40 The American Naturalist. (January, q 
MINERALOGY AND PETROGRAPHY:' 
The Origin and Classification of Igneous Rocks.—Mr. 
Iddings’ has recently published at length the data upon which are 
based his conclusions concerning the causes of the different structures 
exhibited by the igneous rocks of Electric Peak and Sepulchre Moun- 
tain and of their varied mineral composition. The main results 
reached by this study have already been noticed in these pages? It 
may be well again to call attention to the fact that in this region the — 
different conditions attending the final consolidation of the ejected and — 
of the intruded magmas affected not only their crystalline structure, 4 
but also their essential mineral composition; consequently, the mole- 
cules in a chemically homogenous fluid magma combine in various 
ways and form quite different associations of silicate minerals, produc- 
ing mineralogically different rocks. For instance, biotite is an essen- 
tial constituent of even the most basic of the intrusive rocks, while in 
the effusive phases it is rarely found in rocks containing less than 61% 
of SiO, Again, quartz is common in the coarser grained varieties of — 
the former and is absent from those of the latter. Therefore, it is more 
proper to consider intrusive and effusive rocks that havea like chemical — 
composition as corresponding or equivalent rocks, than those forms of 
the two series that have similar mineral compositions. The classifica- 
tion of igneous rocks should recognize the close dependence of struc- n 
ture and mineralogical composition upon geological relations. But, 
since the structure is the best exponent of these relations, structure ; 
should form the basis of this classification. Though giving most 0 
his attention to the general subject of the relation existing between the — 
structure and the geological position of the rocks of the area described, — 
the author devotes a portion of his article to illustrating the inter — 
growths of hypersthene, pyroxene and hornblende that occur so plen- — 
tifully developed in the rocks of the region.——In a second paper the 
same author‘ attacks the great problem of the origin of igneous rocks. : 
He introduces the subject by outlining the growth of the theory first 
enunciated by Scrope, that the varieties of igneous rocks are the result — 
‘Edited by Dr. W. S. Bayley, Colby University, Waterville, Maine. 
*Twelfth Ann. Rep. Di U.S. 
3Cf. AMERICAN NATURALIST, April, 1890, 
360. ie 
‘The Origin of Igneous Rocks. Bull. Philos. Soc. Wash., xii, 1892, p- 89. se 
Geol. Survey, Washington, 1892, p- 569- 
