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Updated: June 9, 2025
But my chief reason for believing that these soft earthy stones, with their extraneous fragments, are not of sedimentary origin, is the extreme improbability of crystals of feldspar, black microscopical specks, and small stains of a darker colour occurring in the same proportional numbers in an aqueous deposit, and in masses of solid trachyte.
The point of the cone was let into a hollow socket prepared for it in a block cut from the hardest stone that could be found. Such a material would resist friction better and take a higher polish than brick, so that it was at once more durable and less holding. Sockets of flint, basalt, trachyte, and other volcanic rocks have been found in great numbers both in Assyria and Chaldæa.
Specific gravity of the constituent parts of trachyte and of basalt, and their consequent separation. Obsidian. Apparent non-separation of the elements of plutonic rocks. Origin of trap-dikes in the plutonic series. Distribution of volcanic islands; their prevalence in the great oceans. They are generally arranged in lines. The central volcanoes of Von Buch doubtful.
Columnar basalt in the Vincentin. Basaltic pillars of the Kasegrotte, Bertrich-Baden, half-way between Treves and Coblentz. The columnar structure is by no means peculiar to the trap rocks in which augite abounds; it is also observed in trachyte, and other feldspathic rocks of the igneous class, although in these it is rarely exhibited in such regular polygonal forms.
The latter are well displayed for twenty miles north and south of Castro; they vary in character from common sandstone to fine-grained, laminated mudstones: all the specimens which I examined are easily fusible, and some of the beds might be called volcanic grit-stones. These latter strata are perhaps related to a mass of columnar trachyte which occurs behind Castro.
The dome-shaped volcanoes are generally composed of felsitic matter, whether domite, trachyte, or andesite, which has been extruded in a molten or viscous condition from some orifice or fissure in the earth's crust, and being piled up and spreading outwards, necessarily assumes such a form as that of a dome, as has been shown by experiment on a small scale by Dr. E. Reyer, of Grätz.
Often, where the water flows over beds of dark grey basalt, masses of trachyte, closely resembling syenite, have formed "potholes," and by mutual action have been worn to pebbles. At Pei-pei there are three circular pools, each about fifty feet in diameter, and separated by walls six feet thick, in a bed of columnar basalt.
I did not meet with a single specimen of true trachyte; though some of the paler lavas, when abounding with large crystals of the harsh and glassy albite, resemble in some degree this rock; but in every case the basis fuses into a black enamel. Beds of ashes and far-ejected scoriae, as previously stated, are almost absent; nor did I see a fragment of obsidian or of pumice.
The first of these mines, which is of great antiquity, is situated in the Cerrillos Mountains, eighteen miles from Santa Fé. The deposit occurs in soft trachyte, and an immense cavity of several hundred feet in extent has been excavated by the Indians while searching for this gem in past times.
The trachyte appears to have been extruded from one or more vents in a viscous condition, the principal vent being probably situated under Tardree mountain, where the rock occurs in greatest mass, and it probably arose as a dome-shaped mass, with a somewhat extended margin, above the floor of Chalk which formed the surface of the ground.
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