Aids in Practical Geology ... With ... illustrations

Front matter

AIDS IN PEACTICAL GEOLOGY. Stanoarb Works. In 8vo, Handsome Cloth, Price 18s. PHYSICAL GEOLOGY AND PALÆONTOLOGY. (On the basis of Phillips.) By H. GOVIER SEELEY, F.R.S., Professor of Geography, King's College, London. "DClitb illustrations. "It is impossible to praise too highly the research which Professor Seeley's Physical Geology evidences, it is far more than a Text-book, it is a Directory to the Student in prosecuting his investigations." — Presidential Address to the Geographical Society, 1886, by Professor Bonney, D.Sc, F.R.S. "Professor Seeley'3 work includes one of the most satisfactory Treatises on Lithology in the English language. ... So much that is not accessible in other works is presented in this volume, that no Student of Geology can afford to be without it."— American Journal of Engineering. >>- f*: —""oil In pufiiHiriitsome-pioth, Price 34s. STRATIGRAPHICAL GEOLOGY AND PALÆONTOLOGY.

(On the basis of Phillips.) By ROBERT ETHERIDGE, F.R.S., Of the British Museum, late Palæontologist to the Geographical Survey. "DClitb Numerous "Cables, ctDap, anb 36 Iplatcs. *** This work affords a complete census of British Organic Remains. " If Professor Seeley's volume was remarkable for its originality and the breadth of its views, Mr. Etheeidge fully justifies the assertion made in his preface that his book ' differs in construction and detail from any known manual.' . . . Must take high rank among works of reference." — Athenæum. " No such compendium of geological knowledge has ever been brought together before." —Westminster Review. LONDON" : CHARLES GRIFFIN AND COMPANY. AIDS IN PRACTICAL GEOLOGY BY GRENVILLE A. J. COLE, F.G.S., PROFESSOR OP GEOLOGY IN THE ROYAL COLLEGE OP SCIENCE FOR IRELAND- W-tb IRumerous Jlluetrations.

LONDON: CHARLES GRIFFIN & COMPANY, EXETER STREET, STRAND. 1891. [AU Eights Reserved.] TO ARTHUR WELLS BAWTREE, F.L.S., tbfs JBook is Inscribes, IX MEMORY OF MANY EXPEDITIONS.

Preface

. This little work is intended as a companion to any ordinary text-book of geology ; and it is hoped that it may be of special service to those students who have made excursions into the field, and who wish to determine their specimens for themselves. Mr. Joshua Trimmer, in 1841, issued his Practical Geology and Mineralogy, with the object of en couraging readers who were beyond the reach of oral in struction. The book necessarily contained some theoretical matter ; but at the present day the abundance of excellent text-books has enabled these Aids in Practical Geology, while originating in the same idea, to be kept within still stricter limits. The section on blowpipe-work has been inserted as an aid to travellers ; while the description of the hard parts of fossil invertebrates will probably assist those readers who find it impossible to distinguish genera by means of mere names and figures.

In arranging the genera thus discussed, those forms have been first dealt with which exhibit most com pletely the characters of their class or sub-division. Hence highly-developed types are often treated of before those which may have preceded them in time, or which may have degenerated from them. By kind permission, I have been able to utilise many of the figures of fossils illustrating Phillips's Manual of Geology, and have supplemented these by a few sketches and diagrams explanatory of special features. A large section of the book has been devoted to rocks and to the ordinary minerals of the earth's crust, since these will

PREFACE.

. VIII always present themselves to the observer during any ex pedition or in any country. As for the names used for igneous rocks, I have endeavoured to retain the comprehen sive terms of pioneers, such as d'Aubuisson, Brongniart, and Haiiy. The more exact determinative knowledge of the present day has introduced us to many new rock-varieties ; but these can be distinguished by the addition of a mere mineral prefix. In 1878 Prof. J. W. Judd, F.R.S., organised the instruction in Practical Geology at the Royal School of Mines in London ; and it is difficult to express briefly how much this book owes, in respect of any merit it may possess, to the courses then instituted and continuously developed from year to year. My great indebtedness, also, to Prof. Judd's published papers, and to the works of Brush, Lacroix, Levy, Rosen busch, Teall, Zirkel, and Zittel, will again and again be apparent in the text.

