refers part of these greenstones to HyperHhene Rock. 
 
 The margin of the granite just west of Great Staple Tor consists 
 of fine-grained rock, which on nearing the tor becomes coarsely 
 porphyritic with orthocla?e crystals twinned on the Carlsbad type. 
 There is no visible contact exposed between the granite and the 
 gabbro of Cock's Tor, a moderately deep valley lying between the 
 two features, Plate IV. The mica in the granite of Gre?t Staple 
 Tor is biotite. The granite composing the Tor exhibits well- 
 marked mural jointing. Plate V. 
 
 This mural jointing in granite has been noticed by Sir Henry 
 De la Beche, Dr. Boase, and others, who state that it is especially 
 characteristic of the large granitic masses where they come in 
 contact with other rocks. The former writer observes : " This 
 variety of cleavage may be regarded as a kind of thick lami- 
 nated structure pervading the masses on the large scale, probably 
 agreeing in form with that of their original surfaces after pro- 
 trusion." Rep. Geol. Cornw., Dcv., and \V. Soiii., p. 163. 
 
29 
 
 PLATE IV. 
 
 c 2 
 
30 
 
 ERUPTIVE BOCKS OF BRENT TOR. 
 
 PLATE V. 
 
31 
 
 PLATE VI, 
 
 Fig. 1, 
 
 Fig. 2. 
 
 ELVAN QUARRY AT LOWER GRENOFEN. BRENT TOR FROM THE NORTH. 
 
 Fig. 3. 
 
 fig. 4. 
 
 BRENT TOR FROM SMEAR RIDGE. 
 
 QUARRY AT GRESTON RIDGE. 
 
32 ERUPTIVE KOCKS OF BRENT TOE. 
 
 PAKT II. 
 
 MICKOSCOPIC EXAMINATION OF THE KOCKS.* 
 
 No. 1. Brent Tor (north side). A grey apbanitic rock containing 
 irregularly-shaped patches, which on a polished surface have a 
 deep reddish brown colour, and under a strong lens appear finely 
 vesicular. These patches are much harder than the surrounding 
 matrix, and seem to approximate to Eisenkiesel. 
 
 Under the microscope, by transmitted light, the red patches 
 appear opaque. They are highly vesicular, and probably repre- 
 sent fragments of pumice which have undergone alteration, so that 
 the fibrous structure is no longer visible. In some parts of the 
 section irregular strings of dark opaque matter traverse the matrix 
 and frequently follow the same direction, thus giving the appear- 
 ance of fluxion structure ; this appearance may in part represent 
 filamentous portions of pumice fragments cut parallel with the 
 direction in which the filaments run. The matrix shows under a 
 magnifying power of about 70 diameters great numbers of very 
 minute doubly-refracting microliths and granules. It is seemingly 
 a devitrified magma (the result being felsitic matter), but it is 
 scarcely safe to express this opinion very decidedly. The 
 microliths lie in all directions, and do not form definite streams. 
 A little calcspar is here and there visible. The rock may be re- 
 garded as a pumice breccia. Fig. 2, PL VII., represents portion 
 of a section of this rock magnified 25 diameters. The dark patch 
 occupying the southern portion of the field is part of a scoriaceous 
 fragment. 
 
 No. 2. Brent Tor (south side near the foot of the Tor). A 
 greenish-grey or buff-coloured rock, with dark grey imbedded 
 fragments, and red stains from peroxidation of ferruginous matter. 
 Polished surface shows minute greyish-white flecks and large 
 spots more or less angular in form and of .a dark green or black 
 colour. 
 
 Under the microscope the rock appears to consist of a devitri- 
 fied magma, now felsitic, which shows fluxion texture and con- 
 tains fragments of vesicular rock (probably pumice). Quartz occurs 
 in granules, globules, strings and nests, and is probably a secondary 
 product, occupying the place of some mineral which has decom- 
 posed. The devitrification of the magma seems in part to be due 
 to the development of doubly refracting colourless microliths. The 
 rock appears to be essentially a pitchstone or hyaline rhyolite 
 (devitrified) containing fragments of a scoriaceous rock (? pumice). 
 It might be called a rhyolitic breccia. Fig. 1, PL VII., represents 
 portion of a section of this rock as seen by ordinary transmitted 
 light under a magnifying power of 25 diameters. The dark portion 
 on the south-west edge of the field is part of one of the scoriaceous 
 fragments, while the little irregular black flecks visible in other 
 
 * The numbers in the margin refer to the sections of these rocks which are 
 deposited in the Museum of Practical Geology, Jennyn Street. 
 
MICROSCOPIC EXAMINATION. 33 
 
 parts of the drawing doubtless represent other minute fragments 
 of similar material included in the section. The white portions 
 are composed of granules- and crystals of quartz, among which 
 globular forms are visible in places, but are not shown in the 
 drawing. They sometimes exhibit, very imperfectly, the inter- 
 ference cross which characterises the polarization of radiate aggre- 
 gates. In the coloured plate the pale yellow part represents the 
 devitrified magma, the fluxion texture being shown by the deeper 
 yellow marking.-. This drawing is a faithful rendering of the 
 actual section, and I have taken pains neither to exaggerate nor 
 to suppress any of the markings. 
 
 No. 3. Brent To r (north-west side at the foot of the tor). The 
 specimen from which the section was cut is greyish in colour and 
 consists of what appear to be angular fragments, ranging up to 
 about an inch in diameter, and resembling fragments of slate 
 imbedded in a paler grey matrix, suggesting at once that the rock 
 is a breccia. On the polished surface it looks rather like a dark 
 green serpentine variegated with whitish speckles and deep red 
 ferruginous spots. 
 
 Under the microscope the section becomes dark between crossed 
 nicols, the dark ground being, however, studded with innumerable 
 doubly refracting microliths which observe no definite direction 
 and are no doubt of secondary origin. These microliths represent 
 far-advanced devitrification of what was, 1 believe, an originally 
 glassy magma. Rounded greenish, feebly-dichroic granules of 
 a mineral which is apparently augite and others which are colour- 
 less and singly-refracting are sparsely scattered through the section, 
 which includes many patches of what also seems a devitrified 
 glass but contains no microliths, only doubly-refracting granules. 
 Some dark opaque matter, probably limonite, is also present 
 in places. The microlithic patches in the section correspond 
 with the dark slaty-looking patches in the hand specimen. Some 
 of them show indications of a vesicular structure, the vesicles 
 being occasionally bordered by dark margins, fragments of much 
 altered minerals and scoriaceous dust, together with minute 
 crystals, which in some cases may represent magnetite or pyrites, 
 and in others a decomposed condition of minute scales of mica. 
 Fluxion texture is clearly discernible in some of the microlithic 
 patches. The rock is essentially a devitrified rhyolite. 
 
 No. 4. Brent Tor (north side). A dark grey rock resembling 
 a highly scoriaceous lava. 
 
 Under the microscope the section of this rock shows a quantity 
 of opaque, finely-vesicular scoria, similar to that described as 
 occurring in the preceding specimens (and which it was suggested 
 might represent pumice). Here and there it seems that the rock 
 is composed of fragments of this material. The vesicles are filled 
 with siliceous matter, which has a very finely granular appearance 
 when seen between crossed nicols, and similar substance surrounds 
 the scoriaceous fragments. It is quite possible that the frag- 
 
34 EKUPT1VE EOCKS OP BRENT TOR. 
 
 mentary aspect of this rock under the microscope may be 
 deceptive, since the interspaces between the seeming fragments 
 of scoria may merely represent sections of large, irregularly 
 shaped vesicles which have like the smaller ones been filled up 
 with siliceous matter. If the latter supposition be correct, 
 and I am disposed to think that it is, we must regard this rock 
 as a scoriaceous lava, while if preference be given to the former 
 hypothesis, it can only be considered as an agglomerate of scori- 
 aceous fragments and dust held together by a siliceous cement, and 
 if so, this rock and No. 1, which is also from the northern side 
 of the Tor, may be looked upon as identical. Under any circum- 
 stances they bear microscopically a very close relation to one 
 another, although the general aspect of the hand specimens is 
 very dissimilar, No. 1 being compact, while No. 4 has a rugged, 
 scoriaceous appearance, highly suggestive of a lava. 
 
 No. 5. Brent Tor (north side). The hand specimen is of 
 very compact texture and of a deep reddish brown or Venetian- 
 red colour, with small dark specks ; this is best seen where the 
 specimen has been cut. 
 
 Under the microscope the section is seen to contain numerous 
 little prisms of felspar which at times show indications of twinning 
 and are no doubt triclinic. With the exception of these prisms 
 little can be made out beyond the fact that the surrounding 
 matter displays a slightly granulated character between crossed 
 nicols, the light not being totally extinguished, while little 
 clear doubly refracting microliths occur in it in considerable 
 numbers. Dark matter is also present, some of which may be 
 pseudornorphous after augite or other minerals, but no definite 
 augite forms are to be discerned. Quartz occurs pseudo- 
 morphous, apparently after felspars and other minerals and also 
 filling vesicular cavities. The rock is no doubt a decomposed 
 basalt-lava. 
 
 No. 6. Brent Tor (north-west side at the foot of the Tor). 
 In general appearance this closely resembles the preceding. . 
 
 Under the microscope it is seen to be also a basalt with an 
 originally glassy magma, which is now devitrified by the develop- 
 ment of clear doubly refracting microliths. The section shows 
 some irregularly shaped patches of brownish-yellow glass which 
 contain comparatively few microliths, and with the exception of 
 these bodies appear dark between crossed Nicols. There are 
 indications of fluxion texture in the section. Any pyroxenic 
 minerals which the rock may have originally contained are now 
 decomposed, and the little prisms of felspar have also undergone 
 considerable change. Although * the evidence for pyroxene is 
 unsatisfactory both in this and in No. 5, I have little doubt that 
 both rocks represent altered conditions of what were once basalts 
 with glassy magmas. 
 
