Re ee SL al ae ache RC er tT NT ee ee ee ts 
J. LeConte—Eatinct Volcanoes about Lake Mono. “87 
sion of King’s ancient lake Lahontan, occur remarkable depos- 
its of ulexite (soda-lime borate) which also deserve separate 
study. 
Lerraces.—I have already mentioned the terraces about Lake 
Mono. Several of these are very distinct and traceable all 
around the lake. But they are seen in greatest number an 
most distinctly on the west side, where the lake approaches the 
Sierra and the hills rise abruptly from the lake-leyel. Five or 
six may here be counted, rising one above the other like level 
benches, the highest being, according to Whitney, 680 ft. high, 
These terraces are undoubtedly the marks of old lake levels, 
and show not only a former greater depth but also a much 
greater extent of the lake waters. The highest level traced 
about the lake would reach the moraines at the foot of the 
Sierra, extend beyond the plains on every side, and enclose an 
same line of volcanic activity. The largest of these islands is 
about 24 miles long, a mile wide and about 300 feet high. It 
is composed mainly of extremely fine, whitish material, beauti- 
fully and very finely laminated, the differently colored laminze 
being very distinct and scarcely thicker than cardboard. is 
material 1s spoken of by Whitney* as volcanic ashes. Under 
the microscope it proves to be composed wholly of diatom shells 
with only an occasional grain of sharp sand. There is no doubt 
therefore that it was deposited very slowly in calm waters, in 
the middle of the lake and beyond the reach of detritus.) The 
stratification is mostly horizontal; only in two or three places 
where the deeper strata are exposed on the cliffs by the action _ 
of waves, I observed a slight dip, and in one place a gentle but 
distinct anticline, showing a quiet upheaval of the whole mass, 
as I think, by volcanic forces. In the highest parts of the 
island, the soft, horizontally-laminated earth is sculptured by 
erosion into sharp pinnacles and turrets like bad-land structure 
on a small scale. On the eastern portion of the island a con- 
siderable area of black basaltic rock is exposed, but this is no 
where more than 50 feet high. Where the diatomaceous earth 
comes in contact with the basalt, the former always overlies 
the latter in undisturbed horizontal layers. I conclude there- 
fore that the basalt preceded the formation of the diatomaceous 
mud, was once entirely covered by the latter, and was subse- 
quently exposed by erosion. 
* Geol. Survey of California, i, 453, 
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