Numerous friends have, in addition, assisted me from time to time. At the risk of passing over some of the most generous, I must express my sincere thanks to Messrs. J. E. Duerden, L. W. Fulcher, J. W. Gregory, and T. H. Holland. And let me add, with Isaak Walton, that " I have found a high content in the search and conference of what is here offered to the reader's view and censure ; I wish him as much in the perusal of it." GRENVILLE A. J. COLE. Dublin, December, 1890. TABLE OF CONTENTS.

Part I

The Sampling of the Earth's Crust. CHAP. PAGE I. — On certain observations in the field, ..... 1 II. — On the collection and packing of specimens, ... 10

Part II

The Examination of Minerals. III. — On the occurrence and some physical characters of minerals, 13 IV. — Simple tests with wet reagents, ..... 30 V. — Examination of minerals with the blowpipe, ... 35 VI. — Simple and characteristic reactions of the constituents of common minerals, ........ 57 VII. — Blowpipe-tests useful in the determination of common minerals, 62 VIII. — Quantitative flame-reactions of the felspars and their allies, .......... 75 IX. — Examination of the optical properties of minerals, . . 81

Part III

The Examination of Rooks. X.— Introductory, ......... 84 XI. — Rock-structures easily distinguished, .... 86 XII. — Some physical characters of rocks, ..... 94 XIII. — The chemical examination of rocks, X CONTENTS. CHAP. PAGE XIV. — The isolation of the constituents of rocks, . . . . 100 XV. — The petrological microscope and microscopic preparations, 111 XVI. — The more prominent characters to be observed in minerals in rock-sections, ........ 121 XVII. — The characters of the chief rock-forming minerals in the rock-mass and in thin sections, ..... 139 XVIII. — Sedimentary rocks, ........ 170 XIX. — Igneous rocks, ......... 196 XX. — Metamorphic rocks, < 251

Part IV

The Examination of Fossils. XXI. — Introductory, 266 XXII. — Fossil generic types. — Rhizopoda (p. 272); Spongiæ (p. 276) ; Hydrozoa (p. 280) ; Actinozoa (p. 282), . . 272 XXIII. — Fossil generic types. — Polyzoa (p. 289); Brachiopoda (p. 291), 289 XXIV. — Fossil generic types. — Lamellibranchiata, .... 299 XXV. — Fossil generic types. — Scaphopoda (p. 319) ; Gastropoda (p. 320) ; Pteropoda (p. 331) ; Cephalopoda (p. 332), . 319 XXVI. — Fossil generic types. — Echinodermata (p. 350); Vermes (p. 364), 350 XXVII. — Fossil generic types. — Arthropoda, ..... 365 XXVIII. — Suggested list of characteristic invertebrate fossils, . . LIST OF ILLUSTRATIONS. FIGOKE PAGE 1. Diagram for determination of true dip, .... 6 2. Abney's Level, ....... 7 3. Miller's simple goniometer, ..... 16 4. Simple reflective goniometer, ..... 18 5.

Walker's specific gravity balance, ..... 25 6. Tube for production of flat-sided flame, . . . .36 7. Letcher's blowpipe-lamp, ...... 37 8. Blowpipe-flames, ....... 42 9. Support for minerals during flame-reactions, . . .76 10. Szabd's flame-reactions, . . . . . .78 11- „ 79 12. Thoulet's apparatus for separation by washing, . . . 104 13. ,, ,, by means of heavy liquids 106 14. Petrological microscope with rotating stage, . . .112 15. ,, ,, with fixed stage and rotating nicols and cross-wires, . . . . . . .114 16. Support for condensing-lens, ..... 121 17. Diagrams of rocks with materials arranged by flow, . . 123 18. Secondary growth of crystals in rocks, .... 125 19. Enclosures in minerals, and schillerisation, . . . 127 20. Aggregate-polarisation between crossed nicols, . . .135 21. Figures given by uniaxial and biaxial minerals in convergent polarised light, ....... 137 22.