 .This is probably the rock described by Sir Henry De la Beche 
 as a greenstone. In the absence of microscopic examination one 
 might also feel disposed to apply the equally vague term nphanite 
 
MICROSCOPIC EXAMINATION. 35 
 
 to it. PL VIII. shows part of a section of this rock magnified 
 55 diameters, as seen by ordinary transmitted light. 
 
 No. 7. Brent Tor (seuth side at the foot of the Tor). 
 This is portion of a rounded, imbedded nodule, which, so far as 
 can be judged from microscopic appearance, is allied to basalt. 
 The section when magnified shows great numbers of rounded 
 vesicles and more or less polygonal patches, all of which are filled 
 with quartz or chalcedony ; the latter forms, however, no doubt 
 represent spaces once occupied by crystals which were normal 
 constituents of the rock. Another imbedded nodule from the 
 same locality shows larger vesicles, some of which are filled with a 
 zeolitic mineral and others with quartz. 
 
 No. 8. From grubbing in roadside, turning to West Langstone. 
 This is a pale brownish-grey rock, and in the hand specimens 
 shows little dark green patches, mostly rounded or lenticular, and 
 moderate sized crystals, and fragments of crystals which once were 
 felspars, but have now undergone so much change that they are 
 easily scratched with the knife blade. They are greyish-white 
 and form light blotches in the matrix. The dark-green patches 
 are greenearth, or some closely allied mineral, and may have 
 resulted from the alteration of fragments of some form of pyroxene ; 
 but in most cases I think they represent the filling up of vesicles. 
 
 A section of this rock when seen under the microscope shows a 
 fine meshwork of felspar crystals considerably altered, and in 
 general appearance it resembles a decomposed basalt, containing 
 rounded fragments of larger felspar crystals, often with a peculiar 
 zone of darker matter constituting their margins, suggestive of 
 fritted or fused surfaces, and amygdaloidal patches of green earth 
 or some allied mineral. This rock, which is definitely bedded, is 
 overlaid by slaty beds containing manganese oxides and a little 
 pyrites, in which is a layer of micro-crystalline pink-coloured 
 quartzite, locally called capel (No. 8a), which contains a little 
 pyrites and manganese oxides. 
 
 In the manganese mines in the neighbourhood capel often over- 
 lies the deposit of ore. It was formerly removed by means of the 
 pick, but the process was a tedious and expensive one and the 
 capel is now usually blasted. 
 
 The exposure from which the above specimens were taken 
 occurs outside the greenstone boundaries marked on Sir Henry 
 De la Beche's map, and is situated on the right-hand side of the 
 main road between East and West Langstone. 
 
 No. 8b. Quartzite also occurs at Week (spelt on the ordnance 
 map Wick), near Brent Tor. It is seen under the microscope to be 
 rather more coarsely micro-crystalline than the capel at West 
 Langstone, and contains minute crystals of decomposed pyrites. 
 
 No. 9. Churlhanger, near Lamerton. The mode of occurrence 
 of this rock has been already described (pp. 23 and 24). Its real 
 character, so far as its origin is concerned, is by no means evident. 
 It has a schistose structure and is of a greenish colour. It is 
 profusely speckled with little amygduloids of calcspar, which when 
 weathered out give the rock quite a vesicular character. It may 
 
36 ERUPTIVE HOCKS OF BEENT TOR. 
 
 be a fine ash, the volcanic dust having been comparatively un- 
 mixed with coarse fragments, but microscopic evidence upon this 
 point is unsatisfactory, and it is hard to conceive how . an ash 
 deposit could become so vesicular.* It is also possible that it is 
 a much altered lava, but this assumption again receives but little 
 support from microscopic examination, while the schistose cha- 
 racter of the rock rather militates against the supposition. The 
 vesicles are in many cases fringed with a green mineral which is 
 apparently chlorite, while the whole of the rock, except the calcspar 
 amygdaloids, when seen by transmitted light, partakes of the 
 same greenish tinge, variegated by dark opaque specks of irregular 
 and ill- defined form (Fig. 6, PI. IX.). The rock is essentially an 
 amygdaloidal schist, and is almost identical in appearance with 
 some of the amygdaloidal Schalstein of Nassau. I have estimated 
 the carbonic anhydride contained in ' the Churlhanger rock from 
 an average sample, and find that it amounts to 15'5 per cent. 
 This gives a per-centage of 35*227 of calcspar, a result which 
 corresponds very closely with the estimate which might be roughly 
 formed from an examination of the microscopic section. After 
 deducting for the loss of carbonic anhydride and hygroscopic water 
 the loss on ignition is found to be 2 per cent., which represents 
 the combined water. This is probably derived from the green 
 mineral, and if we deduct 35 per cent, for the calcspar contained 
 in the rock, and take the remainder to consist of about one third 
 chlorite, the 2 per cent of combined water would suffice to make 
 good the assumption that the green matter is chlorite or some 
 allied mineral. After digesting the powdered rock in hydro- 
 chloric acid, and separating the alumina, iron, and lime, the solution 
 gives the usual reaction for magnesia. 
 
 The rock fuses to a dark-greenish, feebly-magnetic slag with a 
 thin vitreous crust. 
 
 No. 10. Hardwick Quarry, Kilworthy, near Brent Tor. A 
 greenish-grey schistose rock ; in places vesicular, in other parts 
 containing numerous imbedded fragments of a finely-vesicular 
 rock, which even in the thinnest sections transmit no light 
 except through the vesicles, which are filled either with silica 
 or with matter resembling the matrix in which the fragments 
 lie. The following sketch (Fig. 9), indicates the irregular 
 
 forms and disposition of some of these fragments as seen on a 
 smoothly cut surface (natural size). Some of the imbedded 
 
 * Although the amygdaloidal character of some Schalstein is mentioned in 
 Zirkel's Lehrbuch der Petrographie, yet he in no way attempts to account for 
 vesicular structure in schistose rocks when describing the origin and mode of 
 formation of amygdaloids. 
 
MICROSCOPIC EXAMINATION. 37 
 
 fragments are probably of considerable size, judging from the 
 cavities (C, C, 0, C,) shown in the annexed rough sketch 
 (Fig. 10) of a loose specimen, apparently an exceptional one, 
 
 Fig. 10. 
 
 ;v 
 
 picked up in tTie quarry, since their dimensions rather negative the 
 supposition that they are air or gas vesicles or steam pores, 
 although numerous little vesicles of this kind surround them. 
 They are mostly lined with a crust of pyrolusite or wad, but in 
 one instance a recently broken lump contained in this specimen 
 consists of soft white earthy matter like kaolin. The rock is 
 spotted in places with stains of peroxide of iron. Under the 
 microscope, sections from this quarry show well-marked schistose 
 texture, closely resembling the fluxion structure so common in 
 the adjacent rocks of Brent Tor itself, and suggesting the idea 
 that some of the latter may merely represent an altered condition 
 of beds similar to these schists, or what seems more unlikely, that 
 the schistose beds are greatly altered lavas. The appearance of 
 fluxion structure in these schists is probably deceptive, and 
 represents the result of pressure which has compelled loosely 
 coherent particles to adjust themselves in directions varying with 
 the different unequal conditions of pressure and resistance caused 
 by the harder, irregularly-shaped fragments which lie imbedded 
 in the rock. The schist itself is filled with minute doubly- 
 refracting microliths which appear to follow no definite directions, 
 and have probably been developed since the rock was formed. 
 Numerous little whitish granules, almost opaque, are also to be 
 seen under the microscope. Roughly speaking the rock may be 
 called an ash, and it is no doubt mixed to some extent with 
 ordinary sedimentary matter. This stone is used in the con- 
 struction of Kelly College at Tavistock. As a building stone 
 it is likely to suffer more from disintegration by frost and damp, 
 than from decomposition. 
 
 No. 11. Langford Farm, on road from Tavistock to Milton 
 Abbot. Bluish-grey slaty rock, irregularly fissile and schistose 
 in places, the divisional planes coated with little scales of chlorite. 
 The rock contains a few well-marked vesicular bands, which a 
 little way in from the weathered surface become amygdaloidol, 
 the vesicles being filled with calcspar, and the aniygdaloidal bands 
 
38 ERUPTIVE ROCKS OF BRENT TOR. 
 
 thus bearing a close resemblance to the rock at Churlhanger. 
 Regarded en masse the deposit no doubt represents a considerable 
 thickness of ashy matter, mixed probably with ordinary sediment, 
 once mud, now shale. I have not examined this rock micro- 
 scopically, but it doubtless corresponds very closely with that from 
 Churlhanger, already described, so far at least as the vesicular 
 portions are concerned, while some of the more compact parts 
 probably resemble the rock occurring at Kilworthy. The fissile 
 structure corresponds approximately with the direction of the 
 vesicular bands ; the vesicles in these bands do not appear, as a 
 rule, to be much compressed. 
 
 No. 12. Brazen Tor, near Petertavy (exposure mapped as 
 greenstone). A dark close-grained crystalline rock occurring 
 close to the. granite of Dartmoor, and in contact with it. The 
 small imperfectly formed hornblende crystals which mainly com- 
 pose the rock have a somewhat fibrous or silky appearance ; the 
 other components are quartz and a little finely disseminated 
 pyrites. In thin section, when seen under the microscope, some 
 of the hornblende crystals are strongly dichroic, others only 
 feebly so, but the latter mostly lie in a different direction and are 
 spiculse rather than crystals. They are arranged in patches or 
 streams in which they rest side by side in definite directions ; 
 the streams are sometimes slightly tortuous, so that they oc- 
 casionally appear to have a wavy, fibrous texture. In Plate X. 
 (Fig. 1) a drawing is given, showing portion of a thin section of 
 this rock as seen by ordinary transmitted light under a magnifying 
 power of 55 diameters. The irregularly shaped black patches 
 represent pyrites, and the yellowish mineral hornblende. The 
 rock is Amphiboh'te (hornblende rock). 
 
 No. 12a. Close to the beck near Waspworthy, near Brazen Tor. 
 This is another patch of rock mapped in as greenstone. It is 
 identical with the preceding, both in its mode of occurrence and 
 in its mineral composition. The rocks from these two localities 
 appear to differ very slightly in texture. 
 
