6 CALIFORNIA ACADEMY OF SCIENCES. 



rock with more biotite and less hornblende than the pre- 

 ceding. Fairly but not quite fresh. 



A separation of this rock by Thoulet's solution gave the 

 following result, size of grain and quantity being the same 

 as before : 



18%' biotite and hornblende; 66% fell at 2.64-66, con- 

 sisting of quartz and plagioclase, sp. gr. of the latter being 

 nearly equal that of quartz. This portion was not further 

 separated. 16% fell at 2.55-57, consisting of orthoclase. 

 This portion was tested with silico-hydrofluoric acid and 

 gave abundant Ka, no Na reaction. The numbers of per- 

 centage are of course only approximate. Here, then, ortho- 

 clase is present, but not in very large quantity; there is 

 certainly a very large excess of plagioclase, and this rock 

 should also be classified as a quartz-mica diorite. 



5. A specimen, taken from the foot of the supposed fault- 

 scarp, about one-third mile west of Real del Castillo (San 

 Rafael Valley) and near a slate contact, differs considerably 

 from those already described. It is a rather fine-grained, 

 granitic rock of whitish color and containing a few biotite 

 foils. Under the microscope it is seen to consist of some 

 partly idiomorphic plagioclase crystals, a few greenish- 

 brown biotite foils, very little muscovite and a predomina- 

 ting allotriomorphic mass of orthoclase and quartz, oftener 

 showing a tendency to granophyric structure. The feld- 

 spars contain, in consequence of an incipient decomposi- 

 tion, a considerable amount of muscovite. The specimen 

 may be characterized as a granitite. 



The chief interest, however, centers in the marked me- 

 chanical effects of pressure which this section shows. 

 Among the quartz grains undulous extinction is quite com- 

 mon, and many of them are even broken and shattered. The 

 feldspar grains show still more markedly undulous and ir- 

 regular extinction and also frequently peripheric crushing 

 (Randliche Kataklase). Thus, the internal structure of the 



