318 



University of California Publications. 



[Geology 



gates of such grains. There are some dusty inclusions in the 

 quartz, but no liquid inclusions could be detected. 



The modification of the monzonite just described is clearly 

 a contact phenomenon, but it can not be ascribed to the direct 

 influence of the porphyry magma upon the underlying rock. It 

 is with little doubt due rather to a subsequent action in which 

 the chief reagents concerned were derived from the porphyry 

 by a process of downward leaching, though why all the feldspars 

 of the main body of the rock should have been destroyed and only 

 the porphyritic orthoelases left intact, remains a mystery. 



Petro graphical Characters. — The porphyry in general has 

 suffered so much alteration since its solidification that it has 

 proved difficult to find material upon which to make a study 

 of the rock in its original condition. At a few localities, how- 

 ever, specimens were obtained which being less decomposed than 

 the rest of the mass have contributed some information on this 

 point. They are not, however, uniform in character and doubt 

 still remains as to the prevailing petrographical features of the 

 rock prior to its alteration. Some of these will now be briefly 

 described. 



At the head of Lane Valley, on the south side, a tunnel has 

 been run into a mass of the porphyry where it emerges from 

 beneath a limestone capping. Here the porphyry has a bluish, 

 light gray color and may be seen in hand specimens to consist 

 of a dense matrix in which are imbedded numerous small pheno- 

 crysts of dull feldspar up to 3 mm. in length and fewer large 

 phenocrysts of another feldspar of fresh glassy appearance up 

 to 12X6 mm. in size. The rock is shot through with pyrite 

 mostly in disseminated grains but also in nests and small veins. 

 The rock effervesces freely in acid. 



In thin section the ground mass is a microgranitic aggregate 

 of untwinned feldspar with a subordinate amount of quartz 

 and a great deal of secondary calcite. In this are imbedded 

 idiomorphic feldspars of small dimensions thoroughly altered 

 to an aggregate of decomposition products and also much larger 

 phenocrysts of fresh orthoclase containing shreds of calcite, some 

 phenocrysts of bleached biotite and grains of pyrite with blurred 

 outlines. None of the pyrite is enclosed in the feldspar pheno- 