 No. 13. Cock' *s Tor. An iron-grey or dark greenish-grey 
 coarsely crystalline rock. It is in great part composed of a 
 pyroxenic mineral, the imperfectly developed crystals of which 
 have a somewhat bronze-like or metalloidal lustre. By Sir Henry 
 de la Beche this mineral was thought to be hypersthene. Pro- 
 fessor A. Renard, of Louvain, who has kindly examined it for me, 
 believes it to be diallage ; while Professor Rosenbusch, of Strass- 
 burg, to whom specimens were forwarded by Professor Re'nard, 
 takes it to be augite. In presence of two such good authorities 
 I feel some diffidence in expressing any strong opinion upon 
 this subject ; still I am inclined to think with Professor Renard 
 that, from the metalloid lustre, and from the difference in 
 intensity of the two cleavages in some of the crystals, they are 
 probably diallage. The green dichroic matter which is present 
 apparently results from the alteration of some of the pyroxenic 
 mineral into viridite. Titaniferous iron and felspars, both of 
 which have undergone considerable alteration, are the other 
 
MICROSCOPIC EXAMINATION. 39 
 
 components. The nature of the felspars is not evident, owing 
 to the change which they have suffered, but I am inclined to 
 think that they are only plagioclastic in part, one or two of. the 
 crystals exhibiting indications of only a single twinning plane. 
 Where alteration of the titaniferous iron has taken place, a white 
 substance is developed whose precise chemical nature is yet 
 unknown. In a paper, (< Some Results of a Microscopical Study 
 of the Belgian Plutonic Rocks," read before the Royal Microsco- 
 pical Society (April 5, 1876), Professor R^nard speaks of this 
 alteration of the Ilmenite in the Gabbro of Hozemont in the 
 following terms : " The sections of this titanic iron are sur- 
 rounded and covered in some cases with coatings of an opaline 
 substance, perfectly homogeneous, which seems a result of the 
 decomposition of ilmenite. The first stage of this decomposi- 
 tion is represented by the appearance of whitish veins running 
 through the mineral ; a second stage exhibits it enclosed in the 
 opaline substance ; finally, the metamorphosis can be pushed so 
 far that nothing more is visible except a few black specks. Its 
 chemical composition has not been determined, but we have 
 ascertained that it is unalterable by the action of hydrochloric 
 acid, and therefore it is not carbonate of iron, as has been taught 
 by some. We are, however, persuaded that the opinion of 
 Gumbel, who admits that it is not a decomposition product, 
 cannot be sustained."* Fig. 4, Plate X., shows this alteration 
 of titaniferous iron in the Cock's Tor rock. The drawing was 
 made partly by reflected and partly by transmitted light simul- 
 taneously, so that the alteration product appears of a greyish or 
 greenish-grey colour instead of white, as it does by reflected illu- 
 mination only. Believing with Professor Re'nard that the pyroxenic 
 mineral in this rock is diallage, I regard the rock as Gabbro, and 
 have marked it as such on the small map prefixed to this Memoir, f 
 Should the mineral, however, be really augite, as Professor 
 Rosenbusch is disposed to think, the name would probably have to 
 be changed to diabase, but even before this point could be satis- 
 factorily settled it would have to be shown, according to Dathe,J 
 that the plagioclase present was oligoclase. 
 
 No. 14. Indescombe, near Tavi stock. A greenish-grey crystal- 
 line rock containing dark-green, imperfectly-developed crystals of 
 a pyroxenic mineral. The rock weathers with a light-greyish 
 crust, and to the naked eye closely resembles diabase. The com- 
 ponent minerals appear under the microsoope to be decomposed 
 felspars (plagioclase), pyroxene (probably augite), some magnetite, 
 and a little quartz. It is a rock closely allied to, if not basalt. 
 If chlorite were present it might be regarded as diabase. The 
 alteration is mainly restricted to the felspars and the magnetite. 
 
 * The Monthly Microscopical Journal and Transactions of the Koyal Microscopical 
 Society, Vol. XV., page 217. 
 
 f Since this was written, Mr. S. Allport, in a paper " On the Metamorphic Eocks 
 surrounding the Land's End Mass of Granite." Quart. Journ. Geol. Soc., Lond., 
 Vol. XXXII., p. 421, has described the rocks of Cock's Tor and Smear .Ridge as 
 " altered gabbro or dolerite." (See page 49.) 
 
 J Zeitschrift d. deutschen gelog. Gesellsch, Vol. XXVI., page 1. 
 
40 ERUPTIVE ROCKS OF BRENT TOR. 
 
 No. 15. Cottage Inn, on main road from Tavistock to Mary- 
 tavy (about half a mile east of Indescombe). This rock is rather 
 more coarsely crystalline than that occurring at Indescombe, but 
 bears a strong resemblance to it both in its colour and general 
 aspect. The pyroxenic component of this rock has, however, a 
 somewhat bronze-like lustre in places, and exhibits under the 
 microscope cleavages of different intensity, similar to those which 
 occur in diallage. Professors Renard and Rosenbusch appear to 
 differ in opinion upon this subject, the former authority believing 
 he mineral to be diallage, while the latter considers that it is 
 augite. If diallage, the rock is gabbro \. if augite, it is basalt. I 
 am disposed to think with Professor Renard that it is gabbro. 
 Still the rock at Indescombe, although very similar, appears from 
 the cleavages in its pyroxenic component to more closely resemble a 
 basalt. I have nevertheless no doubt whatever that the exposures 
 at Indescombe and at this locality are connected, and that the 
 rocks, if they differ at all, pass by gradation into one another. 
 
 Fig. 3, Plate X., shows the cleavages of unequal intensity in the 
 pyroxenic mineral, which coupled with its metalloid lustre favour 
 the supposition that it is diallage, and consequently that the rock 
 is gabbro. 
 
 No. 16. A rock occurring near Ridge on the opposite side of 
 the Tavy corresponds very closely with the above in microscopic 
 characters, but appears to contain more titaniferous iron, in great- 
 part converted into opacite. This exposure at Ridge is also 
 doubtless connected with those at the Cottage Inn and Indes- 
 combe. 
 
 The following extract from a letter received from Professor 
 Re'nard, after he had examined the specimens and microscopic 
 sections from Brazen Tor, Cock's Tor, and the Cottage Inn, 
 cannot fail to interest those who may subsequently devote attention 
 to these rocks. 
 
 The first paragraph is an extract from a letter which he re- 
 ceived from Professor Rosenbusch : " After a careful examina- 
 tion of the mineral I find that it belongs to the pyroxene group, 
 but I do not think that it is diallage, since it fails to show the 
 cleavage and the characteristic interpositions of that mineral. I 
 am inclined to regard it simply as augite of the type which one 
 meets with in the palaeolithic diabases." 
 
 Professor Renard adds : " In the specimen of your rock, the 
 minerals are not sufficiently well terminated to show their 
 crystalline form. The cleavages exhibit a more or less metalloid 
 aspect, which, to my mind, is more suggestive of diallage than 
 augite. As to the lines of cleavage, such as they appear under 
 the microscope, it is difficult to say to what mineral they belong. 
 The absence of interpositions does not appear to me to prove that 
 the mineral is not diallage." 
 
 With regard to the pyroxenic components of the rocks at Cock's 
 Tor, Brazen Tor, &c. the following notes from the writings of 
 Zirkel and Boricky may also prove interesting : 
 
 Zirkel, in his Mikroscopische Beschaffenheit der Mineralien und 
 
MICROSCOPIC EXAMINATION. 41 
 
 Gesteine, pp. 181-3, states that the very feeble dichroism of diallage 
 indicates that it is allied in optical properties to augite and also 
 renders it easy .to be distinguished, from hypersthene and horn- 
 blende, and he adds that the mineral in the so-called hypersthenites 
 of Penig, Neurode, and the Isle of Skye, which was formerly 
 described as hypersthene must now be regarded as diallage. 
 
 Boricky, in his " Petrographische Studien an den Melaphyr 
 Gesteirie Bohmens," page 19, describes a mineral, which occurs 
 in some of the melaphyres of Bohemia, as an augite-like diallage. 
 The sections of this mineral show, under the microscope, numerous 
 parallel rectilinear fissures which he regards as the edges of 
 delicate twin-like lamellar intergrowths, probably lying parallel to 
 the basal planes of the crystals. In longitudinal sections of the 
 crystals, these fissures are seen to cross the coarser cleavage planes 
 at angles of 70 to 90 and sometimes occur only in one part of an 
 individual augite section. The mineral shows no dichroism. 
 
 No. 17. Lower Grenofen Quarry, near Tavistock. The chief 
 components of this rock are orthoclase, twinned on the Carlsbad 
 type, biotite, a little muscovite, and quartz. 
 
 U nder the microscope the orthoclase crystals are seen in many 
 cases to have undergone considerable change. A granulation or 
 development of opaque-white flocculent matter setting in around 
 the margins of the crystals. Some of the crystals are completely 
 altered in this manner, so that they no longer exhibit any definite 
 optical character. This physical change does not, however, appear 
 to affect the hardness of the crystals, nor to impair the value of 
 the stone for building purposes, for which it is well adapted. 
 The orthoclase crystals range from half an inch in length 
 downwards, giving a porphyritic character to the stone, which 
 renders it suitable for ornamental purposes in architecture. 
 The rock is essentially a fine-grained porphyritic granite with a 
 little felsitic material. It may also be regarded as a quartz 
 porphyry, although the porphyritic character is mainly due to the 
 large development of the felspar crystals. 
 
 No. 18. Shilla Mill Quarry (light porphyritic variety). 
 Under the microscope the components are seen to be orthoclase, 
 mainly decomposed, quartz, and magnesiaa mica. The matrix is 
 felsitic, crystals of magnesian mica are in places seen to lie within 
 the crystals of orthoclase, and also a few crystals of quartz. A 
 re-entering angle in one of the porphyritic crystals of orthoclase 
 is well marked in the section, but decomposition is far advanced. 
 The twining is upon the usual Carlsbad type. Crystals of 
 magnesian mica are also developed within this crystal. They appear 
 to lie parallel to the clinodome. The angles of the quartz crystals 
 are mostly rounded. The rock is a quartz porphyry or elvan. In 
 the same section crystals of magnesian mica are to be seen lying 
 within quartz crystals. From this it would appear that the mica 
 was the mineral which first crystallized. 
 
