154 Naming Rocks. 
thoclase, nepheline and sodalite. It is a dark-colored 
felsitic rock in which biotite is an abundant mineral. 
It occurs chiefly in dykes and intrusive veins. 
Silica 50—65 p. ec. 
Phonolyte-Clinkstone—A hard, compact rock, which 
rings under the hammer. Gray, grayish-blue, to brown- 
ish. It turns white by weathering. It consists of the 
minerals orthoclase (sanidin), nepheline, hornblende 
and titanite. It is a nepheline-trachyte, and often con- 
tains large and well-defined crystals of amphibole. Zeo- 
lites often occur, filling cavities in the mass. Both san- 
idin and nepheline show very clear, and their presence 
admit of approximate indentification of the rock. The 
Cripple Creek District, Colo., is a noted locality of this 
rock, where, in structure, it is more or less slaty. 
Quartz-dioryte—Both a metamorphic and eruptive 
rock, consisting of quartz and plagioclase with horn- 
blende. It is a very tough grayish to greenish-white 
rock, rich in silica. Its texture varies from coarse to 
fine-grained, often porphyritic. Called also greenstone. 
G.=2.66—3. Silica 50—64 p. c. 
Quartz-porphyry—An eruptive grayish to greenish 
rock, consisting of minutely crystalline paste of quartz, 
oligoclase, hornblende, with large crystals of the same, 
and titanite. It occurs in large masses, having probably 
been ejected through fissures. It is tough and coarse in 
fracture. 
Dacyte (quartz-andesyte)—An eruptive, dull, 
grayish-green rock, compact but not hard, and consists 
of feldspar, hornblende, quartz, small crystals of oligo- 
clase, sanidin and magnetite. ~ 
Silica 65—70 p.c. Often graduates into orthoclase 
rocks. 
