238 



University of California Publications. 



| Geology 



Throughout the hand-specimen can be observed small dark- 

 colored patches, approximately half an inch square, which are 

 pepper-and-salt aggregates of ferromagnesian minerals and 

 white. In thin section, the patches consist of panidiomorphic 

 equidimensioned aggregates of the above minerals with the dark 

 equaling the white. It is noted, however, that the triclinic 

 feldspars have become relatively more abundant, and that a 

 notable concentration of apatite has taken place. Each little 

 patch is apparently an integral unit which does not blend into 

 the enclosing rock, though the line of demarcation is not abso- 

 lutely precise, and shows sympathy with the grain of the host. 



At seventeen feet from the contact, the granite does not differ 

 essentially from that at but one foot, except, perhaps, in having 

 recovered completely its appearance of homogeneity. 



From Cliff Creek was obtained a dark medium grained gran- 

 ular inclusion, throughout which a few porphyritic feldspars 

 are scattered. The host, which was not in any near proximity 

 In tin' contact, is a hornblende biotite granite, conspicuous by 

 the porphyritic development of the hornblende and the macro- 

 scopic, character of the titanite. Under the microscope the inclu- 

 sion appears to consist of biotite and green hornblende in equal 

 proportions, oligoclase and the accessories. Quartz is insignifi- 

 cant or absent. The feldspar is characterized by having a core, 

 or more frequently, a zone of sharp crystallographic habit con- 

 sisting of kaolinized feldspar succeeded by a peripheral growth 

 without exterior crystal outline. By far the most striking fea- 

 ture revealed in thin section is the extraordinary concentration 

 of the accessories. Sphene is very abundant in sharp idiomorphic 

 rhombs. In one instance an included cube of magnetite was 

 noted, and in another, a small striated feldspar lath. A horn- 

 blende cross-section was found in parallel intergrowth with a 

 titanite rhomb along their respective cleavage lines. Apatite 

 displays a profuse development of long slender needles and short 

 stout prisms. It is sometimes included in the sphene, and may 

 occasionally be seen jutting into the titanite rhombs. Octahedra 

 of magnetite are unusually plentiful. Zircon, however, is pres- 