 No. 19. Shilla Mill Quarry (dark compact variety). In this 
 section again magnesian mica lies within the quartz crystals. 
 The felspar is in all cases converted into felsitic matter. In this 
 
42 ERUPTIVE ROCKS OF BRENT TOR. 
 
 section quartz, when in juxtaposition with magnesian mica, 
 has asserted its form. In some instances quartz appears to be 
 pseudomorphous after felspars. 
 
 No. 20. Shilla Mill Quarry. In this section again the magne- 
 sian mica is developed within crystals of quartz. The matrix is 
 felsitic. The felspar crystals are all decomposed, but the change 
 which they have undergone is different from that which has taken 
 place in the two sections just described. In the case of No. 18, 
 the crystals appear white by reflected light, the change having 
 apparently resulted from the conversion of the felspar into some 
 substance allied to kaolin. In the section No. 19 the felspars are 
 cnanged into felsitic matter, while in this section they seem to be 
 replaced partly by a micaceous mineral now greatly decomposed, 
 and probably represented by limonite, and partly by felsitic 
 matter. 
 
 Although the three sections of rocks just described are identical 
 in their original composition, yet they differ greatly in their 
 external aspect. 
 
 No. 21. Elvan-course in granite, Gunnis Lake Company's 
 Quarry, Hingston Down. A light-grey rock composed of 
 orthoclase, a little plagioclase, brown and white micas, and quartz. 
 The felspars have for the most part a granulated appearance 
 when seen by polarized light under the microscope. The quartz 
 contains some minute fluid cavities. The rock is essentially a 
 fine-grained granite, and cannot be regarded as a true quartz- 
 porphyry or elvan since there is no micro-felsitic magma. 
 
 No. 22. Top of Brazen Tor, near Petertavy. This is a 
 coarsely-crystalline granite, the chief components being orthoclase, 
 plagioclase, biotite, and quartz, with segregations of schorl. The 
 latter stand out in relief on the weathered surfaces of the rock, but 
 seem to be capriciously distributed through it. The section ex- 
 amined shows some interesting fluid cavities, some of which in 
 addition to bubbles contain minute cubes of rock salt. Some of 
 the best examples are shown in Fig. 5, PI. IX., as seen under a 
 magnifying power of 400 diameters. The orthoclase crystals in 
 this rock sometimes exceed 2 inches in diameter, rendering it very 
 coarsely porphyritic, and this within 100 yards of the junction 
 of the granite with the amphibolite. For the greater part of this 
 distance, however, the rock is very fine-grained and of a pale 
 pink tint, with large black segregations of schorl. In a section of 
 this rock lines are visible in some of the plagioclase crystals 
 cutting at right angles across the twin lamellae,* others are 
 altered by the development of a finely-granular structure. In 
 some cases, although this granulation has extended over the entire 
 crystal, the twin lamellae are still to be recognised. Fig. 4, 
 PI. IX. shows a crystal of biotite lying within a quartz crystal in 
 this section; while in other instances the biotite lies partially 
 
 * In a paper read before the Geological Society, May 12, 1875, the author pointed 
 out the existence of similar transverse bands m plagioclase from the Gabbro of 
 Volpersdorf in Silesia. 
 
MICROSCOPIC EXAMINATIONS. 43 
 
 enveloped within crystals of felspar. Little crystals of quartz 
 are also to be seen enveloped by felspar. From these facts it 
 would appear that the felspar was one of the last minerals to 
 crystallize, and the slowness of this process may in some measure 
 account for the large size which some of these crystals attain. 
 
 No. 23. Brazen Tor (near the contact of the granite with 
 amphibolite). This is the fine-grained pale-pink rock with black 
 segregations of schorl, just alluded to as lying between the top of 
 Brazen Tor, where the coarsely-crystallized porphyritic granite 
 occurs, and the amphibolite. The section when examined under 
 the microscope shows no trace of mica, while the hand specimens 
 appear to be equally deficient in this mineral. The rock is com- 
 posed of plagioclase, orthodnse, quartz, and schorl. The 
 orthoclase crystals are mostly Carlsbad twins. The schorl does 
 not appear to be well crystallized, yet this mineral, as well as the 
 quartz, is seen at times to encroach upon' the areas belonging to 
 the plagioclose crystals. A few minute singly-refracting granules 
 occur in the section, which are probably garnets. As felspars and 
 quartz are the principal mineral components of the rock and micas 
 are absent, we are bound to regard this marginal portion of the 
 granite as granulite or haplite. Fig. 2, PL IX., shows some of the 
 schorl in a section of this rock, magnified 25 diameters and viewed 
 by ordinary transmitted light. The blue and deep yellow portions 
 of the drawing represent schorl, the white spaces quartz, and the 
 pale-yellowish parts crystals, mostly of plagioclase which are some- 
 what altered. 
 
 No. 24. Greston Bridge Quarry. Slate below the lower flow 
 of altered greenstone (? basalt). This rock is an altered slate or 
 Lydian stone veined with quartz. It is of a brownish-black 
 colour, weathering light-brown, and has a splintery or subcon- 
 choidal fracture. 
 
 Under the microscope colourless microliths are visible, and they 
 lie approximately in the direction which the longest axes of the 
 component particles follow. This direction may be either that of 
 cleavage or bedding, but probably the latter, as there is no distinct 
 cleavage. The veins which traverse the slate have very sharp 
 outlines, and were no doubt formed and filled after the rock had 
 undergone most perfect consolidation. The veins are in part 
 composed of crystals or granules of quartz and partly by a finer 
 crystalline and apparently microlithic substance in which quartz 
 predominates. 
 
 In Fig. 2, Plate IX., a portion of the magnified section of this 
 rock is delineated, showing a slight flexure of the direction of 
 lamination in contact with one of the quartz veins ; which implies 
 considerable pressure and consequent molecular disturbance after 
 the veins were filled. 
 
 No. 25. Greston Bridge Quarry. (Slate between the flows of 
 altered greenstone). This is a section of a similar slate to that 
 just described. It shows well-marked lamination, and under the 
 microscope a few colourless microliths are visible, but they do 
 not invariably lie in the direction of the planes of lamination. 
 C. 31. D 
 
44 ERUPTIVE KOCKS OF BRENT TOR. 
 
 Neither in this nor in the preceding rock was there any 
 stronger evidence of alteration at the bottom than at the top of 
 the slate bed. Whatever has taken place, change physical or 
 otherwise, it seems to have affected these little slate beds (only a 
 few feet in thickness) equally throughout. 
 
 No. 26. Chert band in upper bed of altered greenstone^ 
 Greston Bridge Quarry. In this section the only interesting 
 features are the occasional imperfect development of quartz 
 crystals, which in some cases have segregated in the vicinity of 
 crystals of iron pyrites, the latter sometimes having the form 
 of definite cubes. These chert bands are of a deep greenish-grey 
 colour and seem to have been crushed and re-cemented. They 
 run irregularly through the altered greenstone and are only a 
 few inches in thickness. 
 
 No. 27. Greston Bridge Quarry. Under the microscope very 
 little can be made out concerning the original mineral compo- 
 sition of this rock. Elongated prisms which represent what once 
 were felspars alone remain to attest its eruptive character, a 
 supposition which becomes a certainty when the indurated nature 
 of the associated ,4ate beds is taken into consideration. The 
 rock is mainly converted into serpentine. Calcspar fills up the 
 joint-planes and is disseminated through the rock, yet it plays quite 
 an unimportant part in its composition, since an average sample 
 of the stone when ground smooth and immersed in acid underwent 
 only very trifling erosion. A dark mineral is visible in the 
 sections of this rock which is probably limonite, and which shows 
 indications of being pseudomorphous after pyroxene or amphiboly, 
 and also after pyrites or magnetite but no well-developed forms 
 are discernible. Some of the altered felspar crystals still seem to 
 show traces of triclinic banding, but the appearances under 
 polarized light are so hazy that it is unsafe to hazard any definite 
 opinion about the minerals. In one section opaque acicular 
 bodies are developed within nebulous areas which seemingly repre- 
 sent crystals, but the margins of the iatter are ill-defined and they 
 present no trustworthy optical characters. A drawing of portion 
 of a magnified section of this very unsatisfactory rock is given in 
 Fig. 1, Plate IX. 
 
 A section cut from a small specimen procured in the roadside 
 on the north of the quarry shows numerous dark green spots 
 about the tenth of an inch in diameter. These have possibly 
 resulted from the alteration of olivine. 
 
45 
 
 PAKT III. 
 
 DEDUCTIONS. 
 
