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On Quartz, Chalcedony, Agate, Flint, Chert, Jasper, and Other Forms of Silica Geologically Considered

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On Quartz, Chalcedony, Agate, Flint, Chert, Jasper, and Other Forms of Silica Geologically Considered OF THE NORTH WALES BORDER. 439 Mr. Mackintosh, "On a Sea-coast Section of Boulder Clay in Cheshire" (Quart. Journ. Geol. Soc., Vol. xxviii, p. 388. Mr. Shone, "On the Discovery of the Foraminifera in the Boulder Clays of Cheshire, with List of Mollusca by Mr. Gwyn Jeffreys" (ibid, Vol. xxx, p. 181). Mr. Mellard Reade, "On the Drift Beds of the North West of England" (ibid, Vol. xxx, p. 27). Mr. Darbyshire, "On a Deposit of Middle Pleistocene Gravel, near Lay­ land, Laucashire" (ibid, Vol. xxx, p. 38). Mr. Macintosh, "On Boulders near Llanarmon" (ibid, Vol. xxx, p. 711). Mr. De Rance," On the Drifts and River Valleys of the North and South of England" (Proo, Geol. ASSOC., Vol. iv, p. 221). 2. ON THE FIRST IRISH CAVE EXPLORATION. By G. S. BOULGER, Esq. ON QUARTZ, CHALCEDONY, AGATE, FLINT, CHERT, JASPER, AND OTHER FORMS OF SILICA GEOLOGICALLY CONSIDERED. By Professor T. Rl1PERT JONES, F.R.S., Hon, Member and Vice Pres. Geol. Assoc. (Paper read December 3rd, 1875.) TABLE OF CONTENTS. 1. Introduction. 6. Beekites and Potato-stones. 2. Quartz. 7. Honestone. 3. Chalcedony. 8. Jasper. 4. Flint and Homstone, 9. Organic Silica. 5. Sinter, Hyalite, and Opal. 10. Detrital Silica. § 1. INTRODUCTION. This paper is not mineralogical in character, but aims at a general treatment of the subject, in its geological aspect, offering a comprehensive view of well-known facts, rather than any novelties. The relation of minerals to their places of occurrence, their modes of formation, and their relative age as parts of the earth's structure, are 'points of considerable interest, not always treated of by mineralogists, but well worth the study of geologists. 440 T. UUPERT JONES ON QUARTZ AND OTHER FORMS OF SILIOA. It is not enough to know th e composition and mineral characters of many similar and dissimilar substances j the history of th eir development and of their associations should be carefully worked out. In a collection of articles of vertu-whether metal, china, or glass-not only the nature of their substance and their style of art, but their individual and family history, their places of manu­ facture, their successive owners, and other matters are sought after by the connoisseur. In books of mineralogy there are often blanks in the history of even common minerals-such as quartz, chalcedony, and jasper-as to whether the specimens described or mentioned came from vein, or nodule, or massive rock. In Dr. Gustav Bischof's" Elements of Chemical and Physical Geology," translated by B. H. Paul, Cavendish Edition, 8vo., 1855, Vol. ii.,pages 454 et seq.• th e chapter on Silica abounds with valuable facts and inferenc es, original and selected, concerning the origin and collocation of rocks and minerals j and these have greatly influenced geological thought of late years. The following remarks, however, were not written in view of Bischof's extensive repertory of geologico-mineralogical facts j and may therefore be regarded, to some extent, as suppl ementary in respect of the common geological phenomena of Silica, without being chemical or purely mineralogical. § 2. QUARTz.-After noting the great abundance of Silica in the mineral world, both free and in combination, the auth or de­ scribed a few of the salient mineralogical features of its crystallized form-Quartz; and, mentioning some of the largest known crystals. be proceeded to describe its mode of occurrence, and drew especial attention to its presence as a constituent in granite; also in geodes and veins in that and other rocks, mostly old and metamorphic. In the latter, we often have compact veinstones of quartz, showing general crystalline structure on each side, but along th e median line, and in cavities, passiug into hexagonal prisms, with terminal pyramids, more I or less distinct, and varying in translucence. Pyrites, gold, silver, and other minerals are often mingl ed with the quartz ; and rutile, asbestos, hornblende, epidote, mica, chlorite, iron-oxide, bitumen, morsels of anthracite, crystals of calcite, and other substances,· occasionally occur in the crystals, either as fine • Bischofgives a full list 0p . cit. p. 477. Cassiterite, pyrites, chalcopyrite, chlorite, and heematite are the min erals fonnd enclosed in Corni sh quartz, .and schorl and chlorite in the Cornish amethyst, according to Mr. F. H. Collins' "Mineralogy of Cornwall and Devon," 1871, p. 87. T. RUPERT JONES ON QUARTZ AND OTHER FORMS OF SILICA. 