92 
IDDINGS. 
small portion of the whole number of occurrences. They 
appear to owe their origin to causes that are not ordinarily 
in control of the crystallization of these rocks, some of which 
have been discussed by the writer elsewhere.* 
Relation of the Mineral Composition to the Geological Occur¬ 
rence. —The mineral character of rocks of the same chemical 
composition is found to vary greatly with their geological 
occurrence and their mode of crystallization. The variation 
in the mineral composition of rocks derived from the same 
chemical magma and its relation to their geological sur¬ 
roundings is a fact to which sufficient attention has not yet 
been given. It is one of the greatest importance for the 
proper correlation and classification of rocks. 
It is strikingly well illustrated by certain intrusive rocks 
at several localities in the Yellowstone National Park, which 
are the equivalents of the surface lavas. The study of these 
rocks shows that the mineral composition of the coarse¬ 
grained forms of the different magmas is quite different from 
that of the corresponding lava flows, the most noticeable dif¬ 
ference being in the development of biotite and quartz in the 
coarse-grained varieties of those magmas, which, as glassy 
rocks, take the form of pyroxene-andesite or of hornblende- 
pyroxene-andesite. 
The study also shows that a regular mineralogical varia¬ 
tion among rocks of similar grain is found to accompany 
chemical variations. The regularity of these variations, 
however, is modified by the fact that the geological occur¬ 
rence of a magma and the history of its physical experiences 
is different for different parts of one magma. This renders 
the discovery of the laws of rock crystallization the more dif¬ 
ficult, and makes clear the necessity of an extremely close 
geological study of all such occurrences. 
*Am. Journ. Sci., vol. xxxiii, Jan., 1887; ibid., vol. xxxvi, Sept., 1888; 
Seventh Ann. Eeport U. S. Geol. Survey, 1888; also Bull. IT. S. Geol. Sur¬ 
vey, “ On a group of igneous rocks from the Tewan Mountains, New Mex¬ 
ico.” [Not yet issued.] 
