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Updated: July 23, 2025
Whilst the country to the north-east of the lakes is mostly composed of rocks, of great age, geologically, such as schists, quartzites, and old dolerytic rocks, with newer but still ancient trachytes, that to the south-west of them is formed principally of recent volcanic tufas and lavas, the irruption of which has not yet ceased.
The earlier eruptions seem to have been chiefly granitic, though this must not be understood in too wide a sense, since there are granite formations even as late as the Tertiary period; those of the middle periods were mostly porphyries and basalts; while in the more recent ones, lavas predominate.
Guadaloupe, an active volcano, the height of which, according to Leboucher, is 799 toises; according to Amie, 850 toises. Montserrat, a solfatara; fine porphyritic lavas with large crystals of feldspar and hornblende near Galloway, according to Mr. Nugent. Nevis, a solfatara. St. Christopher's, a solfatara at Mount Misery. St.
We have, indeed, to suppose that these steam-impelled lavas, which are driven toward the vent whence they are to go forth in the state of dust or fluid, come underground from distances away, probably from beneath the floors of the sea to the westward.
The decisive character and romantic daring of the famous Watteville had reappeared in the soul of his grand-niece, reinforced by the tenacity and pride of blood of the Rupts. But these qualities or faults, if you will have it so were as deeply buried in this young girlish soul, apparently so weak and yielding, as the seething lavas within a hill before it becomes a volcano.
These are the volcanic lavas which united the two crests forming the eastern and western borders of the Tahoe basin or depression, and through which the Truckee River had in some way to find passage ere it could discharge its waters into Pyramid Lake, resting in the bosom of the Great Basin. Here, then, we have the crude Tahoe basin ready for the reception of water.
Captain Dutton, in describing the High Plateau of Utah, observes that the lavas appear to have welled up in mighty floods without any of that explosive violence generally characteristic of volcanic action. This extravasated matter has spread over wide fields, deluging the surrounding country like a tide in a bay, and overflowing all inequalities.
On the other hand, lavas less fusible, or poured out at a lower temperature, stiffen when they have flowed but a short distance, and accumulate in a steep cone. Trachyte has been extruded in a state so viscid that it has formed steepsided domes like that of Sarcoui.
In the erupted lavas, those substances which are subject to calcine and vitrify in our fires, suffer similar changes, when delivered from a compression which had rendered them fixed, though in an extremely heated state.
It can by no means be inferred that trachyte predominated at one period of the earth's history and basalt at another, for we know that trachytic lavas have been formed at many successive periods, and are still emitted from many active craters; but it seems that in each region, where a long series of eruptions have occurred, the lavas containing feldspar more rich in silica have been first emitted, and the escape of the more augitic kinds has followed.
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