186 QUK'KSILVEU DEPOSITS OF THE PACIFIO SLOPE. 



than from anv direct evidence of fossils." In tl\e second reference tliey are 

 mentioned as " Miocene rocks turned up on edge and in places so much meta- 

 morphosed as to 1)6 converted into mica-slate." No statement of the means 

 of determination of the age of these beds accompanies this remark, which, 

 however, occurs in a brief summary of the geology of the Coast Ranges. 

 Whatever the evidence may be upon which the change of reference was 

 made it can have little bearing upon the age of the metamorpliics in the 

 central Coast Ranges, nor is serpentinization referred to as forming a part 

 of the phenomena. 



So far as is known, tlierefore, no beds in the Coast Ranges of California 

 younger than the Knoxville group have expei'ienced the peculiar magnesian 

 and siliceous metamorphism so characteristic of these ranges. I'liis fact 

 raises a presumption that the metamorphism was effected prior to the depo- 

 sition of the rock resting upon the metamorphic series, and this presumption 

 is confirmed by examination of the conglomerates t)f the later rocks There 

 rest upon the metamorphic series at different localities Wallala beds, which 

 Dr. White regards as middle Cretaceous; Cliico beds, representing the very 

 close of the Cretaceous; and Miocene strata. The fossils of the Wallala 

 series were found in a conglomerate consisting largely of serpentine pebbles, 

 accompanied by siliceous, metamorphic rocks exactly similar to those accom- 

 panjnng the serpentine in the altered rocks of the Knoxville series. At New 

 Idria there is a bed of conglomerate associated with Chico fossils near an 

 extensive metamorphic area. The pebbles are mainly siliceous, as, indeed, 

 is usually the case in conglomerates derived from the metamorphic rock, 

 for the simple reason that serpentine is both easily decomposed and easily 

 abraded. Careful search, however, revealed pebbles in this conglomerate 

 which consisted in part of serpentine, a result confirmed l)y microscopical 

 examination. The Miocene, too, for instance at New Almaden, contains 

 abundant pebbles manifestly derived from the surrounding metamorphic rock. 



No fossils older than the Knoxville group are known to occur in the 

 Coast Ranges and no known fact suggests the existence of older rocks, 

 excepting the character of the limestone and gneisscid rocks of the Clavilan 

 Range already mentioned, for the habitus of the peculiar metamorphic. 

 rocks under discussion is remarkablv uniform. 