 The deductions to be drawn from the foregoing observations 
 serve to show that the views entertained by the late Sir Henry 
 de la Beche are in the main correct, and represent a vast amount 
 of truth derived simply from observations in the field. The doubt 
 which he expresses relative to the boundary line between the 
 Devonian deposits and the culm series, still leaves it an open 
 question whether the eruptive rocks which lie around Brent Tor 
 belong exclusively to the latter period or not. While if the 
 different belts of ash marked on the map represent a repetition 
 of the same beds, which appears highly likely, it precludes the 
 supposition that they were deposited partly in Devonian and 
 partly in carboniferous times. In all probability the boundary 
 between the rocks of these two periods lies somewhat to the south 
 of Tavistock. On the Survey map all the exposures of eruptive 
 rocks are coloured as greenstone, but the " Report " upon the 
 district shows clearly enough that they were not regarded as one 
 and the same rock. Upon the small map prefixed to this Memoir 
 an endeavour has been made to indicate some of the lithological 
 differences which exist, and it is shown that the rocks coloured as 
 one on the Survey map represent Amphibolites, Gabbros, Basalts, 
 Pitchstones, and Schistose ashes or clastic rocks of a doubtful 
 nature. One point is especially worthy of remark, namely, that 
 some of the rocks which in part compose Brent Tor itself are 
 apparently unrepresented in any other portion of the district, 
 while another is that the Tor forms the highest point of ground 
 for some considerable distance around. These are two very 
 significant facts, and a right interpretation of them may do much to 
 elucidate the geological history of this neighbourhood. After care- 
 ful consideration of the question, it seems probable that Brent Tor 
 as it now exists, only represents a portion of the series of volcanic 
 ejectamenta and lava flows which have taken place, as suggested 
 by Sir Henry de la Beche, at or near a crater. What has become 
 of the remainder of these rocks, and to what is the preservation of 
 this vestige of their former existence due ? Brent Tor by no 
 means represents a complete crater, for, if it did represent an 
 entire structure of this kind, it is evident that outcrops of rock 
 of a similar character would occur in the vicinity. Furthermore, 
 since these rocks are much harder and less susceptible of 
 weathering than those which surround them, it is evident that 
 if this were the case any other outcrops of them which existed 
 would form prominent features in the country. Since there is no 
 indication of this, it is clear that Brent Tor represents the last 
 trace of volcanic rocks of that particular character remaining in 
 the district, and we have therefore only to deal with the two 
 questions just enunciated. What has become of the remainder of 
 
 D 2 
 
46 ERUPTIVE ROCKS OF BRENT TOR. 
 
 these rocks and to what is the preservation of this little volcanic 
 legacy due ? It might be due to the Tor having occupied the 
 lowest portion of a synclinal fold, while all the corresponding 
 rocks rested at higher levels and were denuded, but the gentle 
 undulations into-which the surrounding beds are bent negatives 
 such an hypothesis. As a last resource we are thrown back upon 
 a fault, and it appears very probable that one may exist imme- 
 diately to the east of the Tor. 
 
 I can adduce no evidence in support of this supposition other 
 than that suggested by the foregoing remarks, and by the break in 
 continuity of some of the schistose beds shown as volcanic ash on 
 the Survey map. If, however, these, beds represent corresponding 
 beds east and west, the assumption that a fault exists requires little 
 additional proof, while if it be inferred that some of those belts 
 of ash which observe an annular disposition around the eastern 
 and southern aspects of the Tor, dip inwards and form a synclinal 
 fold, it follows that they are superficial representatives of one and 
 the same bed. If this much be granted, it is easy to account for 
 the diminished distance between their assumed continuations on 
 the north and east of the Tor, for if the fault be considered to 
 run in a N.N.W. direction, and the downcast to have taken place 
 on the western side, it is manifest that after denudation the chord 
 of the eastern synclinal arc will be less than that of the more 
 depressed western syncline. It has, however, yet to be demon- 
 strated that the ash belts lying east and west of the assumed line 
 of fault do really represent continuations of the same beds ; this 
 can only be shown, so far as I know, by better and more satisfac- 
 tory exposures of these rocks than those now visible ; moreover, 
 the strip of rock between Bowdon and West Langstone, and pro- 
 bably that between Shallor and Lower Chillaton, dip away from 
 Brent Tor. Many of the changes in dip may, however, be due to 
 very slight undulations. We have then a series of ash beds and 
 clastic rocks girding Brent Tor which are nearly related to it in 
 date of formation, both being of Carboniferous age and occupying 
 closely succeeding horizons. Brent Tor would have been swept 
 away nges ago had it not been nested at a lower level than the now 
 demolished remains of the crater and series of lava flows and 
 agglomerates of which it once formed a part. The balance of 
 evidence, such as there is, tends to show that it owes it preserva- 
 tion to a fault, a fault unmapped and the existence of which is 
 as yet purely a matter of inference. 
 
 The masses of gabbro extending between Crayston and White 
 Tor, Wheal Friendship and Smear Ridge, are no doubt con- 
 nected with one another at some depth beneath the surface, and it 
 is probable that the small patches occurring in contact with the 
 granite at Waspworthy and Brazen Tor and the strip extending 
 from Cock's Tor to Indescombe, are offshoots from the same 
 deep-seated mass. These may even be of later date than the 
 granite of Dartmoor, and so far as evidence goes, there is nothing 
 to show that they are in any way connected with the volcanic 
 series of Brent Tor and its surrounding; ash beds. 
 
DEDUCTIONS. 47 
 
 Upon the rocks at Greston Bridge and Dunterton it is very 
 difficult to offer any opinion,, since they are so much decomposed 
 (and in great part converted into serpentine) that it is scarcely 
 safe to hazard any definite suggestion as to their original com- 
 position. They may have been basalts. One thing, however, is 
 certain, namely, that they represent lava flows, separated by a 
 short period when ordinary sedimentary deposits were formed. 
 Lack of time precluded the possibility of making more extended 
 observations upon those portions of the ash and trap belts which 
 are left blank upon the map which accompanies this Memoir ; but 
 inmost instances, where they were examined, scarcely any evidence 
 could be procured other than that afforded by road metal ; the land 
 was for the most part under cultivation, the few exposures which 
 were visible were not trustworthy, and, unless better evidence 
 were to be procured when the Survey map was made, it is clear 
 that some of the boundary lines must have been drawn from the 
 superficial features of the ground. Still, where reliable outcrops 
 occur and where I have had an opportunity of verifying the work 
 done by the founder of our survey nearly forty years ago, there 
 seems no reason whatever to doubt its accuracy, nor the clearness 
 of perception with which boundary lines were drawn and rocks 
 mapped in, even where the evidence was scant and the record hard 
 to be deciphered. 
 
 The eruptive rocks of Brent Tor and its immediate neighbour- 
 hood apparently bear no relation to those in the neighbourhood of 
 Exeter, on the eastern side of Dartmoor. The latter may repre- 
 sent a continuation of the volcanic action which took place in 
 Carboniferous times, but that they are of much later date there can 
 be no question ; for although in places they are very similar to 
 the rocks of the Tavistock district, the sedimentary deposits with 
 which they are associated belong to a later geological period. In 
 considering the history of this district it is important to bear in 
 mind the stratigraphical relations of the sedimentary rocks, the 
 displacements, vertical and lateral, which they have undergone, and 
 the enormous amount of material which has been removed by 
 denudation. The Devonian and the Culm series apparently rest 
 conformably on one another in this area, and in the absence of 
 any strong palaeontological evidence to the contrary we may 
 regard the two series as representing one uninterrupted period 
 of deposition of sediment under varying bathymetrical conditions ; 
 but the local paucity of fossils greatly diminishes the chance of 
 finding organic evidence in connexion with such changes. 
 
 No serious stratigraphical disturbance in this small area marks 
 the period when these old marine deposits took place, although, 
 from the evidence furnished by the eruptive rocks, it is clear that 
 volcanic action was going on during the depos : tion of the lower 
 portion of the Culm series. It may perhaps be due to the opening 
 of small safety valves that no great flexures were developed in 
 the strata through which these rocks were extruded and with 
 which they are interbedded, and we may with some show of 
 reason believe that this state of things continued until nearly the 
 
48 ERUPTIVE BOOKS OF BRENT TOR. 
 
 whole of the Culm rocks were deposited. At a subsequent period 
 upheaval ensued, and it was probably then that the Devonian and 
 Culm series underwent some lateral pressure which threw them 
 into the major synclinal fold, which now brings the Devonian rocks 
 to the north and south of the Culm area, and at the same time 
 produced those minor undulations which are obscurely indicated 
 by not very numerous and often not very trustworthy dips. 
 
 After or during upheaval marine denudation commenced its work 
 along the margins of the emerging land, but what time elapsed before 
 this important elevation of the district took place it is hard to say. 
 It probably occurred towards the close of the Carboniferous period 
 or in Permian times, and it was most likely during this upheaval 
 that the intrusive granitic masses which constitute Dartmoor, King- 
 ston Down, &c., forced their way into the Culm series, in places 
 uptilting the surrounding beds, as pointed out by Sir Henry De la 
 Beche. 
 
 Tht occurrence of granitic debris in the Triassic rocks seems 
 to indicate that rocks of granitic character were denuded before 
 or during the deposition of the Triassic beds ; yet although the 
 latter have also in great part been removed by similar agencies, 
 it is probable that granitoid rocks connected with those of Dart- 
 moor and some of the other adjacent granite areas may have formed 
 islands in comparatively early Mesozoic times Through subsequent 
 changes of level coupled with periods of marine and of subaerial 
 denudation the country has assumed its present surface configura- 
 tion, but it seems likely that the harder eruptive rocks of this 
 district have at all times resisted the destructive action of the sea 
 as well as that of rain and rivers, far more than the softer rocks 
 which surrounded them ; and consequently it is only reasonable to 
 assume that at whatever period marine denudation took place 
 after these hard rocks were once laid bare, they would invariably 
 be the spots which offered the greatest resistance when upheaval 
 and denudation of subsequent deposits ensued : and that at all 
 events, if the process of upheaval were a slow one, they formed 
 small islands, and may even have existed as such within compara- 
 tively late geological times. 
 
 Tinstone and copper pyrites constitute the chief metalliferous 
 lodes of importance in and around these granitic masses. Man- 
 ganese ores, arsenical pyrites, and hematite also occur in their 
 neighbourhood, but the former are more plentiful at a distance 
 from the granite, the presence of which also seems to be inimical 
 to the development of lead ores. 
 
 Since this work was written a paper was read by Mr. Samuel 
 Allport before the Geological Society (June 21, 1876) ( ( On the 
 Metamorphic Rocks surrounding the Land's-End Mass of 
 Granite." 
 
 In this paper the author compares some of the rocks of Brent 
 Tor and the Tavistock neighbourhood with those which occur in 
 
DEDUCTIONS. 49 
 
 the Penzance district. The following extracts from the paper 
 confirm some of the statements made in this Memoir, but Mr. 
 Allport does not appear to have detected the rhyolitic character 
 of some of the rocks which compose Brent Tor. 
 
