84 
IDDINGS. 
and plagioclase is sometimes surrounded by orthoclase, 
This order is seldom, if ever, reversed. 
Hypersthene is generally the older when it is associated 
with augite or hornblende in parallel growths. 
Augite is generally older than hornblende in the diorites. 
and acts as the nucleus. It is quite frequently inclosed by 
hornblende in the andesites, but it is oftener the younger 
mineral in the glassy rocks. 
(b.) Order of Crystallization of the Different Minerals. 
(1.) Where there has been but one series of crystalliza¬ 
tions—that is, where each species of mineral belongs to one 
uninterrupted act of crystallization—the phenomena are pre¬ 
sented in their simplest form. In this case it is generally 
observed that the oldest minerals are the iron ores, with 
zircon and apatite, followed in turn by one or more of the 
ferro-magnesian silicates, and the feldspars, with a feldspar 
and quartz as the last to crystallize. But when a number 
of different rocks are studied the order is not found to be 
the same in all of them; however, it is generally constant 
in all rocks of one kind. 
In certain diorites the order is: iron ores, zircon and apa¬ 
tite, hypersthene, augite, labradorite, hornblende, biotite, 
oligoclase, and lastly orthoclase with quartz. 
In many granites biotite precedes hornblende. In most 
diabases, and in many gabbros, the feldspars (anorthite or 
labradorite) crystallize before the augite and diallage. When 
olivine occurs in these rocks it is almost universally older 
than the other ferro-magnesian silicates. 
Apatite is generally considered one of the oldest, if not the 
oldest, mineral crystallized in igneous rocks; but in an 
olivine-leucite-phonolite from Ishawooa Canon, Wyoming 
Territory,* this does not appear to be the case. The rock is 
very rich in porphyritical crystals of olivine and augite in a 
* Arnold Hague. “ Notes on the occurrence of a new leucite rock, from 
Ishawooa Canon, Absaroka Range, Wyoming Territory.” To appear in the 
Am. Journ, Sci., vol. xxxviii, Sept., 1889. 
