1 6 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 30 Ser. 



The structure in this case is micropoikilitic. Occasionally 

 sections contain numerous brightly polarizing, microscopic 

 needles, doubtless of hornblende. 



Besides the decomposition products already mentioned, 

 more or less epidote is usually present, generally in small 

 irregular patches. 



All the diorites have doubtless been subject to stresses 

 since they were consolidated, but only the rocks from Silver 

 Canon give any marked microscopic evidence of it. In 

 these rocks the feldspars seem to have been particularly 

 affected, the hornblende and quartz showing little or no evi- 

 dence of strain. The evidence here is of three kinds — 

 altered optical properties, bent crystals and fractures, which 

 ma}^ or may not cause displacements. The extinction of 

 the feldspars is frequently very indefinite and variable. 

 Comparatively few of them show good extinction, and even 

 these are sometimes considerably affected by cracks. In 

 the others strain shadows take the place of the normal 

 extinction. Bent crystals are not common, but a few of 

 the sections show a distinct curvature, and a corresponding 

 alteration in extinction. The fractures referred to are not 

 the cracks frequently found in single individuals, but more 

 extensive ones passing from crystal to crystal, simply as 

 cracks, or forming veins which have been filled with sec- 

 ondary matter. A few veins were found, the most pro- 

 nounced one extending irregularly half through a slide, 

 some of the feldspars on either side having suffered slight 

 displacement. This vein is .05 mm. in width, and is 

 filled principally with calcite, with more or less chlorite and 

 quartz. 



From the extinction angles on either side of the albite 

 lamellce, in favorable sections, the plagioclase appears to lie 

 between a basic oligoclase and an acid labradorite. 



From its relations to the quartz and feldspars, the horn- 

 blende seems to have been the first mineral in the order of 

 the crystallization of the essential constituents of the diorite. 

 The relations of quartz and feldspar show that a part of the 

 feldspar was formed before, and part at the same time with 



