360 The American Naturalist. [April, 
General Notes. 
MINERALOGY AND PETROGRAPHY.! 
Petrographical News.—The granite bosses of Morbihan, France, 
have suffered on their peripheries certain modifications which are 
ascribed by Barrois? to the rate of cooling. These modifications are 
endomorphous contact effects, but are in no way dependent upon 
the nature of the surrounding rocks. ‘Two cases are recognized, ac- 
cording as the boundary lines of the bosses correspond with the strike 
of the enclosing strata or are perpendicular to it. In the first case, the 
granite, which is a muscovite-biotite rock, possesses on its periphery a 
zone of granite porphyry, withits phenocrysts arranged in fluidal lines. 
In the second case, the exterior modification is a fine-grained panidio- 
morphic aplite. Since the aplite and the porphyry both contain their 
constituents in idiomorphic grains, the author concludes that the crys- 
tallization of the magma yielding these and the granite has gone on 
progressively, the porphyritic rocks representing an intermediate stage 
in the formation of a granite from a magma. Schistose granites 
(gneisses) on the peripheries of these same bosses are aplites and 
porphyries that have been crushed by mechanical forces and then re- 
cemented by the deposition of secondary quartz. Since the gneisses 
are found only on the south sides of the bosses, the pressure to whose 
existence they are due is supposed to have come from that direction. 
Mr. Iddings* has continued‘ his study of the cause of different 
structures in rocks produced from the same magma, and has published 
some of the results of his investigations on the igneous rocks of the 
Yellowstone Park. This study is concerned principally with the chem- 
ical relation of different rocks produced by the cooling of a single 
molten magma under different conditions, Electric Peak is a neck of 
diorite cut by numerous dykes of porphyrite. Separated from this by 
a great fault is Sepulchre Mountain, made up in large part of surface 
flows of the magma that was extruded through the orifice at Electric 
Peak. This magma under the conditions surrounding flows formed 

1 Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 
P $ 
3 Bull. Philos. Soc. of Wash., XI., 
4 AMERICAN NATURALIST, Dec., P > 1216, and Aug., 1889, p. 718. 
