



GEOLOGICAL AGE OF THE GRAVELS. 



315 



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 



i 



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 Camozoic 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 



H 



* See Mlk $ pp. 51, 52. 



