PUBLICA TIONS. 851 
carry phenocrysts of olivine and augite, but none of feldspar. The 
groundmass is extremely fine grained and is in part obscured by altera- 
tion products. It is basaltic to a great extent, but contains orthoclase 
in microscopic crystals, or else shows upon analysis a relatively high 
content of alkalies. Mica is also a prominent constituent in some 
instances. Professor Merrill’s remark upon the mutual interference of 
the phenocrysts of augite and olivine, namely, that it “can be accounted 
for only on the supposition that neither mineral is a direct secretion 
from the magma, but that they are residuals of an earlier crystallization 
in which consolidation had proceeded so far that free growth was no 
longer possible,” appears to the reviewer to be greatly in error. One 
need only mention the pegmatitic intergrowth of phenocrysts of quartz 
and orthoclase in certain obsidians and pumices, and the mutual pene- 
tration of pyroxene and hornblende in phenocrysts in some glassy 
andesites. Chemically the rocks belong with lamprophyres, and 
resemble some rocks found in the Absaraka Range, in the Yellow- 
stone National Park. 
The porphyrite-like rocks carry phenocrysts of plagioclase in addi- 
tion to those of augite and olivine, and have a groundmass in which 
orthoclase occurs in connection with plagioclase. The syenitic rocks 
are closely associated with the lamprophyric ones: and in one case the 
chemical composition of the syenite is very similar to that of the 
sodalite-syenite of Square Butte, Highwood Mountains, Montana, as 
pointed out by Professor Merrill. ‘The reviewer hopes to be able to 
present shortly in this JoURNAL an account of the closely related series 
of rocks occurring in the neighboring region of the Yellowstone Park, 
to which Professor Merrill has referred in his article. 
Vodeo lle 
Highwood Mountains of Montana. By Wa.LTER H. WEED and 
Louis V. Pirsson. Bull. Geol. Soc. Am., Vol. 6, pp. 389- 
422. Pls. 24-26. Rochester, April 1895. 
The situation and topographic features of the Highwood Mountains 
are briefly described, and the geologic structure of the district is 
pointed out. The mountains consist of the denuded remains of vol- 
canoes whose rocks show extreme differentiation of a highly alkaline 
magma. There are several volcanic cores now filled with massive 
granular rock. These are surrounded by tuffs and volcanic breccias 
with lava-flows and a great number of radiating dikes. 
