248 



University of California Publications. 



[Geology 



readily evident. It occurs as interdigitating masses with the 

 tuffaeeous conglomerate, occasionally even forming the matrix 

 in which the pebbles lie embedded, and its gradual transition can 

 be traced on the one hand into the normal clay slates and into 

 the tuffaeeous slates on the other. 



The tuffaeeous conglomerate presents an exceptional develop- 

 ment on the section across Timber Gap. Interdovetailing with 

 feldspar tuff are heavy lenses of coarse conglomerate consisting 

 of well rounded pebbles of quartz, red chert, shale and conglom- 

 erate up to the size of a man's head. These occurrences are 

 local and thin out rapidly along the strike. The more tuffaeeous 

 portions show occasional drawn-out pebbles (three-quarters of 

 an inch long) included in a strongly sheared schist of fine grain. 

 The color is slaty, tinged with green, and the foliation planes 

 show but a dull lustre, except where biotite has locally been 

 developed, or where infrequent patches of sericite occur. Some 

 little eyes of glassy quartz can with careful search be discovered. 



Under the microscope, the immediately noticeable feature is 

 the exceedingly large quantity of flaky biotite, with chlorite, 

 which is disseminated throughout the cryptocrystalline ground. 

 Some small lathes of white mica are apparent. Sporadic pheno- 

 crysts of feldspar occur, badly altered, and frequently con- 

 verted to sericitic aggregates. The foliation is around their 

 abraded corners. Between crossed nicols a long slender feldspar 

 rectangle was noted somewhat inclined to the foliation. Towards 

 one end it is bent and fractured, and definitely deflects the stream 

 of schistosity. It is twinned according to the Carlsbad law, 

 and the two halves give simultaneous extinction when the trace 

 of the twinning plane is parallel to the cross-hairs, and is there- 

 fore orthoclase. A few irregular fragments of quartz, showing 

 strain shadows, were found, and proofs of their pyroclastic 

 origin still remained undestroyed. These were found in the sur- 

 vival of pyramid faces, and the occasional presence of straight 

 edge boundaries indicative of their original idiomorphic char- 

 acter. Some tourmaline is present, and was found in three dis- 

 tinct forms : as a small stout prism ; as sector of a spherulite ; 

 and as a spongiform aggregate. This is conclusive proof of the 

 non-allothogenic origin of the tourmaline. 



