348 G. F. Becker — Metamorphic Rocks of California. 



Art. XXXII. — Cretaceous Metamorphic Rocks of California ; by 

 George F. Becker. 



In the course of an investigation of the quicksilver deposits 

 of California, which will form the subject of a monograph of 

 the U. S. Geological Survey, the crystalline and serpentinoid 

 metamorphic rocks of the Coast Ranges have been subjected 

 to an elaborate examination. As the complete report can 

 hardly be distributed in less than a year from the present 

 time, it appears expedient to print an abstract of the results 

 obtained, but all detailed proof of the statements made will be 

 deferred until the final publication. 



The chemical analyses connected with the examination 

 have all been performed by Dr. W. H. Melville, the field 

 work was performed by Mr. H. W. Turner and myself 

 together, and the microscopical examinations were made 

 jointly by Mr. Waldemar Lindgren and mj^self. 



The field studies were made at very numerous points from 

 above Clear Lake to the region of New Idria, thus partially 

 covering a belt of the Coast Ranges of a> out 230 miles in 

 length. Throughout the whole region there is structural and 

 lithological evidence that the sedimentary rocks are underlain 

 by granite of very uniform character, and, wherever they are 

 inconsiderably modified, the slides prepared from them show 

 that they are directly or indirectly derived from granite, or, in 

 other words, that they are arcose. Of this material of known 

 origin a portion has been highly altered. The alteration- 

 processes to which it has been subjected are identical from one 

 end of the region to the other, and innumerable transitions are 

 presented. It is difficult to estimate the area occupied by the 

 metamorphic rocks of the Coast Ranges, because the occur- 

 rences are of very irregular shape. A moderate estimate of 

 the exposures between Clear Lake and New Idria, which con- 

 sist of holocrystalline metamorphic rocks, sandstone 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 

 underlain by metamorphics, and this series is known to extend 

 far to the north and to the south of the limits indicated without 

 substantial change in character. The study is thus not one of 

 mere recrystallization, but of regional metamorphism of irreg- 

 ular intensity and therefore the better fitted for investigation. 

 The age of the altered beds is known from direct paleontological 

 evidence at a number of localities to be Neocomian, or very 

 nearly of that period, and there is no evidence that any consid- 

 erable quantity of older rocks is included within the area. The 



