CHAPTER III. 



THE SEDIMENTARY ROCKS. 



General character. — TliG Coast Rtinges of California presGiit a truly remarka- 

 ble opportunity for the investigation of some of the most important phe- 

 nomena embrac(;d under the general term of metamorphism. To give a clear 

 ideaof the unusual advantages afforded by this area it is necessary to anticipate 

 some of the results reached. Field examinations were made for this memoir 

 at numerous points from al)Ove Clear Lake to the region of New Idria, thus 

 partially covering a belt of the Coast Ranges about 230 miles in length. 

 Throughout this whole region there is structural and lithological evidence 

 that granite of very uniform character underlies the entire country. Ex- 

 cei^ting the belt of schists along the coast from Santa Cruz southward, it 

 is estimated that fM) per cent, of all the rocks of this region ai'e sandstones, 

 altered or unaltered. These sandstones are also extremelv uniform in char- 

 acter, and wherever they are inconsiderably modified the slides prepared 

 from them show that they are directly or indirectly derived from tlie gran- 

 ite, or, in other words, tliat they are arcose. Of this material of known 

 origin a portion has been highly altered. The alteration processes to which 

 it has been sul:)jected are identical from one end of the region to the other 

 and innumerable transitions are presented. It is difficult to estimate the 

 areas occupied by the metamorphic rocks of the Coast Ranges, because the 

 occurrences are extremely irregular. A moderate estimate of the exposures 

 between Clear Lake and New Idria, which consist of holocrystalline meta- 

 morphic rocks, sandstones in which recrystallization has made considerable 

 progress, phthanites, and serpentine, is 3,000 square miles. Large areas, 

 covered by late Cretaceous and Tertiary strata, are also known to be un- 

 derlain by metamorphics, and this series extends far to the north and to the 



