ANKALS NEW YORE ACADEMY OF SCIENCES 



Per cent 



Anorthite 83 



Grossularite 1.35 



AlmaDdite 1.49 



Hornblende 2.83 



Biotite 1.17 



Hematite 1.28 



101.27 



Diorite Phase 



From this highly acidic type, the Bavenswood rock varies through inter- 

 mediate stages to a quartz-diorite. It is then darker gray in color, 

 medium in grain and slightly gneissic in structure. The mass of the rock 

 is made up of oligoclase, green hornblende, brown biotite and garnet. 

 Under the microscope, orthoclase, augite, zircon, titanite, apatite, pyrrho- 

 tite, pyrite, magnetite and kaolin were also identified. Here, as in the 

 more acid type, the rock is granitoid with the component grains irregular 

 in outline and interlocking. In size, they average about .8 to 1 mm. in 

 diameter. 



Oligoclase is the predominent feldspar. Polysynthetic twinning is 

 shown, sections parallel to which have extinction angles of about seven 

 degrees. Pericline twinning and Carlsbad twinning also occur, and in 

 several cases all three are in combination. Strong wavy extinction is 

 shown. Inclusions of hornblende, biotite, apatite, zircon and pyrrhotite 

 are common. 



The hornblende and the augite are of the same type as described in the 

 granite phase and show the same alteration. The alteration of the augite 

 is especially well shown. This takes place from the edges of the grains 

 toward the center, but in some cases the change has taken place through- 

 out the entire grain, the surface of which is spotted and blotchy from the 

 formation of hornblende. 



A chemical analysis of the typical diorite rock gave the following 

 result : 



Per cent 



Si0 2 64.98 



A1 2 3 20.51 



Fe 2 3 50 



FeO 2.35 



MgO 93 



CaO 4.23 



K 2 1.84 



Na 2 5.22 



H 2 n 



100.67 