Altered Andesite-Tuff. Snead, near Bishop's Castle, x 14, . 180 23. Crushed oolitic limestone passing into schist. Mofttiers-en- Tarentaise. x 12, . . . . .189 24. Granite. Near Dublin, x 12, . . . . .199 25. a, Graphic Granite. Stanner Rock, Herefordshire, x 8. b, Micropegmatitic intergrowth of quartz and felspar in Eurite, Stanner Rock, x 160, ...... 200 26. Syenite. Dresden. x 8, . . . . . .204 27. Altered Pyroxene-Mica-Diorite. Stanner Rock, Herefordshire. x 12, . . . . . . . .210 28. Granular Pyroxene-Diorite. Near Huntley, Aberdeen. x 35, . 210 29. Altered Dolerite (Diabase). Mynydd-y-Gader, Cader Idris. x 24, Xll LIST OF ILLUSTRATIONS. FIGURE PAGE 30. Olivine-Gabbro. Near Huntley, Aberdeen, x 7, . • 214 31. Gabbro rich in Olivine (Troctolite). Coverack, Cornwall, x 12, 214 32. A, Rhyolite.

Hlinik, Hungary, x 24. B, Rhyolite. Dyke, Broadford Bay, Skye. x 24, . . . • .223 33. Trachyte. Ischia. x 12, . . . . .226 34. Nepheline-Trachyte (Phonolite). Briix, Bohemia, x 35, . 228 35. Hornblende-Andesite (trachytic type). Summit of Beinn Nevis, Scotland, x 7, . . ■ ■ • .232 36. A, Pyroxene-Andesite (basaltic type). Kremnitz, Hungary. x 14. B, Glassy Pyroxene-Andesite (basaltic type). Dyke, Eskdale, Dumfries. x 40, . . . .234 37. Basalt. Lion's Haunch, Edinburgh. x 8, . . .238 38. ,, passing into Ophitic Dolerite. Tobermory, Mull, x 25, . 239 39. Leucite-Basalt. Vesuvius, x 12, . . . . 240 40. Nosean-Leucitite. Rieden, Eifel. x 7, . . . . 243 41. Spherulitic Obsidian. Beaver Lake, Yellowstone Park. x 12, 245 42. Altered Spherulitic Obsidian ("Pyromeride"). Wuenheim, Vosges. x 7, . . . • • ■ • 245 43.

Mica-Schist. Saxony, x 7, . . . • .259 44. Ancient Conglomerate. Charlton Hill, Shropshire, x 7, . 262 45. Gabbro-Gneiss. Le Chenaillet, Mt. Genevre, . . . 264 46. Nummulites lœvigatus, ...... 274 47. Sand containing abundant spicules of Siliceous Sponges. Hythe beds, Tilburstow HiU, Surrey, x 40, . . . . 277 48. Diagrams of a Tetracorallan [Lithostrotion) with radial grouping of the septa, ....... 282 49. Heliolites interstincta, ...... 283 50. Calyx of Zaphrentis, ...... 284 51. Cyathophyllum truncatum, ...... 284 52. Lithostrotion basaltiforme, ...... 285 53. Omphyma turbinatum, . . . . . 285 54. Calceola sandalina, ....... 286 55. Halysites catenvlaria, . . . . . ' . 288 56. Fenestella retiformis, ...... 290 57. ,, „ (enlarged), . . . . .290 58. Eschara monilifera, ...... 291 59. Brachiopod {Terebratula vitrea), ..... 291 60.

