OCR Text |
Show STEUCTUEE OF THE AQUAEIUS. 293 5,000 feet. In some localities the denudation has been much greater, in others considerably less. The preservation of the Aquarius has no doubt been due to its immense roof of hard lava. The eastern part of the plateau is the loftiest, being about 11,600 feet above sea-level. Its platform here is believed to be nearly horizontal, as indicated by the projection of its summit against the sky from every point of view around the horizon. When seen from Thousand Lake Mountain, which is very nearly as high, no peak, nor even a hill, breaks the monotony of the almost level Crest. But the summit is so densely forest-clad that no effort was made to penetrate its interior spaces. The upper wall of dark volcanic rock is seen to extend completely around the eastern third of the plateau. A little east of the center of the plateau a fault throws down the platform west of it from 600 to nearly 1,000 feet. This fault is a southward extension of the one which runs along the western base of Thousand Lake Mountain and across the Red Gate. South of the Gate its throw gradually diminishes, and on the southern slopes of the Aquarius, a few miles south of the lava-cap, it runs out. This fault is comparatively recent for the most part, and is probably coeval with the other great displacements of the Pliocene-Quaternary system. On the northern slopes it splits into two branches, which reunite near the southern verge.* This movement has produced a sag in the central part of the plateau, but the altitude of the summit is nearly all regained towards the west by a gradual ascent. Of the rocks upon the summit I can say but little, having traversed only the central part of the plateau. Those which were observed were chiefly dark hornblendic trachytes commingled with very extensive masses of augitic andesites. In their general aspect they resemble those which are found on Thousand Lake Mountain and northward as far as Mount Hilgard, but with a somewhat larger proportion of augitic lavas. The bedded lavas exposed edgewise in the upper cliffs are highly varied within their limits of chemical and mineral constitution. No acid rocks were observed, and only a few very basic ones. But the sub-acid and sub-basic * Mr. Gilbert is of the opinion that the displacement is much more complicated. Ascending the face of this fault and reaching the summit, he found a narrow valley near and parallel to the fault, which valley he believes was caused by the sinking of a narrow wedge. He has also suggested to me several other minor features of inequality in the surface which he regards as due to minor faulting. |