 " At Peter Tavy the rock forming Smear Ridge and its pro- 
 longation to Cock's Tor, near the granite, is an altered gabbro or 
 dolerite, some specimens having a coarsely crystalline texture, 
 while others are fine-grained ; both varieties are rather less altered 
 than the Penzance rocks, and are, therefore, important for 
 comparison. 
 
 " In the coarsely crystalline variety the alteration is of 
 precisely the same character as that described in the Tolcarn 
 rock ; the diallage has been partially converted into hornblende, 
 and this substance also fills veins and former cavities ; the felspar, 
 though highly altered may be readily recognised ; and there are 
 the same pseudomorphs after magnetite. 
 
 " In a fine-grained specimen of the same mass the pyroxene 
 (apparently augite) is very well preserved, but the felspar is com- 
 pletely decomposed, being represented by pale green pseudo- 
 morphs; the same substance also fills cavities, several of which 
 contain groups of radiating blades of tremolite. 
 
 " Brentor, four miles north of Tavistock, presents many of the 
 features of a volcanic mass, being chiefly composed of purple 
 bedded ash, together with scoriaceous and compact trap of a 
 greenish-grey colour. 
 
 ef The latter is an altered basalt, in which the augite occurs in 
 small well-formed crystals ; it has suffered little or no alteration, 
 while the felspar is converted into pale green pseudomorphs. 
 
 " The mass of ' greenstone,' half a mile north-east of Brentor,. 
 is chiefly ash of the same character as that of Brentor itself." 
 Quart. Journ. Geol. Soc., Lond., vol. XXXII., p. 421. 
 
 The last statement, here quoted from Mr. Allport's paper, 
 seems to me to refer to an exposure of schistose rock visible in a 
 narrow lane near Brent Tor. The rock did not impress me with 
 the idea that it was identical with any of those which actually 
 constitute the Tor ; and, although I collected a few specimens, 
 only one of these has yet been examined microscopically. This is 
 a vesicular basalt, with a matrix which, in part, is singly re- 
 fracting. The augite is mainly decomposed. If Mr. Allport's 
 conclusion be a correct one, it may serve to indicate the down- 
 throw-side of the problematical fault, the existence of which I have 
 endeavoured to demonstrate. I should, however, have anticipated 
 that, where rocks identical with those of Brent Tor occurred, a 
 more marked feature would have been developed. 
 
 POSTSCRIPT. 
 
 During the revision of this Memoir I have read an interesting 
 paper by Dr. Harvey B. oil, <s On the Older Rocks of South 
 
50 
 
 ERUPTIVE HOCKS OF BRENT TOR. 
 
 Devon and East Cornwall," published in the Quarterly Journal 
 of the Geological Society for November 1868. In this paper 
 Dr. Holl states that there are numerous faults in the vicinity 
 of Brent Tor, one of them crossing the Kiver Lyd, south-west of 
 Cory ton, and ranging by Monkstone to the west of the Tor. 
 ({ Another skirts the north-east side of the volcanic rocks of the 
 Tor, extending from near Monkstone to South Brent Tor. These 
 faults carry the country further to the south or rather south-east, 
 and reverse the dips along the valley of the Tavy, where the beds 
 rise to an anticlinal axis, which crosses the railway in a N.N.E. 
 and S.S.W. direction, midway between Ford Gate and the 
 Marytavy Railway Station." 
 
 Although Dr. Holl does not appear to have investigated 
 the important bearing which these faults have upon the question 
 of the preservation of Brent Tor itself, I am glad to find that his 
 statements, so far as the directions of these faults are concerned, 
 coincide approximately, and in one instance perfectly, with the 
 conclusions at which 1 had independently arrived. 
 
 APPENDIX. 
 LIST OF MINERALS FROM THE NEIGHBOURHOOD OF TAVISTOCK. 
 
 Actinolite.* 
 
 Anatase. 
 
 Augite.* 
 
 Axinite.* 
 
 Bismuthine. 
 
 Bitumen. 
 
 Blende. 
 
 Brookite. 
 
 Calcite. 
 
 Chalcotrichite. 
 
 Chalybite. 
 
 Childrenite. 
 
 Chlorite. 
 
 Clinoclase. 
 
 Copper, Native. 
 
 Copper Pyrites. 
 
 Copper-uranite . 
 
 Cuprite. 
 
 Diallage. 
 
 Erythrine. 
 
 Fahlerz. 
 
 Fluor. 
 
 Galena. 
 
 Garnet. 
 
 Jasper.* 
 
 Kupfernickel. 
 
 Limonite. 
 
 Manganite. 
 
 Marcasite. 
 
 Mispickel. 
 
 Olivenite. 
 
 Psilomelane.* 
 
 Pyrites. 
 
 Pyrolusite.* 
 
 Quartz. 
 
 Rhodonite. 
 
 Scheelite. 
 
 Silver, Native. 
 
 Titaniferous Iron, 
 
 Vivianite. 
 
 Wolfram. 
 
 * These minerals have been stated to occur at Brent Tor. The immediate 
 neighbourhood is probably meant, not merely the tor itself. 
 
INDEX. 
 
 Acids, application of to microscopic sections 
 Age of Granite of Dartmoor 
 
 Kingston Down 
 
 Allport, S. - 
 
 Alteration around the granite of Dartmoor 
 Altered greenstone defined 
 
 at Greston Bridge 
 
 rocks near Cock's Tor 
 
 slate at Greston Bridge Quarry 
 Amphibolite of Brazen Tor 
 Amygdaloidal schist at Churlhanger 
 Apatite, decomposition of - 
 
 microscopic characters of - 
 Aphanite, mineral constitution of - 
 Arsenical pyrites - 
 Ashes, volcanic - 
 Augite, nondichroism of - 
 Augitoid diallage - 
 
 Page 
 
 - 48 
 
 - 48 
 39,48 
 
 - 14 
 
 - 13 
 
 - 44 
 
 - 15 
 
 - 43 
 15,38 
 
 - 35 
 
 - 6 
 
 - 4 
 
 - 12 
 
 - 48 
 
 - 6 
 
 - 3 
 
 - 41 
 
 B. 
 
 Balls in schistose ash at Kilworthy 
 Basalt of Brent Tor 
 
 Indescombe - 
 Baveno type, twinning of felspars on 
 Beale, Dr. Lionel S. 
 
 Beale's reflector ----- 
 Belgian gabbros similar to those of Devon 
 Boase, Dr. - 
 
 Boricky, Prof. E. - 
 
 Brazen Tor ----._ 
 
 amphibolite of - ... 
 
 granite of ..." 
 
 rock-salt in fluid cavities in quartz of granite of 
 schorl in granite of - 
 Brent To basalt of - .... 
 
 devitrified rhyolite of - 
 imbedded fragments in rocks of - 
 
 preservation of, due to faulting - 
 pumice, breccia of 
 rhyolite, breccia of 
 
 scoriaceous lava of - 
 
 Bronzite -----__ 
 Building-stones, remarks on 
 Burnford Farm - - ... 
 
 - 23 
 34,49 
 16,39 
 
 - 2 
 
 - 11 
 
 - 11 
 
 - 15 
 8,28 
 
 - 41 
 
 - 26 
 15,38 
 42,43 
 
 - 42 
 - 26, 42, 43 
 
 39,49 
 
 - 33 
 
 - 35 
 
 - 46 
 
 - 32 
 
 - 32 
 
 - 34 
 
 - 4 
 
 - 6 
 
 - 17 
 
 c. 
 
 Camera for microscopic drawing 
 
 Capel 
 
 Carlsbad type, twinning of felspars on 
 
 - 11 
 
 - 35 
 
 o 
 
52 ERUPTIVE BOOKS OF BRENT TOR. 
 
 Page 
 
 Cavities, fluid, in quartz of Brazen Tor - -42 
 
 in schistose ash at Kilworthy - 23, 37 
 
 Chert in altered greenstone at Greston Bridge - - 44 
 
 Chowen, Mr. G. - - 25 
 Churlhanger, schalstein of- - 17, 23, 35 
 Clastic rocks - ... 13 
 Cock's Tor, gabbro of 15, 17, 38, 39 
 
 Colourless bands in polarization of felspars - 2 
 
 Combe, ash at - 23 
 
 Constituent minerals of rocks, table of - 12, 13 
 
 Cottage Inn, gabbro of 16, 40 
 
 Craystone - - 27, 46 
 
 Culm, boundary of 17, 45 
 
 conformable on Devonian rocks - - 47 
 
 D. 
 
 Dartmoor, granite of - 46, 47, 48 
 
 Dathe, J. F. E. - - 39 
 Debdon, plagioclase in basalt of 
 
 Decomposition of titan iferous iron - - 39 
 
 De la Beche, Sir Henry - 7, 8, 14, 17, 18, 21, 23, 28, 40, 45, 48 
 
 Devitrified rhyolite of Brent Tor - -_ 33 
 
 Devonian rocks, boundary of 17, 45 
 
 Diabase, mineral constitution of - 12 
 
 (?) at Indescombe - 39 
 
 Diallage - - 4 
 
 augite-like, of Boricky - - 41 
 
 in gabbro of Cock's Tor - - 38 
 
 Dichroism of minerals 4, 41 
 
 Diorite, mineral constitution of - 12 
 
 (?) of Stiles Wick - 
 
 Downhouse Farm - - 22 
 
 Drawings of microscopic preparations 1 1 
 
 Dunterton, greenstone at - 17, 47 
 
 E. 
 
 Elvan. See quartz porphyry - 13 
 
 Elvan dykes - 24 
 
 in granite Gunnis Lake Quarry - - 42 
 
 ,, Lower Grenofen Quarry - - 41 
 
 Shilla Mill Quarry - - 41 
 
 Enstatite - - 4 
 
 Etched surfaces of rocks - - 10 
 
 Exeter, eruptive rocks around - 4/ 
 
 F. 
 
