152 Naming Rocks. 
absent. G. = 2.6—2.7. Silica 70—80 p.c. It contains 
no mica. 
Greisen is a foliated soft granite with little or no 
feldspar. It is a schistose micaceous quartz rock. 
Granite-porphyry is a granite in which, through 
some influence not understood, large, isolated crystals 
separate themselves from the mass of felsite, which 
remains in a pasty magma, Composition, felsite, quartz, 
feldspar, mica or chlorite. 
Quartz-porphyry—Composition same as above, with 
no mica or chlorite, it is a hard, compact rock of various 
colors, generally gray to brown. . 
Felsyte—A many-colored rock, from gray to bluish 
to brown and red; compact and very hard. It is a rapidly 
cooled granite paste, containing crystals of quartz and 
feldspar. 
Pitchstone—A dark, glassy rock, with numbers of 
small crystals of glassy feldspar, and sometimes crys- - 
tals of sanidin, quartz and mica. It is a glassy feldsite 
which has cooled rapidly. 
Rhyolyte, or quartz trachyte—A compact feldsite 
with glass in the base, and. quartz crystals and occa- - 
sionally mica. Rhyolytes are of many colors and text- 
ures, breaking with a rough fracture. They frequently 
exhibit wavy lines of structure (fluidal), caused by the 
arrangement in lines or layers of mineral aggregates or 
colored obsidian. They contain a large amount of 
quartz, which has resulted from the excess of silica not 
. required to complete the feldspar crystals. 
Pearlyte is a rapidly cooled rhyolyte. 
