74 New York State Museum 



word indicates the composition. In general a gneiss is a 

 metamorphic rock composed of feldspar with other min- 

 erals, and having a certain texture. The color of gneisses 

 depends upon the color of the quartz and feldspar and the 

 relative amount of dark minerals present. They vary 

 from almost white through light shades of red and gray 

 into darker shades, to browns and greens and even black. 

 Mica-schist is related to gneiss on the one hand and 

 quartzite on the other. Gneiss often grades into it. Mica- 

 schist is the most widely distributed and important of the 

 schists. The essential minerals are quartz and mica. 

 Different varieties of mica occur. Dark or biotite mica is 

 common but the silvery muscovite mica is most common. 

 The micas occur in irregular leaves with their cleavage 

 planes in line with the cleavage or schistosity of the rock, 

 giving the rock a very fissile character. The sheen of 

 such schists is due to the high luster of the mica surfaces. 

 In color they vary from very light through grays, yel- 

 lows or browns, and may even become very dark if there 

 is much dark mica present. Other schists are known as 

 talc-schist, hornblende schist etc. Quartzite and slate 

 have been discussed under sedimentary rocks (page 34). 

 Quartzite is normally white or light-gray or yellowish to 

 brown in color, but green, blue, purple, black etc. colors 

 may be developed through included material that acts as 

 pigment. The colors of slate are chiefly gray to dark- 

 gray or black, depending upon the amount of carbonace- 

 ous material present. They may, however, show green 

 colors and the presence of iron oxides gives red, purple, 

 yellow or brown colors. The name phyllite ("leaf 

 stone") has been given to a group of rocks closely con- 

 nected with slates. These rocks split into exceedingly 

 thin sheets and differ from ordinary slates in containing 