Slringocephalus Burtini, ...... 294 61. Bhynchonella, ....... 294 62. Pentamerus galeatus, ...... 295 63. Spirifer pinguis, ' . ■ . ' . . . . 295 64. ,, trigonalis, ....... LIST OP ILLUSTRATIONS. XIII *'GUEE PAGE 65. Spiriferina Walcotii, ...... 295 66. Productus giganteus, ... . 297 67. Discina Forbesii, ...... 298 68. Crania parisiensis, ...... 298 69. Sinupalliate Lamellibranch {Cytherea incrassata), . . 299 70. Tellina, ........ 302 71. Mya truncata, ....... 304 72. Corbida pisum, ....... 304 73. Hinge of Cardium edule, ...... 305 74. Conocardium aliforme, ...... 305 75. Cyrena cuneiformis, ..... 306 76. Astarte elegans, , , . , . , 306 77. Hippurites, . , , ■ . , , 308 78. Area Noæ, ....... 309 79. Cucullœa Hardingii, ...... 309 80. Nucvla Cobboldiœ, ...... 310 81. Posidonomya Becheri, ...... 313 82.

Inoceramus Cuvieri, . . . • . 313 83. Inoceramus {Actinoceramus) sulcatus, .... 313 84. Perna Mulleti, ...... 313 85. Modiola Fittoni, . . . , , . . 314 86. Hippopodium ponderosum, ..... 315 87. Ostrea expansa, ....... 316 88. Alectryonia frons, ...... 316 89. Exogyra sinuata, . . . . . . .317 90. Hinge of Lima, ....... 317 91. Pecten islandicus, ....... 318 92. Janira quinqueeostata, . . . , . .318 93. Plicatula, spinosa, ....... 319 94. Gastropod (Fusus), ...... 320 95. Pleurotoma rotata, ...... 323 96. Voluta athleta, . . . . . . 323 97. Buccinum undatum, ...... 324 98. Ficula reticulata, ....... 324 99. Rostellaria {Hippochrenes) ampla, .... 325 100. Cerithium plicatum and Cerithium elegans, . . . 325 101. Turritella incrassata, ...... 326 102. Nerinœa Goodhalli, ...... 327 103. Helix occlusa, ....... 330 104.

Planorbis euomphalus, ...... 330 105. Aptychus of an Ammonite, ..... 333 106. Gomphoceras pyriforme, ...... LIST OF ILLUSTRATIONS. FIGURE PAGE 107. Suture-line of an Ammonite (Harpoceras), . . • 337 108. Clymenia, . .'.'..'. . • .338 109. Goniatites sphæroidalis, . . . ■ ■ • 338 110. Suture-line of Ceratites nodosus, . . . ■ • 339 111. Natural cast of Arcestes Bœclchi, ..... 340 112. a, First lateral saddle of Monophyllites Simonyi. b, First lateral saddles of two adjacent septa of Phylloceras heterophyllum, ........ 340 113. Ægoceras capricornus, ...... 342 114. Stephanoceras modiolare, . . . ■ ■ ■ 343 115. Acanthoceras rothomagense, . . . . . 344 116. Baculites anceps, . . . . . . ■ 345 117. Scaphites œqualis, . . . . . . 346 118. Guard of Belemnites, cut open above to show phragmocone, . 347 119.

Guard of Belemnitella mucronata, .... 348 120. Guard of Actinocamax plenus, . . . . . 348 121. Encrinus liliiformis, . . . . . . 353 122. Cyathocrinus, . . ... . . . 354 123. Collyrites bicordata, ...... 362 124. a, Cypris purbtckensis. b, Cypridea punctata, var : gibbosa. c, Cythere retirugata, var : rugulata, . . . . 366 125. Beyrichia, .... . . . . 366 126. Leperditia inflata, . . ... . . 366 127. Trilobite (Dalmania caudata), . . . . . 369 128. Olenus micrurus, . . . . . . 371 129. Agnostus, ........ 372 130. Trinucleus concentricus, . . . . . . 372 131. Harpes Flanagani, . . . . . . 372 132. Homalonotus, . . . . . . . 373 133. Asaphus Powisii, . . . . . . . 374 134. Phacops (Chasmops) conophthalmus, .... 374 135. Bronteus flabellifer, . . . . . . 375 136. Prestwichia anthrax, . . . . AIDS IN PRACTICAL GEOLOGY.

