GEOLOGICAL AGE OF THE GRAVELS. 31 
or 
along the flanks of the range, are such as to demonstrate that there can be 
no metamorphic rocks of that age included within the crystalline formations 
underlying the gravels. Unmetamorphosed Cretaceous rocks, representing 
the later stages of that epoch, rest in an unaltered and almost undisturbed 
condition on the upturned edges of the bed-rock series, at a sufficient num- 
ber of points along the base of the Sierra to prove that there was a great 
break at the close of the Jurassic, having as its result a complete change in 
the orography of the western side of the continent, as well as in the organic 
life of that region. This break between the Jurassic and the Cretaceous 
seems to have been the most important era in the geological history of North 
America west of the Wahsatch Range. It was pre-eminently the mountain- 
building epoch of that region. 
It follows, therefore, that the detrital masses resting on the bed-rock may 
contain representatives of the various geological groups higher in the series 
than the Jurassic. The first question would naturally be whether the Cre- 
taceous epoch was represented among these. There is, as has been stated in 
the preceding pages,* a very large development of the rocks of this period 
in the Coast Ranges, and a much smaller one along the western base of the 
Sierra Nevada. These, however, are exclusively of marine origin, and their 
position on the flanks of the Sierra is so low down as to show that the range 
had essentially its present elevation — so far, at least, as the effect of oro- 
graphic causes is concerned — during the Cretaceous epoch. This statement, 
however, would not be true except for the mining region described in the 
present volume. Near Folsom, at the point where the American River issues 
from the foot-hills, the Cretaceous strata are but very little elevated above 
the level of the Great Valley. They rise, however, as we go north, and near 
Shasta City are more than a thousand feet above the sea-level. Farther on 
in the same direction, beyond Mount Shasta, in the Cottonwood Valley, they 
are found in considerable force at an elevation of over three thousand feet. 
It would seem, therefore, that there has been a decided uplifting of a 
region of large extent in the northern part of the State since the Cre- 
taceous strata were deposited. This is in harmony with other known facts 
connected with the geology of that region, where the Coast Ranges, built 
up by orographic disturbances which have all taken place in Cenozoic time, 
come so close into contact with the Sierra Nevada, which is essentially a 
Mesozoic mountain-range, that it has not yet been possible to draw the line 
* See ante, pp. 51, 52. 
