1:36 J. E. MILLS — ROCKS OF THE SIERRA NEVADA OF CALIFORNIA. 



much farther they extend north and south of these limits T do not know. 

 The serpentines show themselves on the railroad between Nevada city 

 and Grass valley, and at the crossing of Greenhorn creek, and between 

 there and the crossing of the Rear. On the same railroad, about a mile 

 north of the Central Pacific railroad, is massive talc of the same horizon 

 and very similar in character to that on the eastern side of the thinly 

 laminated shales near the serpentines at the Middle fork of the American. 

 Serpentines also occur west of the thinly laminated shales between the 

 North and Middle forks of the American at a locality which is probably 

 in section 13, T. 13 N., R. 9 E. 



West of these serpentines and slates are exposures of the rocks of the 

 lower Mesozoic subgroup, and they continue westward to the pre-Mesozoic 

 gneiss and granite. The Central Pacific railroad crosses them from the 

 contact with the gneiss about a mile southwest of Auburn to near Cape 

 Horn. They consist largely of eruptive rocks (diabase), which have not 

 here, as already stated, the prevailing chloritic character, but are of gray 

 and black colors, sometimes porphyritic, and often resembling, to the 

 naked eye. the Tertiary andesites. They often occur in dikes, traversing 

 both slates and eruptive masses. East of Colfax, between it and ('ape 

 Horn, limestones occur, as they also do under Cape Horn, near the river. 

 These limestones hold the same relative position at the head, of the lower 

 subgroup between the diabases or greenstones below and the serpentines 

 above as at the northern end of the range. 



Ammonites colfaxii. — One mile west of Colfax Professor Whitney found 

 specimens of an ammonite which Gabb describes as Ammonites colfaxii, 

 and referred with certainty to Mesozoic time and with some hesitation to 

 the Liassic epoch. Whitney calls it a " secondary fossil."* It was found 

 in the slates and diabases which underlie the limestones at the head of 

 the lower Mesozoic subgroup. It is therefore from a somewhat lower 

 horizon than the fossils found in the limestones at the northern end of 

 the range, and this affords confirmation of the Mesozoic age of the lime- 

 stones near Pence's on the West branch of Feather river. 



Mesozoic Exposures south of the American. — From the South fork of the 

 American to Sutter creek I have not had opportunity to examine the 

 rocks. 



From Sutter creek to the Tuolumne the area of Mesozoic exposures 

 lying between the pre-Mesozoic rocks on the east and the Great valley on 

 the west is approximately 12 to 15 miles wide. Within the area are two 

 prominent axes of uplift, having the general trend of the main range, and 

 along these axe.-, between the Calaveras and Stanislaus rivers, are two of 

 the minor mountains above mentioned, the western one rising from the 



:: Auriferous Gravels of the Sierra Nevada, 1819, pp. 37-41. 



