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It was carefully studied with a 6 1/4 inch Cooke-achromatic by the late Professor Phillips, the geologist, who compared it to the dolomitic or trachytic mountains of the earth. The buttresses and spurs which it throws out give its base a digitated outline, easily seen under suitable illumination.
Can it be due to a less perfect degree of fluidity, which is generally supposed to be characteristic of the allied trachytic lavas? Soft calcareous sandstone occurs in extensive, though thin, superficial beds, both on the northern and southern shores of the island.
The maps which accompany Von Buch's work are remarkably good, and were partly constructed by himself. The island of Ascension, formed entirely of volcanic matter, rises from a depth of 2000 fathoms in the very centre of the Atlantic. As described by Darwin, the central and more elevated portions are formed of trachytic matter, with obsidian and laminated ash beds.
As we advanced the Mal Pais became more broken: the 'bad step' was ugly climbing, and we often envied our men, who wore heelless shoes of soft untanned leather with soles almost as broad as they were long. The roughness of the trachytic blocks, however, rendered a slip impossible. At 6.45 we reached the second floor of this three-storied volcano, here 11,721 feet high.
The term was subsequently made to include other rocks, such as clinkstone and obsidian, which have the same mineral composition, but to which, owing to their different texture, the word in its original meaning would not apply. The feldspars which occur in Trachytic rocks are invariably those which contain the largest proportion of silica, or from 60 to 70 per cent of that mineral.
Another very lofty volcanic mountain is Mount Rainier, in the Washington territory, consisting of three peaks of which the eastern possesses a crater very perfect throughout its entire circumference. This mountain appears to be formed mainly of trachytic matter.
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.
Now we have plain evidence of crystals being embedded in many lavas, whilst the paste or basis has continued fluid. I need only refer, as instances, to the several, great, pseudo-porphyritic streams at the Galapagos Islands, and to the trachytic streams in many parts of the world, in which we find crystals of feldspar bent and broken by the movement of the surrounding, semi-fluid matter.
It was found to be of volcanic origin, and the soil, formed from the decomposition of trachytic lava, was evidently very fertile. From Pisang the corvette made her way among islands, till then scarcely known, to Rawak, where she cast anchor at noon on the 16th of December.
Thus, basalt from Staffa, containing 47.80 per cent of silica, has a specific gravity of 2.95; whereas trachyte, which has 66 per cent of silica, has a specific gravity of only 2.68; trachytic porphyry, containing 69 per cent of silica, a specific gravity of only 2.58.
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