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Updated: June 13, 2025
These rocks possess an extremely varying character; they consist of black, brown, and grey, compact, basaltic bases, with numerous crystals of augite, hornblende, olivine, mica, and sometimes glassy feldspar. A common variety is almost entirely composed of crystals of augite with olivine. These compact lavas alternate with tuffs, amygdaloids, and wacke, and in some places with coarse conglomerate.
The rest of the evening they spent in wandering through the village, charmed with its bizarre mixture of quaintness and commonplaceness; in hanging about the shop-Windows with their monotonous variety of feather fans, each with a violently red or yellow bird painfully sacrificed in its centre, moccasins, bead-wrought work-bags, tobacco-pouches, bows and arrows, and whatever else the savage art of the neighboring squaws can invent; in sauntering through these gay booths, pricing many things, and in hanging long and undecidedly over cases full of feldspar crosses, quartz bracelets and necklaces, and every manner of vase, inoperative pitcher, and other vessel that can be fashioned out of the geological formations at Niagara, tormented meantime by the heat of the gas-lights and the persistence of the mosquitoes.
Thin layers of feldspar, swelling at intervals into well crystallised kernels, are sometimes included in these black schists; and I observed one mass of the ordinary black variety insensibly lose its fissile structure, and pass into a singular mixture of chlorite, epidote, feldspar, and mica.
Frequently, between the granite and the hornblende-slate above- mentioned, grains of mica and crystalline feldspar appear in the schist, so that rocks resembling gneiss and mica-schist are produced. Fossils can rarely be detected in these schists, and they are more completely effaced in proportion to the more crystalline texture of the beds, and their vicinity to the granite.
The red granite, black sienite, and red porphyry of Egypt, which are seen at Rome in obelisks, columns, and sarcophagi, are amongst the most durable compound stones; but the grey granites of Corsica and Elba are extremely liable to undergo alteration: the feldspar contains much alkaline matter; and the mica and schorl, much protoxide of iron.
FIRST, a white syenite, streaked and mottled with red; it consists of well- crystallised feldspar, numerous grains of quartz, and brilliant, though small, crystals of hornblende. The feldspar and hornblende in this and the succeeding cases have been determined by the reflecting goniometer, and the quartz by its action under the blowpipe.
Lavas are chiefly composed of three varieties of feldspar, varying in specific gravity from 2.4 to 2.74; of hornblende and augite, varying from 3.0 to 3.4; of olivine, varying from 3.3 to 3.4; and lastly, of oxides of iron, with specific gravities from 4.8 to 5.2.
Of the different minerals composing granite, quartz alone remains unaltered. Mica weathers to detached flakes which have lost their elasticity. The feldspar crystals have lost their luster and hardness, and even have decayed to clay.
The sides of the gallery assumed a crystallized tint, with a somber hue; white mica began to commingle more freely with feldspar and quartz, to form what may be called the true rock the stone which is hard above all, that supports, without being crushed, the four stories of the earth's soil. We were walled by an immense prison of granite!
As a general rule, quartz, in a compact or amorphous state, forms a vitreous mass, serving as the base in which feldspar and mica have crystallised; for although these minerals are much more fusible than silex, they have often imprinted their shapes upon the quartz. This fact, apparently so paradoxical, has given rise to much ingenious speculation.
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