^74 
THE ORIGIN OF IGNEOUS ROCKS. 125 
to the law of Gouy and Chaperon, be more abundant in the 
lower than in the upper portion of a magma basin.” 
“ Differences of temperature operating according to Soret’s 
principle will cause the same • molecules to accumulate in 
the colder portions of the same magma basin.” 
Another possible cause of concentration considered by 
Vogt is magnetic attraction, which may act after concentra¬ 
tion has been started by some other force. 
In a paper by the present writer on the eruptive rocks of 
Electric peak and Sepulchre mountain, which was prepared 
for the Annual Report of the United States Geological Survey 
for the year ending with June, 1891,* the opinion is expressed 
that “ the chemical differences of igneous rocks are the result 
of a chemical differentiation of a general magma, and in a 
very special manner all of the igneous rocks of any locality 
are so intimately related to one another chemically that there 
is far more reason for considering them as a complex chemi¬ 
cal unit than as a number of independent well defined 
magmas.” 
In the Report on the Geology of the Eureka District, Ne¬ 
vada, by Arnold* Hague, now in press and about to appear 
as a monograph of the United States Geological Survey, in 
chapter VIII, on the volcanic rocks, Hague reaches the 
conclusion that all the volcanic rocks of that district have 
been derived from one general magma by a process of dif¬ 
ferentiation. 
In a paper “ On the Plutonic Rocks of Garabal Hill and 
Meall Braec,” by J. R. Dakyns and J. J. H. Teall, which has 
just been published t and which reached the writer after the 
present paper had been read before the Society, May 7th, the 
investigators describe a series of granular rocks that form a 
connected group of intrusions ranging from ultra basic to 
• * Iddings (J. P.) The Eruptive Rocks of Electric Peak and Sepulchre 
Mountain, Yellowstone National Park. Twelfth Annual Report of the 
U. S. Geol. Surv. 8°. Washington, 1892, pp. 569-664. 
t Dakyns (J. R.) and Teall (J. J. H.) Quart. Journ. Geol. Soc. 8°. 
London, 1892, May 2, vol. 48, part 2, No. 190, pp. 104-120. 