J? A K T I. THE SAMPLING OF THE EARTH'S CRUST. "A fossil shell may interest a conchologist, though he be ignorant of the locality from which it came ; but it will be of more value when he learns with what other species it was associated, whether they were marine or freshwater, whether the strata containing them were at a certain elevation above the sea, and what relative position they held in regard to other groups of strata."— Charles Lyell, Principles of Geology, vol. i., 1830.

Chapter I. On Certain Observations In The Field

The examination of the features presented by the Earth's crust in any locality, with the object of learning something of its past liistory, must always be one of the most delightful of occupations ; and the material advantages arising from a correct determination of minerals and rocks are obvious to every traveller. Such aids in determinative geology as are given in the following pages may be applied in any halting-place, or in cities after the return from an expedition ; but in any case observations made on specimens are of slight importance if uncoupled with knowledge of their true position in the field. The Museum-Curator, for instance, has duties of an invaluable character. He brings together, collates, and arranges the types and varieties described by authors, adding to them by his own special knowledge, and thus forming a series with which any new specimen can be easily and accurately compared.

To him the Earth becomes a great reality, for he surveys it through the extent of his collections and his studies ; but the ordinary student, gathering together a few relics from the curiosity cabinets of relatives and friends, finds that they appeal to him but little; they have no associations, they have long been separated from their kindred, they are "fossil" in the worst of senses. But let him, having a knowledge of first principles and of museum-types, go out to see things for himself. Furnished •with the maps and books of experienced workers, let him re-examine the evidence on which they have relied. A week's holiday thus spent amid varied surroundings, as on the Welsh border, or in Antrim, or around Edinburgh or Bristol, will provide material for long and careful study. Once in the field, the complexity of the subject will dawn upon him ; but at the same time he becomes assured that, wherever he may wander, he will find congenial work.

The first visit to a district commonly raises numberless questions, when the specimens gathered are examined at one's leisure ; and the suggestions of the laboratory or the microscope must be tested in a second or third excursion by re-examination of the relations of the rock-masses in the field. In the field itself broad names are assigned to objects, detailed determination being left for comparative and instrumental work ; but in these after-hours of study every scene comes back vividly before one, and even the lichens that may yet cling in hollows and betray the collection of an imperfect and weathered specimen, serve their turn with the naturalist and remind him of the wide, open-air, and eminently natural character of his work. The art of observing in the field, and of balancing the evidence of various exposures, must be to a great extent learnt by oral tradition and personal guidance ; and the study of any geological map, with its outliers, its sinuous outcrops, its inliers, its repetitions by faults or foldings, should be carried on, wherever possible, in the actual district that has been mapped.

The practical construction of maps, and of sections from them, is discussed in A. Geikie's Outlines of Field Geology (Macmillan & Co.), and Penning's Field Geology (Bailliere, Tindall & Cox). The examination of a country like Britain, or any part of western Europe, from the point of view of a student anxious to grasp the salient features, the connexion between underground and surface characters, has been immensely facilitated by the modern development of cycling. Traverse after traverse of a country may be made with some handy geological map carried in a capacious tool-bag, together with a hammer, heavy enough to investigate each wayside exposure. The physical geology, dip-slopes and escarpments, alluvial plains or mountain-gorges, become very real to travellers on the road ; and the repetition of the same features in the same order in successive traverses comes upon one with admirable distinctness, and gives a key to the structure of wide areas.

The general grouping of the strata once grasped, attention can be paid to some limited district. But even here a bicycle or tricycle ready to hand is of considerable value. Though days and weeks may be spent on foot up and down rugged exposures, or across broad cultivated lands set with little quarries, as in the Cotteswolds, every now and then comparison becomes desirable with some distant point, and the road is taken without delay. The intervening country may be reviewed in passing ; when local work is completed, a different route can be taken in returning; and thus hints are received and sections examined which otherwise might have entirely escaped. And this matter applies also to geological observations in districts inviting by their wildness. If the student of the Grampians, the Juras, or the Alps, can find time to approach them mile by mile along the highways, following up the rivers that flow from them, tracing afar off the limits of the lowland, the first curvings of the foot-hills, the change from pasture to moorland, from moor to desolate crag, he gains a most vivid appreciation of his surroundings when he arrives at the locality of his work.

