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 seadevel. 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 south- 

 ward extension of the one which runs along the western base of Thousand 

 Lake Mountain and across the Red Gate. South of the Grate 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 ho believes was caused by the sinking of a narrow wedge. He has also suggested to me 

 soveral other minor features of inequality in the surface which he regards as due to minor faulting. 



