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Updated: June 25, 2025
In Great Britain, those members of the series which approach most nearly to granite in their composition, as gneiss, mica-schist, and hornblende-schist, are confined to the country north of the rivers Forth and Clyde.
There can be no doubt in these cases that strata once composed of mud, sand, and gravel, or of clay, marl, and shelly limestone, have for the distance of several yards, and in some instances several hundred feet, been turned into gneiss, mica-schist, hornblende-schist, chlorite- schist, quartz rock, statuary marble, and the rest.
Possibly it may denote the existence of some strata of hornblende-schist covered by the waters, in which schist the sand is disseminated. Cape Codera forms on the north an immense spherical segment. A shallow which stretches along its foot is known to navigators by the name of the points of Tutumo and of San Francisco.
But if we investigate different mountain chains, we find gneiss, mica-schist, hornblende-schist, chlorite-schist, hypogene limestone, and other rocks, succeeding each other, and alternating with each other in every possible order.
In some cases, dark limestones, replete with shells and corals, have been turned into white statuary marble; and hard clays, containing vegetable or other remains, into slates called mica- schist or hornblende-schist, every vestige of the organic bodies having been obliterated.
At Rancie the lias nearest the granite is not only filled with iron-ore, but charged with pyrites, tremolite, garnet, and a new mineral somewhat allied to feldspar, called, from the place in the Pyrenees where it occurs, "couzeranite." "Hornblende-schist," says Dr.
MacCulloch, "may at first have been mere clay; for clay or shale is found altered by trap into Lydian stone, a substance differing from hornblende-schist almost solely in compactness and uniformity of texture."
Gneiss in geology is commonly used to designate not merely stratified and foliated rocks having the same component materials as granite or syenite, but also in a wider sense to embrace the formation with which other members of the metamorphic series, such as hornblende-schist, may alternate, and which are then considered subordinate to the true gneiss.
We observe, for instance, gneiss alternating with layers of black hornblende-schist or of green chlorite-schist, or with granular quartz or limestone; and the interchange of these different strata may be repeated for an indefinite number of times. In the like manner, mica-schist alternates with chlorite-schist, and with beds of pure quartz or of granular limestone.
The rocks are still hornblende-schist and granite, with a covering of alluvium, full of quartz pebbles. I remained two days with Lieutenant Beadle, enjoying in his society several excursions to the hot springs, etc.
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