And just as the district in which he finally settles acquires dignity from its wide associations, so his very specimens and chips, whenever studied, come to have a truly geological, not merely mineralogical or palæontological significance. Even a microscopic slide, amid such memories, seems to assume its place in nature. The instruments used in the field should be noticed here. Eirst in importance is the hammer, which may vary much in size and weight with the work proposed, and which may easily err in being too heavy, as well as in being too light. Before under taking any long expedition, the head and shaft should be well tested, and the form of handle that cramps the hand least should be selected. A handle too small in circumference is liable to cause blisters, or at any rate to pain the hand, during long use on refractory materials.

Eor most kinds of work, a flat end to the head about 1 inch square seems most suitable ; the other end should be chisel shaped, and there are many reasons, easily seen in practice, why the chisel-edge should run horizontally, not vertically, when the hammer-handle is held upright. A strong side-reason when working in hill-districts is that a secure hold can often be obtained in ascending or crossing steep grass-slopes, by driving the chisel-end at each step into the soil. A sharp pick-like termination may sometimes be useful, in place of the chisel-edge, for splitting open lumps of soft rock when seeking fossils, or in laying hold of and bringing down materials that are beyond the reach of the arm alone. But for general purposes, trimming of specimens, wedging out blocks, and so forth, the chisel-edge, some 1 inch or so broad, is in valuable.

Where much collecting is to be done, weight becomes an object; and the reduction of specimens to a convenient minimum size in the field is always desirable, since any accidental fracture can be remedied by at once securing another specimen. Hence a light trimming hammer proves a great additional convenience, and the risk to specimens during trimming, particularly when they contain fossils, is thus very much reduced. Though many geologists prefer to dispense with a chisel, there is no doubt of its convenience where blocks of rock have to be worked out from a cliff-face, or in any place where the hammer fails to get an easy hold. A good " cold chisel," some 4i to 5 inches in length, is suitable. If it is too short, it may become driven in down joint-cracks before its work is done and before the block is wedged away from the parent-mass.

Elaborate hammer-belts seem quite unnecessary. The speci men-bag is commonly slung by a strap passing over the right shoulder, so that it can be steadied and partly supported by the left hand when it becomes full and heavy. An additional strap for the hammer cumbers the chest, and even in a belt the head has to be prevented from touching and wearing through the clothes. It is simple enough to slip the hammer into the side bag itself, the handle projecting from the forward end under the flap. The left hand, by resting on the handle, can then easily, during long walking, keep the bag from rubbing unpleasantly on the hip. The bag itself should be light and strong, with two strongly attached buttons, rather than straps, to close the flaps, so that no time is lost in opening. On moving from each collecting-place it must invariably be closed, as a slight slip or twist when climbing may deprive the observer of valuable spoils.

In rounding rocky slopes it is best to keep the bag slung well upon the back; if on the outside, it tends to destroy the balance in the wronc direction; if on the inside, it thrusts the body away from the rocks, and is also liable to catch during any rapid movement. In jumping, as across little stream-cuts, it will soon be found that the hand should steady it from below. A receptacle that proves extremely light and satisfactory is the rubber " game-bag," covered with thin cloth, which is pro curable at all waterproof-dealers. The price is about 6s., and the supporting rings and straps are generally strong. Though liable in old age to be finally cut through by sharp rocks, the material, even when perforated, does not fray away. It is scarcely necessary to mention a walking-stick as part of the geologist's equipment, for it is indispensable on steep or roughish ground, lt should, at any rate, be never left behind where long slopes and taluses are in question, since its use will make observations possible that might otherwise involve genuine risk.

