DEFINITION OP THE SIERRA NEVADA. 415 



have not heretofore been defined, nor have the rocks within the groups 

 been described with the order of their succession. 



Division into two 'unconformable Groups. — By detailed examination of 

 the rocks of one district within the range and comparison with those of 

 other parts of it, I have been enabled to distinguish two unconformable 

 groups definitely, and to determine the succession of rocks within the 

 later of the two and partially within the older one, and, so far as my 

 surveys have extended, to map the areas of exposure of each. The later 

 group includes the rocks determined by Whitney to he Mesozoic, and, as 

 will be shown hereafter, includes none other than Mesozoic. I shall call 

 this group, for the purposes of this paper, the Mesozoic group, excluding 

 from consideration the unaltered Cretaceous strata exposed along the 

 western foot of the range. 



The older group lias thus far yielded no fossils within the Sierra proper, 

 and 1 will designate it simply as the pre-Mesozoic group. 



( rENERAL STRATIGRAPHY. 



General Features of the Sierra. — Before entering upon a detailed consid- 

 eration of the two groups and the succession of rocks within them, it will 

 he well to present some general features of the stratigraphy of the range, 

 for they throw much light upon the order of succession ; and among strata 

 so tilted, faulted and altered it is necessary to use all the means at hand 

 to determine which are the higher or lower in the series. 



The Sierra Nevada, as now defined, extends about o7<> miles in a north- 

 westerly direction, with the general trend of the coast of this part of the 

 continent, from near latitude 34° 48' to near latitude 40° 12' north. At 

 its southerly end it curves westward around the southern end of the val- 

 ley of California, and coalesces with the Coast range. At its northern 

 end it might he difficult, on purely geographical grounds, to distinguish 

 it from the Cascade range'; hut geological considerations leave no doubt 

 thai the Sierra ends northward where its metamorphic rocks pass be- 

 neath the Lavas of the Lassen peak district ; for that mountain and the 

 lava field stretching out southward from it occupy an area where, as 

 late as the Chico (upper Cretaceous) epoch, the sea passed around the 

 northern end of the Sierra, and where, as late as Miocene time, there 

 was still a depression occupied by fresh water.* Other reasons, from 

 structural geology, for thus limiting the range northward will be given 

 hereafter. 



Dual Character of the Range. — In its northern portion the Sierra is 

 double, consisting of eastern and western divisions. The eastern division 



*Geologyof the Lassen Peak District, by J. S. Diller, in 8th Annual Report of the IT. 3 

 i 5 . pari i. 1889, pp. 30 ■ I 12 



