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Updated: June 9, 2025
The whole range is for the most part composed of various kinds of trachytic conglomerate. "From the midst of these vast tufaceous deposits, the tops of the hills, composed of trachyte, a rock which forms all the loftiest eminences, here and there emerge.... The trachyte is ordinarily reddish, greyish, or blackish; it mostly contains mica.
It must have taken months, if not years, to have fashioned the statue I have figured out of the trachyte without tools of iron, and it strikes one with wonder to think of the patience and perseverance with which the details were worked out.
The coarse granite of Harney Peak indicating that, as the central point of the earliest upheaval, and the three porphyries known as rhyolite, trachyte, and phonolite, showing the uplifts of later periods to have had their centers a little more to the north, but the entire area is said to be only about sixty miles long and twenty-five miles in width.
These rocks do, indeed, sometimes alternate partially, as in the volcano of Mont Dor, in Auvergne; and in Madeira trachytic rocks overlie an older basaltic series; but the trachyte occupies more generally an inferior position, and is cut through and overflowed by basalt.
The igneous rocks of the Westerwald, and of the mountains called the Siebengebirge, consist partly of basaltic and partly of trachytic lavas, the latter being in general the more ancient of the two. There are many varieties of trachyte, some of which are highly crystalline, resembling a coarse-grained granite, with large separate crystals of feldspar. Trachytic tuff is also very abundant.
The hill is composed of trachyte, and the upper portion is perforated in all directions by galleries and vaults that served formerly as a quarry for the extraction of stone of which the Romans formed their sarcophagi, in consequence of its powers of absorption of the moisture exuding from the bodies laid in their stone chests.
Savannahs, sparingly timbered, were next crossed; then we reached one of those level plains, with black soil and blocks of porous trachyte lying on the surface, which are swamps in the rainy season, and have for vegetation sedgy grasses and scattered jicara trees, cactuses and thorny acacias.
Basalt is the most fusible of volcanic rocks, owing to the augite and magnetite it contains, so that it spreads out with a very slight slope when highly fused. Trachyte, on the other hand, is the least fusible owing to the presence of orthoclase felspar, or quartz; so that the volcanic domes formed of this material stand at a higher angle from the horizon than those of basaltic cones.
The trachyte is generally of a pale brown colour, stained with small darker patches; it contains broken and bent crystals of glassy feldspar, grains of specular iron, and black microscopical points, which latter, from being easily fused, and then becoming magnetic, I presume are hornblende.
Thus the terms Trachytic porphyry, Trachytic tuff, etc., merely refer to the same rock under different conditions of mechanical aggregation or crystalline development which would be more correctly expressed by the use of the adjective, as porphyritic trachyte, etc., but as these terms are so commonly employed it is considered advisable to direct the student's attention to them. Porphyry.
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