Gneiss Enlarge
Sep 1 1813 published by Jas Sowerby London.
British Mineralogy
CCCCXCIII

Gneiss

  • Syn.
    • Gneiss. Jameson, Lucas, &c.
    • Gneüs. Werner, &c.

Perhaps it would he hardly pardonable not to give some hint of what is the popular idea of the formation or periodical series of this our Empire, considered as related to the whole globe.

Granite, being the foundation or oldest rock, consists chiefly of the three substances comprehended at present under the terms Quartz, Feldspar or Kaolin, and Mica. These are exemplified in many parts of this work, but, those that relate to the more primitive granite formation are tabs. 180, 181, 332, &c. and the others are more and more subordinate, as tabs. 211, 212, 224, &c.

The next rock is Gneiss, in winch are the same constituents, Quartz being in general the most abundant, and the Mica lies in strata or layers, so that the rock is stratified: the strata varying much in thickness make it in some cases resemble slate; sometimes it seems rolled one part over another or undulating, &c.; indicating a tremendous and mighty agency, beyond the comprehension of us mortals, yet pointed out by the seeming effect on the vast and massive granite, having separated it into immense blocks, logan stones, granules, and even into dust to astonish and amuse us.

The strata of Gneiss, approaching more or less to vertical, form peaks and towering craigs. The black mica in many specimens is so brilliant as to be compared to the fine splendent metallic lustre sometimes found on the laminated glittering substance from the chimneys of iron furnaces. My friend Mr. Meason sent me a specimen from one of the Western Isles of Scotland, something the the upper specimen, which is passing into Mica Slate. The lower specimen is from the neighbourhood of Aberdeen, and contains small bright cubes of Pyrites. Garnets, Topaz, Tourmaline and most of the metals are found in Gneiss.

The Moor stone of which the crosses in Devonshire are made may perhaps be the beginning of the Gneiss formation, or the latter of the Granite: blocks are apparently chosen of a somewhat continuous form; and I have one 6 feet long and 10 inches in diameter, in which the crystal* of Feldspar are 31/2 inches long.

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