Prof. Blake on the Diluvial Deposits in California. 387 
eighteen ‘hundred feet high, formed entirely of the older rocks, 
no traces of deposits being found on their surface, nor in the 
ravines that lead. from. them. 
epth of these deposits is extremely variable. Sometimes 
nothing more than a trace of them in the presence of a few round 
pebbles lying on the top of a ridge, is found; the valleys and 
ravines in the neighborhood containing their disintegrated ele- 
ments in considerable quantities. Jn other instances, particularly 
where spread out over the elevated flats, they are of a moderate 
and pretty uniform thickness for a considerable distance, varying 
from two or three feet to a few inches, and this too in positions 
where the surface could not have been exposed to any great 
amount of denndation. They are again found many hundred 
feet in thickness; composed of superimposed strata of different 
mineralogical ‘constitution, generally horizontal and conformable 
with each other. ct afk ‘ 
The localities where these deposits are met with most exten- 
sively disclosed aud that have been worked, are at Nevada and at 
Mokelumne Hill. At the former place they form the crest of a 
high mountain called the Sugar Loaf, full 2000 feet above the 
level of Deer Creek, the upper 6U0 feet being formed entirely of 
diluvial strata. At Mokelumne Hill they are also some 200 feet 
deep, forming ‘here also the summit of a high and isolated 
mountain. The elements of which they are composed, differ 
considerably in different localities, although there are through the 
whole series many points of resemblance. In the lower valleys, 
and flats, between the ranges of the lower hills they appear to 
consist of beds of gravel, containing occasional boulders of quartz, 
and the harder rocks. On the elevated flats higher up in the 
mountains, the surface of these deposits is generally covered by 
a reddish loam; mixed with small gravel, whilst reposing on t 
bed rock, and a few inches above it, is found a stratum contain- 
ing large boulders and gravel, the boulders being. principally 
quartz... On the top of the hills and the crests of the ridges, 
‘where they generally attain their greatest thickness, we find 
them composed of many distinct strata lying nearly horizontal, 
and conformable with each other and generally also with the 
surface of the whderlying rock. In these situations the most 
superficial stratum is composed of a mass of extremely hard con- 
glomerate, containing principally trachytic rocks, imbedded in a 
hard argillaceous cement 
different localities. ~ At Nevada and Mokelumne Hill there are 
found extensive beds of white and blue clay, containing 
of quartzose sand, and occasionally few small pe 
