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Updated: June 9, 2025
It is composed of fine tufa, in which is imbedded a great number of large angular fragments of trachyte, some of which are more than three feet in diameter. It is the last bed but one, the surface being composed of lightly coherent strata of tufaceous ash, worn into an undulating surface by the action of the elements.
If the ore were derived from the trachyte, it should be at least somewhat alike in the two mines, should be more generally distributed in the Horn Silver fissure, and might be expected to give out at, no great depth.
Then as they walked slowly homeward, he took Sophia's hand, and swayed it gently to and fro to the old fiddler's refrain, "'Little I thought what love could do." "Oh, how this spring of love resembleth The uncertain glory of an April day!" "Hammering and clinking, chattering stony names Of shale and hornblende, rag and trap and tuff, Amygdaloid and trachyte."
Near the junction of the R. Colorado with the main stream, there is a hill of whitish, brecciated, partially decomposed feldspathic porphyry, having a volcanic aspect but not being really of that nature: at Tolla, however, in this valley, Dr. Meyen met with a hill of pumice containing mica. Meyen found true trachyte.
We may be permitted to meditate a little on the origin of mountains, after having seen the position of the Mexican volcanoes, and of trachyte summits on an elongated crevice; having found in the Andes of South America primitive and volcanic rocks in a straight line in the same chain; and when we recollect the island, three miles in circumference, and of a great height, which in modern times issued from the depths of the ocean near Oonalaska.
Leonard Horner states that it would be easy to form a suite of specimens showing a gradation from a white trachyte to a black basalt; but we must recollect that when Mr. Horner wrote, the microscopic examination of rocks by means of thin sections was not known or practised, and an examination by this process might have proved that this apparent transition is unreal.
In another part of the island, there are layers of pearlstone and pitchstone, which in many respects resemble those of Ascension. The zones in the columnar trachyte are generally contorted; they extend uninterruptedly for a great length in a vertical direction, and apparently parallel to the walls of the dike-like mass.
These tuffs of the southern shores of the Grand Canary, containing the Upper Miocene shells, appear to be about the same age as the most ancient volcanic rocks of the island, composed of slaty diabase, phonolite, and trachyte.
Sir A. Geikie has disputed the correctness of the view, which I advocated as far back as 1874, that the trachytic lavas of Antrim are the earliest products of volcanic action; but at the time he wrote his paper on the volcanic history of these islands, it was not known that pebbles of this trachyte are largely distributed amongst the ash-beds which occur in the very midst of the overlying basaltic sheets, as I shall have to explain later on.
The lava consists of dark- greyish, harsh rocks, intermediate in character between trachyte and basalt, containing glassy feldspar, olivine, and a little mica, and sometimes amygdaloidal with zeolite: the basis is either quite compact, or crenulated with air-vesicles arranged in laminae.
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