90 
IDDINGS. 
and frequently are ill-shaped, while rhyolitic obsidian oc¬ 
curs in great abundance and sometimes in large bodies. 
The andesitic glasses are generally found to be more sili¬ 
ceous than the normal andesites to which they are related. 
In the case of intruded rocks, which occur under similar 
geological conditions, that are about the same sized bodies 
and appear to have been intruded into rocks haying about 
the same temperatures, it is also observed that the basic ones 
are more coarsely crystalline than the acidic. 
(2.) The character of the structure also is intimately re¬ 
lated to the chemical composition of the magma. Thus the 
ophitic structure is peculiar to certain basic rocks, such as 
the diabases and the dolerites, and a somewhat similar 
structure is characteristic of highly feldspathic rocks like 
syenites and certain diorites, where the feldspars crystallized 
in advance of some of the other minerals, and are more 
nearly idiomorphic. The evenly granular or granitic struct¬ 
ure is more characteristic of the acid rocks, while a com¬ 
bination of these two extreme structures occurs in rocks of 
intermediate chemical composition. 
Relation of the Mineral Composition to the Chemical Compo¬ 
sition. —The relation between the mineral composition and 
the chemical composition of rocks is found to vary within 
certain limits, as already stated. The extent of this varia¬ 
tion must continue fora longtime to be the subject of careful 
investigation. 
(1.) The study of a great number of suites of rocks from 
a large part of this continent shows that this relation is less 
variable among rocks of similar geological occurrence. Thus, 
when the volcanic rocks of a large area are compared, it is 
observed that the basalts, which represent the basic end of 
the series, in ninety-nine cases out of a hundred, have a uni¬ 
form mineral composition. Over a very large portion of 
Western America they consist of lime-soda feldspars, augite, 
olivine, and iron oxides (magnetite or ilmenite). The less 
basic members of the series, the pyroxene-andesites , consist of 