 Faults at Brent Tor 46, 50 
 
 Felspars, orthoclase, large crystals of, in granite of Brazsn Tor - - 42 
 
 ,, polarization in 2 
 
 twinning of 
 
 weathering of 3 
 Ferrite 
 
 Fine-grained rocks, examination of 
 
 Fluid-cavities in quartz of granite at Brazen Tor - - 42 
 
 Fluxion structure,' deceptive appearance of, in s hists - 37 
 
 in rhyolites of Brent Tor - 33 
 
 Forbes, D.- 11 
 
 Fruchtschiefer - 26 
 
INDEX. 53 
 
 G. 
 
 Page 
 
 Gabbro of Cock's Tor - 15, 38 
 
 Cottage Inn - 16, 40 
 
 Ridge - - - - ' - . 23; 40 
 
 Goniometers for microscope - -. . * - 10 
 
 Granite, mineral constitution of 13 
 
 mural jointing in - - 27 
 
 of Brazen Tor - 42,43 
 
 of Great Staple Tor - - 28 
 
 of Gunnis Lake Quarry > 26, 42 
 
 Granulite, mineral constitution of - - ".* - - 13 
 
 Great Staple Tor, granite of - " - 28 
 
 mural jointing in granite of - 28 
 
 Greenstone name how applied altered, at Greston Bridge I/, 44 
 
 ash __-_-- -16 
 
 broad signification of term - - 16 
 
 coloured, on sheet 45 - - 45 
 
 Greston Bridge, altered greenstone at - 1 7, 44, 47 
 
 Lydian stone at - - 17 
 
 ,, Quarry, altered slate at - 17>43 
 
 Gunnis Lake Quarry, granite of - 26, 42 
 
 H. 
 
 Haplite of Brazen Tor - - 43 
 
 Hardwick Quarry, schistose ash of - ,23, 36 
 
 Hematite - - - - . *- - - - -48 
 
 High magnifying powers, employment of - 
 
 Kingston Down, granite of - 26, 48 
 
 elvan course in granite - - - 42 
 
 Holl, Dr. H. B. - - - 49 
 
 Hone-stone at Kilworthy - - - 23 
 
 Hornblende, dichroism of - 3 
 
 Hornblende rock, mineral constitution of - - 12 
 
 of Brazen Tor - 15,38 
 
 Horndon, blocks of gabbro at - 27 
 
 Hydrous alkaline silicates - - - 5 
 
 Hypersthene - - 4 
 
 -rock - - - 28 
 
 I. 
 
 Ilmenite, decomposition of- - 39 
 
 Imbedded fragments in rocks of Brent Tor - 35 
 
 microscopic appearance of 7 
 
 Indescombe, basalt of 16. 39 
 
 J. 
 Jordan, Mr. J. B., section-cutting machine of - 11 
 
 K. 
 
 Kelly College, stone used for building of - 23, 37 
 
 Kilworthy, balls in schistose ash at 
 
 schistose ash at 
 
 Kingsford Corner - - 28 
 
54 ERUPTIVE ROCKS OF BRENT TOR. 
 
 Page 
 
 Lamerton, decomposing rock at - - 23 
 
 Langford Farm, section shown at - - 22, 37 
 
 Lead ores - - - 48 
 
 Leucite - 5 
 
 Limestone fragments in basalt - 10 
 
 List of minerals from Tavistock neighbourhood - - 50 
 
 rocks and their constituent minerals - 12 
 
 Literature, micro-petrol ogical - - - 11 
 
 Lower Grenofen Quarry - * -41 
 
 Lydianstone at Greston Bridge - 17, 43 
 
 M. 
 
 Machines for cutting sections of rocks - - 1 1 
 
 Magnesian mica, determination of, under the microscope r 4 
 
 Magnetite, alteration of - 3 
 
 Manganese mines, capel in - 35 
 
 oxides - 23, 35, 37, 48 
 
 Margins of granite masses - 25 
 
 " Master rock " at Horndon - 27 
 
 Meavy, alteration of slates at - 14 
 
 Microscopic characters of Apatite - - 4, 6 
 
 of Leucite - 5 
 
 of Nepheline - 4 
 
 of Olivine - - 4 
 
 Microscopic drawings - - 11 
 
 Micro-structure of felspars 
 
 Mineral constitution of rocks, table of - 12 
 
 Minerals, list of, from neighbourhood of Tavistock - 50 
 
 Morwell down tunnel, section in - - 25 
 
 Morwell rocks, altered slates - 25 
 
 Mount Tavy - 22 
 
 Mural jointing in granite - - 27 
 
 - N. 
 
 Nassau, schalstein of - 36 
 
 Nepheline -- _ - _ - - - 4 
 
 Neutral-tint reflector - - - - - - -11 
 
 o. 
 
 Olivine - - - 4 
 
 Opacite _-- -----5 
 
 Origin of rocks - - 6 
 
 Orthoclase, large crystals of, in granite of Brazen Tor - - 42 
 
 twinning of - - 2 
 
 P. 
 
 Perthite, inter-lammellation of 3 
 
 Petertavy - - 42,49 
 Pillands - 
 
 Pitchstone, definition of - - 13 
 
 Plagioclase in basalt of Debdon - - 3 
 
 twinning of 2 
 
INDEX. 
 
 Page 
 
 Pocket lens, use of - 1 
 
 Polarised light, employment of -2 
 
 Porphyritic granite of Brazen Tor - - 42 
 
 Pumice-breccia of Brent Tor - - 32 
 
 Q- 
 
 Quartz of Brazen Tor Granite, inclosures in - 42 
 
 Quartzite of West Langstone - - - - - 35 
 
 Quartz-porphyry, mineral constitution of - - - 13 
 
 " of Lower Grenofen Quarry - 41 
 
 of Shilla Mill Quarrv - - - - - 41 
 
 R. 
 
 Reagents, application of under the microscope - - - - 10 
 
 Reflected light, employment of in microscopic examination - 3 
 
 Reflector, Beale's neutral tint - - - - - 11 
 
 Renard, Prof. A. - - 6, 15, 38, 39, 40 
 
 Rhyolite, definition of- - - - - - -13 
 
 devetrified, of Brent Tor ... -33 
 
 Rhyolitic-breccia of Brent Tor - - - - - - 32 
 
 Ridge, gabbro at ------ 23, 40 
 
 Rocks, origin of - - - - - 6 
 
 Rock-salt in fluid cavities in granite of Brazen Tor - - - 42 
 
 Rosenburch, Prof. H. ... -38,40 
 
 Schalstein ......... - 24 
 
 definition of - - - - - - 13 
 
 Schistose ash of Kilworthy - - 36 
 
 rocks -- ....-8 
 
 Schorl in granite of Brazen Tor - * 26, 42, 43 
 
 microscopic characters of - 4 
 
 Schorlaceous granite of Brazen Tor - - 34 
 
 Scoriaceous lava of Brent Tor - - - 34 
 
 Shilla Mill Quarry - 9, 41, 42 
 
 Shortabura ... .23 
 
 Smear Ridge, gabbro of - - 15, 46, 49 
 
 Sorby, Mr. H. C. - - - 6 
 
 Sowtentown - - - - 16 
 
 Stiles wick, diorite at - 22 
 
 T. 
 
 Table of mineral constituents of rocks - - - - 12 
 
 Tamar Granite Quarry - - - 26 
 Tinstone - - - - - - - - -48 
 
 Titaniferous iron, alteration of - - 39 
 
 in gabbro of Cock's Tor - - 38 
 
 Trachydolerites of Wales and Lake District - - - 9 
 
 Tschermak, method of distinguishing hornblende from augite - - 3 
 
 Tuffs . . 6 
 
 Twinning in felspars - ... 2 
 
 Twowell 23 
 
56 . . ERUPTIVE R.OCKS OF BRENT TOR. 
 
 V. 
 
 Page 
 
 Vesicular fragments in schistose ash - - 36 
 
 Viridite - - 5 
 
 Volcanic ashes - 6 
 
 tuffs - 6 
 
 W. 
 
 Ward, Mr. J. C. 9 
 
 Waspworthy, amphibolite of 15, 38 
 
 Weathering of felspars - - 3 
 
 Week, quartzite of - 35 
 
 West Langstone - - 35 
 
 Wheal Friendship - ..... 45 
 
 White Tor 27, 56 
 
 Wolf rock, nepheline in 4 
 
 Wollaston's prism - - 11 
 
 Wrington Warren, limestone in basalt of - - - 10 
 
 z. 
 
 Zirkel, Prof. F. - 40 
 
Jeological S-urv&y c England. & Wales 
 
 vn. 
 
 Brent Tor. (South, side, near the ba6&.J*25. 
 2 
 
 Brent Tor (North, sidej *25. 
 
 Vinseut Brook* Davi Son, Lith 
 
Geological Survey oc England. & Wales 
 
 400 
 
 1. Decomposed Greenstone, Greston Bridge. 2. Slate beneath, lower Greenstone, Greston Budge. 
 S.Etvan S. end of Shilla. Mill Quarry, (Polar d ) 4 Ma^MicsL&cavities in Quartz of Granite, Brazen Tor 
 5. Fluid cavities contl cubes of NeuCLBraLzenTor. 6. Amygdaloidal Sdialstem,Churlhan|ern r Leunerton 
 
 Vincrat Brooka.Day &Son.Lith. 
 
Geological Survey o England & Wales 
 
 N?l Homblcnde.Pjmtfls and Quartz m Amphibolite, Brazen tor, Devon 
 2. Schorl in the Granulvtic margin of the Granite in contact with N 1. 
 3. Pyroxene fprohabty- Diallage) in&abbro^ Cottalehin.MainRoaxitD Majytayy 
 4. Titaiuferous Iron, partly altered, fGabbro) Cocks Tor.near Tavistock. 
 
 Nl, *S&. N os 2,3 and 4*25. Ordinary transmitted Ught, Onthe surface of 
 N94atittU Ught was also reflected 
 
 Vaent Brook.. Dv& Son.Uth 
 
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 Clee Hill, 53 (NE, NW). 
 Denbighshire, 74 (NE & SE^, 79 (SE). 
 Derby and Yorkshire, 71 (N'W, NE, & SE), 82 (NW & SW), 
 
 81 (NE), 87 (NE, SE), 88 (SE). 
 Flintshire, 79 (NE & SE). 
 Forest of Dean, 43 (SE & SW). 
 Forest of Wvre, 61 (SE), 55 (NE). 
 