Among rocks it may sometimes be necessary to throw it over in advance ; but to descend dry grass-slopes without a stick is undoubtedly time-consuming and vexatious. Mountaineers will forgive our reminding the geological student, who will often find himself in situations all the more pleasant for being un familiar, that a steep hill-side should be traversed with the stick in the inside, not in the outside hand. A compass is a necessity for the pedestrian. It may be com bined with the clinometer, as in the convenient box-instruments often made.* Many of these, however, do not allow sufficient length for the edge which is to be held coincident with the line of dip observed. Any one can construct a clinometer fuom an ordinary protractor, a swinging index, or even a weighted thread, being hung from the centre of the straight edge so as to reach the graduated arc.

Of course the 90° marked on the pro tractor reads as 0° when a dip is to be taken ; thus, if the index points to 84°, the dip is 6°, and so on. In observing a dip, the plane of the graduated arc of the clinometer must be held parallel to a vertical rock-face on which the beds appear exposed, and the distance between the eye and the rocks should be reasonable, in order that the straight edge may appear coincident with a considerable length of the clipping strata. The instrument is tilted until this edge appears to lie along some well-marked line of stratification; the plummet or index then points to an angle equal to the angle of dip observed. Several observations are desirable as checks to one another; any evidences of lenticular or current-bedding must be noted; and the compass bearing of the face of rock utilised must also be observed. * An excellent simple form of compass-clinometer is made by Millar, Optician, Keighley, at about 5s. 6d. The dip thus found is very probably only an apparent dip, and is less than the true dip, which runs in some other direction.

Two or more observations taken near to one another will settle this point. Thus where there are two dips seen on different walls of the same quarry, or in closely adjoining quarries, and these can be relied on as not being due to mere local slippings or to the very common creep of the higher beds down the slope of a hill-side, then the direction and amount of the true dip can be found by the simple geometrical method of Mr. W. H. Dalton.* The directions of the walls, or rock-faces, on which the dips are seen are determined with the compass, and two lines are drawn to represent them on paper, giving the angle r ab (fig. 1). Should one dip in the actual quarry-sections incline towards a and the other away from a, one of the lines drawn must be produced, so that the dips represented in direction by the lines a b and a r both either incline towards or away from a. Draw a c perpendicular to a b, and of any convenient length, Fig. 1. say, for greater accuracy, about 3 inches ; and draw a s per pendicular to a r and equal to a c. From c and s draw lines making with a c and a s respectively angles equal to the comple ments of the observed angles of dip, and cutting a b and ar in d and t. Then the angles ad c and al s represent the angles of observed dip along the directions a b and a r respectively.

Join d t ; this line represents the strike of the beds. a e, drawn from a perpendicularly to it, gives us the direction of true dip. Draw as perpendicular to a e and equal to a c or st s ; join/e. The angle a ef, when measured with a protractor, gives the amount ofthe true dip. The matter is clear if the three triangles a st, ac d, and a fe are imagined as bent up so as to stand perpendicularly to the plane a t d, which remains horizontal. The points s, c, and f coincide, and a plane laid upon the dipping lines s t, fe, and c d will represent truly a surface of one of the strata observed in the field, when both the apparent clips were inclined away from a. * Geol. Mag., 1S73, p. 333. dt is a horizontal line in this surface, and is therefore the strike; the line f e, perpendicular to it, and also in the same surface, represents the true dip both in compass-bearing and in inclination to the horizontal.* If both the observed dips are inclined towards the point c, it is clear that the same construction suffices, only the arrow set down upon the map to indicate direction must point along a e towards a and not away from it.

Graphic methods like the foregoing serve the geologist far hetter than any system of elaborate tables. Provided the scale of the drawing is sufficiently large, the errors of observation in the field, owing to the small exposures studied, will be greater than any that can be introduced afterwards by measurement from a carefully constructed drawing. To find the relation of the point where observations are being made to features marked upon the map, and thus in one's notes to localise the observation, is often difficult in a wide and open country. Even the map on the scale of G inches to a mile cannot represent every rock and projecting boss, and measure ments must be made extending from some recognisable point to Fig. 2. the place of observation. The tape-measure, so important in determining the thicknesses of beds on faces of a quarry, is often of use in direct measurement on the surface of the ground, for which purpose it should be at least 40 feet in length.