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The evidence in support of this view is very clear and conclusive; for, while the volcanic craters formed of ash, lapilli, and scoriæ, together with the rounded domes of trachytic rock of which the Puy de Dôme group is composed, preserve the form and surface indications of recently extinguished volcanoes, those which we may assume to have been piled up in the region of Mont Dore and Cantal have been entirely swept away by prolonged rain and river action, and the sites of the ancient craters and cones of eruption are only to be determined by tracing the great sheets of lava up the sides of the valleys to their sources, generally situated at the culminating points of their respective groups.
The few still active volcanoes in Chile are confined to the central and loftiest ranges of the Cordillera; and volcanic matter, such as appears to have been of subaerial eruption, is everywhere rare. According to Meyen, there is a hill of pumice high up the valley of the Maypu, and likewise a trachytic formation at Colina, a village situated north of St. Jago.
Obsidian is of a black or ash-grey colour, and though opaque in mass is transparent in thin edges. Among the rocks of the trachytic family, or those in which the feldspars are rich in silica, that termed Clinkstone or Phonolite is conspicuous by its fissile structure, and its tendency to lamination, which is such as sometimes to render it useful as roofing-slate.
The abrupt wall had salient and re-entering angles, not unlike the Palisades of the Hudson River, with intercalated strata and a smooth glacis at the base, except between the east and north-west, where the periphery has been destroyed. It is apparently basalt, as we may expect in the lower levels before reaching the trachytic region.
In the highest and steepest parts of the mountain chain may be found marine petrifactions of every variety the sea-hedgehog, the oyster, the mussel, and the star-fish; and in the beds of trachytic rock, deposited in such order that one might fancy they had been placed there by a careful and tasty housewife, are layers of the most curious shells, univalve, bivalve, sublivalve and multivalve, madrepors, and shapeless remnants of creatures now no longer known, and petrified fish.
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.
Since then there have been several eruptions; and so late as 1857 it threw out volumes of smoke, and probably ashes. The whole country is volcanic. For scores of miles every rock is trachytic, and the earth decomposing tufas. The lake itself is like an immense crater with its perpendicular cliffs.
The clouds, whose volumes were disposed like the leaves of a camellia, lay far down to the north-east, as if unable to face the fires of day. And now the great trachytic dome, towering in the translucent air, was the marking feature.
Mountains of gneiss-granite in the province of Oaxaca. Latitude 18 1/2 to 19 1/2 degrees. Trachytic knot of Anahuac, parallel with the Nevados and the burning volcanoes of Mexico. Latitude 19 1/2 to 20 degrees. Knot of the metaliferous mountains of Guanaxuato and Zacatecas. Latitude 21 3/4 to 22 degrees. Linked by spurs to the maritime Alps, or mountains of California.
PUMICE is a light, spongy, fibrous substance, produced by the action of gases on trachytic and other lavas; the relation, however, of its origin to the composition of lava is not yet well understood. Von Buch says that it never occurs where only labrador-feldspar is present.
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