438 J. E.MILLS— ROCKS OF THE SIERRA NEVADA OF CALIFORNIA. 



mass of both mountains consists, as before said, of greenstones and slates 

 of the lower Mesozoic subgroup. The greenstones are largely conglom- 

 eratic and are largely altered to quartzite. In mount Bullion, at the 

 west of the principal mass of greenstones, is a series of slates with lime- 

 stones. Both slates and limestones are exposed on the Merced, and also 

 nine miles southeast of the river. The limestones are siliceous, and no 

 fossils have been found in them. Next west of these slates and lime- 

 stones outcrop serpentines on both sides of the Merced and at points for 

 10 miles southeast. Farther southeastward they are replaced by talcose 

 rocks, which probably belong to the same horizon as the serpentine, and 

 these continue southeastward to the contact with pre-Mesozoic gneiss. 

 The serpentines on the Merced are in part altered to quartzite, and tins 

 alteration is exhibited unmistakably and on a large scale on the right 

 side of the river. Thinly laminated slates follow next west of the ser- 

 pentines on the Merced, and they continue southeastward at least 11 

 miles. They form the floor of the narrow valley between the two moun- 

 tains ; at Bear Valley village the area of outcrop of these shales is about 

 a mile wide. Here in these shales were found the Amelia and other 

 fossils by which Professor Whitney established the Mesozoic age of this 

 part of the metamorphic rocks of the Sierra. He, on the identification 

 of Meek and Gabb, considered them Jurassic; while, as already stated, 

 White places them at the confines of .1 urassic and Cretaceous, and Becker 

 places them still later in the Cretaceous. 



There is faulting at the western foot of mount Bullion, as shown by 

 excavations on the great quartz lode there. Professor W. P. Blake men- 

 tions a fault in the Princeton mine, which is on this lode 9 miles south- 

 east of the Merced, in a report on the mine which I have not now at 

 hand to refer to. It is plain from maps and reports of the mines, as well 

 as from interruptions of the exposures at the surface, that the lode occu- 

 pies a fissure at a fault plane. But the succession of the rocks, although 

 obscured in places by the faulting, is essentially the same as at the 

 northern end of the range. 



THE MESOZOIC SERIES. 



Natural Divisions. — The Mesozoic series is essential^ the same through- 

 out the two great areas of exposure, and is as follows in descending 

 order : 



T ^ , ( Thinlv laminated slates: 



Upper subgroup < Q1 , J j <■ 



11 t I folates and serpentines. 



r , ( Slates and limestones with some greenstones; 



Lower subgroup < Q1 , , ,• , 



1 Mates and greenstones or diabase. 



