430 J. E. MILLS — ROCKS OF TLIE SIERRA NEVADA OF CALIFORNIA. 



peak mountain, but higher up on the face isolated areas of greenstones 

 and slates occur and, as hereafter shown, the greenstones, slates and lime- 

 stones come next to the same area of pre-Mesozoic exposures oh the west 

 between it and the ( ! reat valley, and in by far the greater number of eases 

 throughout the Sierra the rocks of what I have called the lower Mesozoic 

 subgroup outcrop between the serpentines and slates of the upper sub- 

 group and the pre-Mesozoic rocks. 



Unconformity on Claremont. — The fossiliferous limestones and accom- 

 panying slates lie unconformably on the pre-Mesozoic slates of Clare- 

 mont. The contact and unconformity are plain to the eye where the 

 road from Quincy to Oroville crosses the neck of the " Devil's elbow," on 

 the left bank of Spanish creek, at the mouth of Rock creek, in section 18, 

 T. 24 N., R. 9 E. The unconformity on Claremont is plainly by erosion, 

 as no corresponding difference in dip and strike is apparent. There are 

 greenstones and limestones and a little serpentine in isolated areas on 

 and next east of the pre-Mesozoic area of this faulted northwestern end 

 of the mountain mass. 



UPPER MESOZOIC SUBGROUP. 



Thinly laminated Slates and Serpentines. — The upper Mesozoic subgroup 

 is the highest in the series of metamor'phic rocks. Its exposures therefore 

 lie generally in positions midway between the axes of greatest uplifting 

 and between exposures of the lower subgroup on either side, the latter 

 adjoining the pre-Mesozoic rocks still farther toward the right and left 

 and nearer the axes of uplift. Tins prevailing order of succession on the 

 surface is, however, often interrupted locally by faults. In the district 

 here under more immediate consideration, the northeastern crest of the 

 range is, as already described, of the lower Mesozoic subgroup; the south- 

 western crest and the face of the escarpment immediately below it on 

 the east are of pre-Mesozoic rocks, with isolated areas of the lower Meso- 

 zoic greenstones and slates. Between the two mountains the greater part 

 of the space is occupied with serpentines and slates of the upper sub- 

 group. The slates occupy the eastern part and the serpentines the 

 western part, and the two are separated by the long, narrow belt of pro- 

 truding older Mesozoic and pre-Mesozoic rocks brought up by the Clare- 

 mont uplift already described. As this belt approaches the Middle fork 

 of the Feather it narrows and ends near the river, where the slates and 

 serpentines of the upper subgroup come together. The area of exposure 

 of the serpentines is from 1.6 to 3.5 miles wide, and that of the slates 

 from 6.5 to 7.5 miles wide. 



There is a narrow strip of serpentine outcropping on the easterly side 

 of the exposure of slate, between it and the older Mesozoic greenstones 



