254 



University of California Publications. 



[Geology 



origin. The matrix is eryptocrystalline, and contains a great 

 deal of chloritic matter flecked through it. Some accessory mag- 

 netite with pyrite cores, and some secondary epidote are also 

 found. 



THE CONTACT. 



It has heen pointed out that the sedimentary rocks form an 

 elongated belt surrounded by granites. The vigorous erosive 

 activity has laid bare the contact at numerous points and has 

 revealed clearly the intrusive nature of the plutonic rocks. 



Porphyritic modifications, and in one instance, a rude gneissic 

 structure, are occasionally developed as marginal facies. The 

 porphyritic modification. is best displayed at the upper Monarch 

 Lake. The granite, here a coarse quartzose microcline granite 

 of normal hypidiomorphic habit, is converted at the contact 

 into a closely crowded assemblage of microcline phenocrysts 

 embedded in a fine grained mesostasis consisting of quartz and 

 small flakes of biotite. A similar modification is found on the 

 western side of the belt in the White Chief cirque. 



The irruptive is accompanied by extensively ramifying sys- 

 tems of apophyses, which are, however, never very long. The 

 longest found was 20 feet thick and followed the strike of the 

 schists nearly a thousand feet. The larger veins, as a general 

 rule, cut across the schistosity at random, but the anastomosing 

 system of capillaries to which they give rise, tend to seek out 

 the planes of weakness. Their fineness, and acutely serpentine 

 character, argue a great degree of fluidity for the intrusive 

 magma. 



In addition to the thermal metamorphic influence of the 

 granite, it has at points been active dynamically also. West of 

 the Timber Gap, the irruptive has locally bent the schists from 

 N. 45° W. around to N. 25° E. and back again to the normal 

 strike. Extensive shattering of the strata, especially in the 

 Silver Lakes region, has taken place, but the results are compli- 

 cated by the internal movements which the belt has undergone. 



The granite at the contact frequently contains included large 

 numbers of angular fragments of schists. Occasionally they 



