CRYSTALLIZATION OF IGNEOUS ROCKS. 
83 
than quartz and the plagioclases, while quartz has seldom, 
if ever, been found intergrown with the ferro-magnesian sili¬ 
cates in igneous rocks. 
Orthoclase is commonly interlaminated with albite and 
other plagioclases. 
Hornblende and the pyroxenes (hypersthene and augite) 
are frequently intergrown both in glassy and in holocrystal- 
line rocks. 
Hornblende and biotite are often intergrown in the crys¬ 
talline rocks. 
The intergrowth of the ferro-magnesian minerals with the 
feldspars is of very rare occurrence. 
The intergrowths of quartz and orthoclase with pegmatoid 
and granophyre structures is particularly common in the 
medium-grained acid rocks, where they are usually constitu¬ 
ents of the groundmass; but they also occur in isolated and 
porphyritical groups, as in the porphyries at Tryberg, in the 
Schwarzwald,* and the single occurrence at Eureka, Nev., al¬ 
ready alluded to. They also occur in very small groups scat¬ 
tered through the glassy groundmass of many rhyolites in 
the Western United States and in the obsidian of Obsidian 
Cliff, Yellowstone National Park.t In these instances they 
are nearly the oldest secretions from the molten magma. 
Hence there may be in some magmas an initial tendency to 
secrete crystals of two different minerals at the same time. 
(3.) As already remarked, the intergrowth or interpenetra¬ 
tion of two minerals indicates their synchronous crystalli¬ 
zation ; but parallel growths often occur where one mineral 
has acted as a nucleus for the other, and is evidently older. 
In the majority of cases there is a regular order of succession. 
Thus it is frequently observed that the basic plagioclases are 
surrounded by a parallel growth of more alkaline species, 
* George H. Williams. Inaugural Dissertation. “ Die Eruptivgesteine 
derGegend von Tryberg im Schwarz wald.” Stuttgart, 1883, p. 23. 
f J. P. Iddings. “Obsidian Cliff, Yellowstone National Park.” Sev¬ 
enth Ann. Rep’t U. S. Geol. Survey, 1888. 
