GLAUCOPHANE AND ASSOCIATED SCHISTS 743 



masses seems certain. It seems improbable, however, that the 

 main portion of the normal glaucophane and actinolite schists is 

 a result of contact action. That the schists have not resulted 

 from contact action by peridotite masses seems probable, for at 

 some points the same masses have certainly produced but slight 

 alteration in adjoining sandstones and shales, and the thickness 

 and character of the schists is such that they could only have 

 been produced by metamorphosing agents acting on a large scale. 

 It seems difficult to believe, also, that the schists could be formed 

 by serpentine dikes which are smaller than the schist masses 

 themselves. Besides, the inclusions of schist in the Healdsburg 

 and Angel Island serpentines render it almost certain that the 

 schists are the older of the two rocks. In addition, the evidence 

 points to the massive glaucophane rocks and normal schists being 

 older than the Golden Gate or Franciscan series of rocks, for the 

 schists are unconformably beneath what appears to be the Golden 

 Gate or Franciscan rocks in the Calaveras Valley, and they prob- 

 ably have similar relations at Healdsburg. Finally, serpentinized 

 dikes are frequently found intrusive in Golden Gate or Franciscan 

 rocks, while at Mount Diablo, 1 near Gilroy, 2 and in San Luis Obispo 

 county, 3 there are serpentine dikes intrusive in the Knoxville 

 beds. This would of course make the dikes younger than the 

 schists, if the schists are older than the Golden Gate or Franciscan 

 series. 



In some cases the schists have been formed directly out of 

 sedimentary rocks, and probably in some cases out of tuffs or 

 other igneous material. The writers have observed cases in 

 which basic igneous dikes have had glaucophane and other sec- 

 ondary minerals developed in them, and have become more or 

 less schistose. 



It can hardly be doubted that glaucophane schists have 

 been developed in rocks of different ages, and older than the 

 Knoxville. It seems probable, also, that there is a series of 



x Loc. cit., p. 390. 



2 Communicated by Dr. J. P. Smith, Stanford University. 



3 "The Stratigraphy of the California Coast Ranges," H. W. Fairbanks, Jour. 

 Geol., Vol III, p. 428. 



