422 J. E. MILLS ROCKS OF THE SIERRA NEVADA OF CALIFORNIA. 



laminated, but more often in distinct layers half an inch to an inch and 

 more in thickness. The alteration occurs in all stages from that of some- 

 what siliceous slate to slaty quartzite and complete quartzite. The slates 

 and quartzite arc frequently contorted ; the contortion being local and 

 not caused by any general movement of the mass or by .pressure from 

 without, but by some locally acting force within the mass, probably 

 molecular force accompanying the chemical and mineralogical changes 

 of metamorphosis, causing alteration of volume and consequent displace- 

 ment. 



These pre-Mesozoic slates and quartzites lie on the granite, and were 

 probably deposited upon it. as 1 have found no intrusions of the erup- 

 tive rock in the strata. They outcrop between the granite and Mesozoic 

 rocks, and Mesozoic strata come in contact both with them and with the 

 granite. It will he shown hereafter that fragments of both the granite 

 and quartzite are found in a conglomerate of the lower Mesozoic group, 

 and therefore that these rocks are older than those of that group and 

 unconformable with them. 



The Claremont uplift has brought to the surface a series of pre-Meso- 

 zoic strata. There are no granites or other eruptives among them, and 

 they consist of highly metamorphosed slates. They retain more or less 

 of slaty structure, though rarely cleavable into lamina' or sheets of consid- 

 erable size, and they break with irregular, often more or les^ conchoidal, 

 surfaces and into more or less prismoidal fragments. They are curled 

 and contorted in much the same way as the slates and quartzites before 

 described, but much more generally than they. There is a very general 

 deposition or segregation of silica in the mass, evidently chemical. The 

 silica is in part disseminated through the slate, but much of it is lodged 

 in films on surfaces of cleavage or lamination, or in irregular hunches 

 and lenticular bodies or veins, sometimes crossing, sometimes lying par- 

 allel with the surfaces of lamination. There are micaceous surfaces, and 

 the mica and also an arrangement of the siliceous grains in the slaty 

 lamina' sometimes give a gneissoid form to the rock: hut there is not 

 enough of mica or micaceous felting to form a true gneiss. The rock is 

 sometimes chloritic, and some of the chloritic ledges have a massive form 

 that suggests eruptive origin. 



It will he shown hereafter that limestones and slates of the oldest 

 Mesozoic subgroup of the district rest unconformably on these rocks. 

 They are therefore older than the oldest Mesozoic rocks of the dis- 

 trict. They are nowhere within this district exposed in contact with the 

 granites or quartzites of Spanish peak mountain, and there are not any 

 means here of determining directly the relative age of these and the 

 Spanish peak pre-Mesozoic rock-: hut farther northwestward, near the 



