432 J. E. MILLS — ROCKS <>]' THE SIERRA NEVADA OF CALIFORNIA. 



from the structure as seen under the microscope that the serpentine 

 has replaced olivine and enstatite.* 



Mr. J. S. Diller kindly gave me the results of microscopic examination 

 of typical specimens which I took from the le/t bank of Spanish creek 

 above the mouth of Rock creek and below Spanish ranch. He wrote of 

 these January 25, 1887 : 



"Specimens numbered 1 ami i' are undoubtedly peridotites. Number 2 contains 



a great deal of olivine, but most <>t it lias been altered to serpentine. Originally 

 there was evidently a rhombic mineral, probably enstatite, associated with the 

 olivine, hut now it has all disappeared and serpentine with oxide of iron have 

 taken its place. In specimens 1 and 4 no trace of olivine could he found ; all has 

 been altered to serpentine and magnetite; but the peculiar reticulated structure of 

 the serpentine indicates clearly that it was derived from olivine. I have no doubt 

 that these serpentines are altered eruptive rocks, peridotites." 



These rocks can be found in all stages of alteration, from that of a dark 

 gray or black trappean rock, sometimes porphyritic, massive, cleaving 

 into irregular prisms, to that of an oil-green serpentine with conchoidal 

 fracture and smoothed and rubbed or " sliekensided " surfaces. It is 

 sometimes fibrous. In a geological sense, the whole mass can most con- 

 veniently be designated as serpentine, hut in a detailed lithological de- 

 scription it would be grouped under different heads according to original 

 minor differences in the lava and to different degrees of alteration. A 

 small proportion of the serpentine shows schistose structure and is more 

 or less micaceous. Whether this is sedimentary lava or detritus of other 

 rocks has not been determined. 



The serpentine is in places altered to quartzite. Such quartzite after 

 serpentine occurs at the outlet of Spanish ranch valley; also on Rock 

 creek about three-quarters of a mile above its mouth. 



Upper Slates. — These slates, as already stated, are at the head of the 

 series of the metamorphic rocks of the Sierra. Wherever 1 have seen 

 them freshly exposed by recent erosion or by artificial excavation they 

 are of dark blue or bluish-black color and very commonly pyritous. 

 The first effect of weathering is to cover the surfaces with red and 

 yellow oxides of iron, frequently with efflorescences of alum; in later 

 stages of weathering the red and yellow staining is removed and a light 

 gray, nearly white, often powdery surface is left on the laminae of the 

 slate. When thoroughly weathered the slates show themselves very 

 thinly laminated and fragile. At the outcrop this thin lamination is a 

 distinguishing characteristic. They are very largely altered to quartzite, 

 and the alteration is of a characteristic kind in this district. The result- 



* Lithological Studies: A Description ami Classification of the Rocks of the Cordilleras, 1884, 



p. 158. 



