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Vast sheets of trachytic and basaltic lavas successively invaded the tracts surrounding the central orifices of eruption, now constituting the more ancient of the lava-sheets of the Auvergne region, and, invading the waters of the neighbouring lake, overspread the lacustrine deposits which were being accumulated therein.
As we have seen that crystals of feldspar, in the instance described by Von Buch, sink in obsidian, in accordance with their known greater specific gravity, we might expect to find in every trachytic district, where obsidian has flowed as lava, that it had proceeded from the upper or highest orifices.
Over the marine lavas and tuffs trachytic and basaltic products of subaerial volcanic origin, between 4000 and 5000 feet in thickness, have been piled, the central parts of the Grand Canary reaching the height of about 6000 feet above the level of the sea.
From this station I can perceive the western termination of the Trachytic range, and I am now about to explore the country between it and the Nammoy, and the further course of this river; and in the event of its continuance in a favourable direction, I shall fix my depot on its right bank, whence I now write, and descend the stream in the portable boats.
It would be impossible in a work of this kind to attempt a detailed description of the Eifel volcanoes, often of a very complex character and obscure physical history, as in the case of the basin of Rieden, where tufaceous deposits, trachytic and basaltic lavas and crater-cones, are confusedly intermingled, so that I shall confine my remarks to the deservedly famous district of the Laacher See, which I had an opportunity of personally visiting some years since.
AFTER crossing the trachytic plain, we reached a large cattle hacienda, and beyond, the river Chocoyo, on the banks of which was some good, though stony, pasture land. We saw here some fine cattle, and learnt that a little more care was taken in breeding them than is usual in Nicaragua.
About here, some of the trachytic porphyry which forms the substance of the hills had happened to have cooled, under suitable conditions, from the molten state into a sort of slag or volcanic glass, which is the obsidian in question; and, in places, this vitreous lava from one layer having flowed over another which was already cool was regularly stratified.
From this station I can perceive the western termination of the Trachytic range, and I am now about to explore the country between it and the Nammoy, and the further course of this river; and in the event of its continuance in a favourable direction, I shall fix my depot on its right bank, whence I now write, and descend the stream in the portable boats.
After the filling up or drainage of the ancient lakes, huge piles of trachytic and basaltic rocks, with volcanic breccias, accumulated to a thickness of several thousand feet, and were superimposed upon granite, or the contiguous lacustrine strata.
As the later eruptions, however, from most volcanic mountains, burst through their basal parts, owing to the increased height and weight of the internal column of molten rock, we see why, in most cases, only the lower flanks of the central, trachytic masses, are enveloped by basaltic streams.
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