A natural history of fossils / By Emanuel Mendes da Costa ... Vol. I, pt. I.

  • Emanuel Mendes da Costa
Date:
1757
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Licence: Public Domain Mark

Credit: A natural history of fossils / By Emanuel Mendes da Costa ... Vol. I, pt. I. Source: Wellcome Collection.

    289/316 page 273
    OF BOB SLL e. | ure a OB en Bi nds ges bg CHAP. IV. Genus Il. The Granites. G7 ONES of an irregular texture, concreted of feparate large grains or parts, not blended or intermixed together into one mafs or fubftance, fo as to be compact and uniform, asthe marbles, €?c. are, but diftinét from each other, rudely concreted, and only cohering firmly together; thefe parts likewife are not homogeneous, but are different concretions of quartz and mice. The bodies of this genus are of fuch great hardnefs, as freely to ftrike fire with {teel, they ferment not with acids, and are vitrifiable, I. Granita fere nigra, precipue e micis nigris parvaque intermixta quartet quantitate compofita. This fpecies takes a fmooth furface, but no good polifh, its ground is black with a flight tinture of green fpotted with white. The chief fubftance or bafis of this granite is a black mica, the quantity whereof is fo great as to be full three quarters parts of its compofition, and to give it a black ground. In other refpects this fpecies has great affinity to the granite called granitello, defcribed fpecies II]. and feems to differ from it chiefly in the quantity of the mica it contains, therefore by fome would be rather reckoned a variety than a diftinét kind, butas it carries fo fpecifie a difference, I think it deferves to be ranked asa diftin& fpecies. The guariz in this ftone is equally difperfed in it; it is opake, glofly and flightly tinged of a dull dark green colour, fo as tobe in very few fpots of a whitifh hue; in fome places it appears of a tabulated ftruéture, but moftly lies in folid grains or parts; the micaceous matter lies in large bundles or con- tremely glofly and black, witha caft of dark dull green. I have never yet heard of ftrataof this kind of granite, but vaft large mafies of it abound in the pavements of the ftreets of this metropolis, and nodules of Il. Granita albifima micis magnis nigris argentei{que notata. Moorftone, Woodward, Cat. G.e. 3. © Granita alba, durifima, nigro variezata ; que incolis Cornubienfibus Moorttone, Hill’s Hift. Foff. p. 498. N° a. . sea _ This ftone generally takes a very high polifh and good furface; is white, thinly fpotted with large black fpots or flakes of mice, and fome of a dazzling filver colour, and is a very fine fpecies of granite. It is heavy, of a large grofs, rude, ftructure, though the different grains or parts firmly cohere together, and form a ftone of a very hard texture; the bafis Nona is
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