441 fibres, flakes, or spots : hence the " Oheveux de Venus, de Thetis, et de Mars," &c., some" cat's eyes," avanturines,* &c. Minute cavities, partially occupied with water, with or without salts of iron, calcium, sodium, &c., are often present; sometimes as films and clouds, called" feather" by the jewellers. Sorby and others have investigated these" fluid cavities."t The countries where the best specimens of Rock-crystal are found are enumerated in the many books on Mineralogy.j The " druses " in the micaceous and other metamorphic rocks of the Alps" are among the best known sources; but similar schists, clay-slates, gneiss, and granites of all mountainous countries yield them. The Carrara marble has cavities lined with rock­ crystal; also the Paheozoic limestone of Oape Diamond, near Quebec, and at Fairfield, St. Lawrence county, New York State, and the altered Chalk of Donald's Hill, Antrim, &c. In the Kentish Rag (Neocomian sandy limestone) some fissures are lined with quartz on calcite, as in the very much older rocks at Snailsbeach mine. The Dolomitic conglomerate of the New Red series in Somerset and Gloucestershire contains numerous drusy cavities, and hollow nodules or geodes of quartz, chalce­ dony, and jasper, together with calcite and other minerals.§ In some nests and veins of quartz the crystals are free in the matrix, and frequently perfect with terminal pyramids; when such depositories have been opened out, their scattered contents supply the numerous crystals known as Irish, Oornish, Bristol, Dungarven, Quebec, Aleneon, Paphian (Cyprus), and other "diamonds." Occasionally masses of minute crystals, with or without 'a matrix, .. Avanturine quartz, having particles of mica disseminated throughout, may possibly have been a micaceous sandstone, subsequently altered by hydrothermal agency into the more homogenous and crystalline avanturine ; or it may have been some form of mica-schist, of metamorphic origin, but further altered into a more crystalline rock, or, lastly, the mica may have been developed by chemical changes whilst an impure siliceous sandstone or schist was becoming highly crystalline. t Even pseodomorphic quartz crystals (octahedrons and cubes) are found to contain these Iiquid-cavities : Collins' "Mineralogy of Cornwall and Devon," p. 87. :t For British quartz, see. Greg and Letsom's "Manual of Mineralogy of Great Britain and Ireland," 1858, p. 82, &c.; and J. H. Collins' "Miner­ alogy of Cornwall and Devon," 1871, p. 86, &c. II Many years ago a great drusy cavity was broken into, on the Alps at Zinken, Savoy, which is said to have yielded 1000 cwt. of rock-crystals , one crystal weighed 800lbs. (Shepard], The crystal-hunters hang over the cliffs by ropes, and strike the rocks for the hollow sound of the druses (" ovens '~) containing the sought-for crystals. § "Memoirs of the Geological Survey Great Britain," Vol. i., p. 246. 442 T. RUPERT JONES ON QUARTZ AND OTHER FORMS OF SILLCA. occur aggregated, like grains of sandstone. The crystals are sometimes found at considerable distances from their place of origin; for instance, the "Bagshot Diamonds," or subangular pieces of quartz.crystal, in the Post-Tertiary Gravel of Berks and Surrey, have been drifted far, by many stages, whether from the Breton, Cornish, or Welsh area of old rocks. The discrimination of quarts fromjlint and other siliceous materials in sandstone-not only by external aspect under a lens, but by the polariscope-is very useful in geological research. When siliceous sand has not only been cemented together as sandstone, but further agglutinated and compressed, after partial softening of the grains, into quartzite, we are led back to pure quartz again, as a variety of quartz-rock distinct in origin from vein-quartz. The crystal globes of China and Japan, and the Brazilian "pebbles," cut for lenses, mostly come from large waterworn crystals brought down by mountain torrents. The beautiful purple quartz, or Amethyst (differing somewhat from Rock-crystal in its "rippled" fracture, and its optical pro~ perties) is not unfrequent in drusy cavities of old rocks (as in Brittany), in agates, and even in the" potato-stones" of Somerset. A faintly purple quartz lines some fissures in the Kentish Rag at Fairbourne, near Maidstone. Smoky Quartz,- varying from yellow (cairngorum) to black (merion), is found in the crystalline, metamorphic, and more or less granitoid rocks of mountain districts, just like the pellucid, the milky, and other varieties. Iron, manganese, titanium, and other metals, besides carbon, are supposed to be concerned in giving different colours to quartz. Prase is said to be quartz with green hornblende disseminated in it; rose quartz is pro­ bably reddened by manganese; deep-blue quartz is called siderite. Some quartz, when burnt and dipped in a solution uf cochineal, becomes beautifully reddened and crackled.j All these forms of quartz have been deposited by aqueous (pro­ bably hydrothermal) agency. § 3. CHALCEDoNY.-Chalcedony, which is crystalline silica, with *' Some smoky quartz is made yellow and of a rich orange tint by burning. t Witb indigo, the heated quartz becomes bluish. See Dr. Billings' "The Fcience of Gems, Jewels," &0. 8vo. London, 1867, p. 57. Thus also heated fluor-spar is dipped in a hot solution of "dragon's blood" to improve the purple tints. T. RU PEItT JONES 0 :-1 QUA RT Z AN D OT HE R FORM8 OF SIL IC A.
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