STRATIGRAPHIC GEOLOGY. 
DEFINITIONS OF IGNEOUS ROCKS. 
The various igneous rocks which occur in the area under discussion 
are simply defined below for the benefit of the n nical reader. 
Alaskite is a granular rock of white or light-gray eoloi . It is com 
posed almost entirely of quartz and orthoclase. It is a granite luck- 
ing mica and hornblende. 
Andesite is a porphyritic rock; that is, it has visible crystals 
(phenocrysts) embedded in a finer-grained, often glassy groundmass. 
The most conspicuous features are the phenocrysts of hornblende or 
biotite or augite and plagioclase. 
Aplite is here applied to a finely granular rock which usually oc- 
curs in dikes cutting, for example, a granite. The i plite, which as a 
rule is more siliceous than the granite, in many instances has solidified 
from a portion of the granite mass that is still liquid at great depth.-. 
Basalt is a black or dark-gray rock which is often porous through 
the presence of steam holes. Crystals of olivine, plagioclase, and 
pyroxene may or may not be visible in the dense or finely granular 
groundmass. 
Dacite may be briefly defined as a quartz-bearing andesite. The 
more basic dacites closely resemble andesites, while siliceous dacites 
are closely connected by transition rocks to rhyolite and might be con- 
fused with rhyolite were it not for the presence of plagioclase crys- 
tals. 
Diorite is a gray granular rock which is composed essentially of 
plagioclase and hornblende, while biotite or augite may also be pres- 
ent. Diorite and andesite are similar chemically, but differ textur- 
ally. 
Diorite porphyry is a rock of the composition of diorite with abun- 
dant crystals of plagioclase. hornblende, and biotite. The ground- 
mass is finely granular. 
Granite is a granular rock of pink or gray color. It is composed of 
orthoclase, quartz, and either muscovite, biotite. or hornblende. 
Granite porphyry is a rock of porphyritic habit having the same 
composition as granite. Quartz and orthoclase are the more common 
phenocrysts, and these are embedded in a finely granular groundmass. 
Latite is a porphvritic rock with more or less glassy groundmass. 
Phenocrysts of orthoclase and plagioclase are present in equal 
amounts, while either hornblende, augite, biotite, or olivine is usually 
also present. Chemically, latite is the equivalent of monzonite. 
Monzonite is a granular rock resembling both granite and diorite, 
but containing orthoclase and plagioclase in approximately equi 
* Orthoclase (alkali feldspar) is unstriated ; plagioclase (soda-lime feldspar) is stri 
ated and when light strikes it alternate tiny parallel bands reflect the lighl simul 
taneously. 
