Naming Rocks. 155 
Dioryte—An eruptive, coarse to fine-grained, com- 
pact rock; like syenyte in structure; color, light-gray 
and green to dark greenish black. Its typical constitu- 
ents are: Orthoclase, feldspar, oligoclase and horn- 
blende, with sometimes labradorite, apatite, magnetite 
and pyrite. When containing quartz, it is known as 
quartz-dioryte. The hornblende is easily recognized in 
the form of small needles, and the feldspar is more often 
flesh-colored than white. It occurs chiefly in wide dykes 
and fissures. G.=2.66—3. Silica 50—64 p. c. 
Andesyte—An eruptive, bard, compact rock, con- 
sisting of the minerals, andesvte, crystals of augite or 
hornblende, biotite and magnetite, with labradorite as _ 
the chief feldspar. Augite-andesyte is a dark-gray near- 
ly black rock, with dark-colored crystals of augite. 
Hornblende-andesyte is a pale-gray, compact rock, 
where the dark-green hornblende crystals occur in small 
columnar forms. Occasionally the hornblende appears 
surrounded by pale-green stains, indicating its alteration 
into chlorite. Andesytes are of wide occurence. They 
have been poured out from dykes and fissures. G.=2.6— 
2.7. Silica 59—63 p. c. Has more or less the aspects 
of trachyte. 
Gabbro—An eruptive rock, occurring in intrusive 
dykes and sheets. Its color varies from dark-gray, 
blackish to brown, rusty red, and sometimes bright 
spangles of mica. Its chief mineral constituents are: 
Labradorite, diallage or pyroxene. Magnetite and mica 
are accessory minerals. The kind known as olivine- 
gabbro contains the mineral olivine as a constituent. 
Hypersthenite is a closely-related rock. Gabbro is wide- 
ly distributed and often is associated with serpentine. 
Granite-like in texture. G.=2.7—3.1. 
