24 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



lock. Occasionally small, ragged flakes of hornblende or 

 small patches of chlorite occur. The minerals of the 

 ground-mass, in those slides showing the most pronounced 

 porphyritic structure, have a diameter of from .03 mm. to 

 .1 mm. As the size of the grains increases the rock 

 assumes the structure of diorite-porphyrite, with occasional, 

 though rare, porphyritic quartzes. All gradations were 

 found between porphyrite and diorite. 



The quartz varies considerably in amount in the porphy- 

 ritic rocks. Usually it is rather subordinate, but occasionally 

 it is quite abundant, forming perhaps one-fourth of the 

 minerals of the slide. These rocks, however, are not 

 common, and are doubtless only local developments. One 

 slide of the diorite-porphyrite is remarkable for the manner 

 in which the quartz is developed with respect to the other 

 minerals. This rock (from the slopes to the west of the 

 entrance to Silver Canon) contains abundant phenocrysts 

 of feldspar, with porphyritically developed quartz. Under 

 crossed nicols the quartz appears as scattered and more or 

 less rounded grains. Occasionally several of these are 

 found near together, showing similar polarization colors and 

 a common extinction. On revolving the stage it is seen that 

 the rounded borders do not mark the limit of the sections 

 of quartz; for the extinction of the mineral shows that it 

 has an outer zone which has a pronounced micropoikilitic 

 structure, being closely packed with finely polarizing feld- 

 spars. The quartz occasionally shows a crystal form. The 

 ground-mass of the rock is coarsely crystalline and consists 

 largely of micropoikilitic quartz similar to the larger sections, 

 but without clear centers. A ver}^ few of the larger sections 

 also are wholly filled with feldspar aggregates. The clear 

 centers together with the micropoikilitic margins indicate 

 arrested development of the quartz, which began to form 

 before the growth of the minute feldspars, the latter form- 

 ing before the final crystallization of the quartz. The rock 

 is much altered, and except in one or two instances the 

 traces of twinning in the feldspar phenocrysts are wholly 