 Lancashire, 80 (NW),8i (NW), 89 (SB.NE, NW, & SW), 88 
 (SW). (For corresponding six-inch Maps.see detailed list.) 
 Leicestershire, 71 (SW), 63 (NW). 
 Newcastle, 105 (NE & SE). 
 North Staffordshire, 72 (NW), 72 (SW), 73 (NE), 80 (SE), 
 
 81 (SW). 
 
 *8outh Staffordshire. 54 (NW). 62 (SW). 
 Shrewsbury, 60 (NE), 61 (NW & SW). 
 South Wales, 36, 37. 38, 40, 41, 42 (SE, SW). 
 Warwickshire, 62 (NE & SE), 63 (NW & SW), 54 (NE), 53 
 
 (NW). 
 Yorkshire, 88, 87 (SW), 93 (SW). 
 
 SCOTLAND. 
 
 Edinburgh, 32, 33. * Haddington, 32, 33. 
 
 Fife and Kinross, 40, 41. 
 
 IRELAND. 
 
 auturk, 174, 175. *Castlecomer, 128, 137. 
 
 jllienaule (Tipperary),146. 
 ff or Sections illustrating these Maps, see detailed list.) 
 * With descriptive Memoir. 
 
 GEOLOGICAL MAPS. 
 
 ^ Scale, six inches to a mile. 
 
 ^Rie Coalfields of Lancashire, Northumberland, Cumberland, 
 Westmorland, Durham, Yorkshire, Edinburgh shire, Had- 
 dington, Fifeshire, Renfrewshire, Dumbartonshire, Dum- 
 friesshire, Lanarkshire, Stirlingshire, and Ayrshire are 
 surveyed on a scale of six inches to a mile. 
 
 Lancashire. 
 
 47. Clitheroe. 
 
 48. Colne, Twiston Moor. 
 
 49. Laneshaw Bridge. 
 
 55. Whalley. 
 
 56. Haggate. 6s. 
 
 57. Winewall. 
 
 61. Preston. 
 
 62. Balderstone, &c. 
 
 63. Accrington. 
 
 64. Burnley. 
 
 65. Stiperden Moor. 4s. 
 
 69. Layland. 
 
 70. Blackburn, &c. 
 
 71. Haslingden. 
 
 72. Cliviger, Bacup, &c. 
 
 73. Todmorden. 4s. 
 
 77. Chorley. 
 
 78. Bolton-le-Moors. 
 ,79. Entwistle. 
 
 80. Tottington. 
 
 81. Wardle. ' 
 
 89. Rochdale, &c. 
 
 92. Bickerstafl'e, Skelmers- 
 
 dale. 
 
 93. Wigan, Up Holland, &c. 
 
 94. West Houghton, Hind- 
 
 ley, Atherton 
 
 95. Radcliffe, Peel Swinton, 
 
 &c. 
 
 96. Middleton, Prestwich, 
 
 &c. 
 
 97. Oldham, &c. 
 
 100. Knowsley, Rainford, &c. 
 
 101. Billinge, Ashton, &c. 
 1<>2. Leigh, Lowton. 
 
 103. Ashley, Eccles. 
 
 104. Manchester, Salford, &c. 
 
 105. Ashton-under-Lyne. 
 
 106. Liverpool, &c. 
 
 107. Presoott, Huyton, &c. 
 
 108. St. Helen's, Burton 
 
 Wood. 
 
 $4. Ormskirk, St. John's, &c. 109. Warwick, &c. 6s. 
 
 85. Standish, &c. 111. Cheedaie, part of Stock- 
 
 86. Adlington, Horwick, &c. port, &c. 
 
 87. Bolton-le-Moors. 112. Stockport, &c. 4s. 
 
 88. Bury Hey wood. 113. Part of Liverpool, &c. 4s. 
 
 Durham. 
 
 Scale, six inches to a mile. 
 
 Sheet. Sheet. 
 
 1. Ryton. 4s. 8. Sunderland. 
 
 2. Gateshead. <te. 9. 4s. 
 
 3. Jarrow. 4s. 10. Edmond Byers. 4s. 
 
 4. S. Shields. 4s. 11. Ebchester. 
 6. Greenside. 4s. 12. Lantoydy. 
 
 6. Winlaton. 13. Chester-le-Street. 6s. 
 
 7. Washington. 14. Chester-Je-Street. 
 
 MINERAL STATISTICS 
 
 . Embracing the produce of Tin, Copper, Lead, Silver, Zinc, Iron, Coals, and other Minerals. By ROBERT HUNT F.l 
 Keeper q/ Mining Records. From 1853 to 1857, inclusive, Is. fid. each. 1858, Part I., Is. 6d. ;_ Part II., 5s. ^1859, Is. 
 186U, 3s. 6d., 1861, 2s. ; and Appendix, Is. 1862, 2s. 6d. 1863, 2s. 6cZ. 1864, 2s. 
 
 THE IRON ORES OF GREAT BRITAIN. 
 
 Part I. The IRON ORES of the North and North Midland Counties o f England (Out of print). Part II. The II 
 ORES of South Staffordshire. Price Is. Part III. The IRON ORES of South Wales. Price 1*. id. Part TV. 
 IRON ORES of the Shropshire Coal-field and of North Staffordshire. 1*. 3d. 
 
 .Durham cent. 
 
 Sheet. Sheet. 
 
 16. Hunstanworth. 25. Wolsb.jSjham. 
 
 17. Waskerley. 26. Branc -peth. 
 
 18. Muggleswick. 32. White Kirkley. 
 
 19. Lanchester. 6s. Vertical 33. Hamsrerley. 
 
 Section, 39. 34. Whitworth. 
 
 20. Hetton-le-Hole. 41. Cockfield. 
 
 24. Stanhope. 
 
 42, Bishop Auckland. 
 
 47. Coquet Island. 4s. 
 56. Druridge Bay, &c. 
 63. Netherwitton. 
 65. Newbiggin. 4s. 
 
 68. Belingham. m 
 
 69. Redesdale. * 
 
 72. Bedlington. 
 
 73. Blyth. 4s. 
 
 77. Swinburn. 
 
 78. Ingoe. 6s. 
 
 80. Crarnlington. 
 
 81. Earsdon. 
 
 84. Newborough. 
 
 85. Chollerton. 
 
 86. Matfen. 
 
 87. Heddon-on-the-Wall. 
 
 Northumberland. 
 
 Scale, six inches to a mile. 
 
 88. Long Benton. 
 
 89. Tynemouth. 
 92. Halt .whistle. 
 
 95. Corbridge. 
 
 96. Horsley. 4s. 
 
 97. Newcastle-on-Tyne. 
 
 98. Walker. 4s. 
 101. 
 
 102. Allendale Town. 
 105. Newlauds. 
 
 107. Allendale. 
 
 108. Blanchland. 
 
 109. Shofleyfield. 
 
 110. Wellhope. 
 
 111. Allenheads. 
 
 100. Limley. 
 184. Kelbrook. 
 201. Bingley. 
 204. Aberford. 
 
 216. Bradford. 
 
 217. Calverley. 
 
 218. Leeds. 
 
 219. Kippax. 
 
 231. Halifax. 
 
 232. Birstal. 
 
 233. Ea*t Ardsley. 
 
 234. Castleford. 
 246. Huddersfield. 
 260. Honley. 
 
 272. Holm firth. 
 
 273. Penistone. 
 
 Yorkshire. 
 
 274. Barnsley. 
 
 275. Darneld. 
 
 276. Brodsworth. 
 
 281. Langsell. 
 
 282. Wortley. 
 
 283. Wath upon Dearne. 
 
 284. Conisborough. 
 
 287. Low Bradford. 
 
 288. Ecclesfield. 
 
 289. Rotherham. 
 
 290. Braithwell. 
 
 293. Hallair: Moors. 4s. 
 
 295. Hands worth. 
 
 296. Laughton-eu-le-Mortl 
 299. . 
 
 300. Harthill. 
 
 SCOTLAND. 
 
 Scale, six inches to a mile. 
 Edinburghshire. 
 
 2. Edinburgh, &c. 12. Penicnick, Coalfielc 
 
 3. Portobello, Mussel- 
 
 burgh, &c. 
 
 6. Gilmerton, Burdie 
 
 House ,&c. 
 
 7. Dalkeith, <fcc. 
 
 8. Preston Hall. 4s. 
 
 Lasswade, &c. 
 
 13. Temple, &c. 
 
 14. Pathead. 4s. 
 
 17. Brunston Colliery, 
 
 18. Howgate. 
 
 Haddingtonshire. 
 
 Six inches to a mile. 
 8. Prestonpans, &c. Price 4s. 
 9. Trenent, Gladsmuir, &c. Price 6s. 
 
 13. Elphittsione, &c. Price 4s. 
 
 14. Orihiston, East Salton, &c. 
 
 Fifeshire. 
 
 Six inches to a mile. 
 
 24. Markinch, &c. 33. Buckhaven. 
 
 25. Scoonie, &c. 35. Dunfennhne. 
 
 30. Beath, &c. 36. Kinghorn. 
 
 31. Auchterderran. 4s. 37. Kinghorn. 4. 
 
 32. Dysart, &c. 
 
 Ayrshire. 
 Six inches to one mile. 
 19. Newmilns. 36. Grieve Hill. 
 
 26. Glenbuck. 4s. 40. Chiltree. 
 
 27. Monkton, &c. 41. Dalleagl^r. 
 
 28 Tarbolton, &c. 42. New Cumnock. 
 
 30. Aird's Moss. 46. Dalmellington. 
 
 81. Muirkirk. 4s. 47. Benbeock. 
 
 33. Ayr, Ac. 50, Daily. 
 
 34. Coylton. 52. Glenmoat. 
 
 o, .., - - > .., . 
 
 1865, 2s. 6d. 1866 to 1876 2s. each.