42(j J. E. MILLS — ROCKS OF THE SIERRA NEVADA OE CALIFORNIA. 



horizon in the lower subgroup. I have not found any certain uncon- 

 formity between these subgroups. The whole group seems to be one 

 long series of sediments and lavas deposited during a period of pre- 

 vailing though perhaps not uninterrupted subsidence of the region. 



LOWER MESOZOIC SUBGROUP. 



Slates, Greenstones and Limestones. — The greenstones or diabases of this 

 subgroup are of eruptive materials, hut these materials have quite com- 

 monly been transported to their present position and deposited there by 

 water. Stratification is not infrequently visible, and the transition from 

 massive greenstone to slate is sometimes gradual. The greenstone is 

 very often and over wide areas conglomeratic, made up of bowlders and 

 pebbles in a cement or groundmass of the same material, all of altered 

 lava exeept at times a small proportion of fragments of quartz and other 

 rocks. The bowlders and pebbles and groundmass have undergone much 

 the same kind and degree of alteration, and the surfaces and outlines of 

 the bowlders and pebbles are more or less obscure, but still are readily 

 recognized on fresh fracture, and often more plainly on weathered sur- 

 faces. The bowlders and pebbles are well rounded. The mechanical 

 condition and admixture of these materials are very similar to those of 

 much of the Tertiary andesite, which has been transported by water and 

 deposited in the same district, often on the greenstones. Between the 

 South Yuba and the American, as well as between the Mokelumne and 

 Calaveras rivers and elsewhere, lavas of this subgroup are exposed in 

 dikes, where, to the naked eye, at least, they are not chloritic, hut of dark- 

 gray colors or black, sometimes porphyritic, and often very similar to 

 Tertiary andesite. Professor Whitney says of this rock: "It appears 

 from Mr. Wadsworth's (not yet completed) examination to be a diabase 

 tufa, a much metamorphosed volcanic deposit. * * * Mount Bullion, 

 Juniper ridge, Bear mountain (on the Merced) and Merced mountain are 

 made up of this rock."* I have ^ven the exposures on mount Bullion 

 and .Juniper ridge, and the rock there is chloritic and largely conglom- 

 eratic, like the greenstones of the district under more immediate consid- 

 eration here. On mount Bullion they are also largely altered to quartzite. 



The greenstones and slates of the lower Mesozoic subgroup form the 

 crest of Hough mountain and of the greater part of Grizzly ridge, though 

 covered in part by Tertiary deposits. At the southeasterly vm\ of Grizzly 

 ridge the}' come in contact with pre-Mesozoic granite. The main eastern 

 crest of the range is of these rocks from its northwesterly end south of the 

 Middle fork of the Feather to the northwesterly Hank of mount Haskell. 



♦ Auriferous Gravels oflhe Sierra Nevada, 1879, p, 44. 



