HOOSAC MOUNTAIN. 61 



they lie in planes roughly parallel to the schistosity of the ruck, but their 

 crystallographic directions have do such relation. 



Some varieties of the rock at the shaft arc filled with red garnets in 

 dodecahedral crystals. 



The surface rock has the same characters, but with certain variations 

 due in part to weathering. The shiny black variety is found here and there, 

 but the rock is commonly greenish, indicating a certain amount of chlorite; 

 it varies from light to dark green. Garnets are sometimes contained in the 

 rock, especially at the base, where a gametiferous horizon occurs. Feldspar 



i 





Fio. 22.— Albite schist (B i - -t Don , Central shaft. One-sixth natural size. 



Here the quartz lenses are more irregular and thicker: the lit t !•■ white specks dotting tin 



of albite. 



is often present with the garnet. These schists are identical in every detail 

 with the schists of Mount Greylock. 



The porphyritic albites are prominent in the slide. Simple twins are 

 common, but polysynthetic twins rare. Single crystals are common. They 

 have a rounded lenticular or flat shape. The groundmass outside the feld- 

 spar is composed of muscovite and biotite, or muscovite alone, chlorite, 

 grains and aggregate lenses of quartz, magnetite in octahedra or grains, 

 apatite, tourmaline, and rutile. ( Htrelite is found in some localities The 



