168 THE JOURNAL OF GEOLOGY. 
Andes, which was @ priori not to be expected, since their solidi-_ 
fied differentiation products (the lavas) so much resemble each 
other when examined individually by the usual petrographical 
methods. From this point of view such a comparison as has 
been here attempted is of special interest as going to show that 
the various bodies of magma beneath the earth’s crust which 
supply the different volcanic centers are not identical,—parts of 
only one large interior mass,—as some have maintained who 
base their opinion on the superficial resemblance; though future 
research may show that these separate bodies of magma are them- 
selves differentiation products of one original magma mass. But 
only a beginning has been made in the investigation of the differ- 
entiation of rock magmas, and, with Professor Iddings, I must 
put in a plea for fuller sets of analyses of the eruptive rocks from 
the various volcanic centers of the globe, as only such complete 
sets can furnish the data requisite for solving some of the prob- 
lems that confront us. 
Henry S. WASHINGTON. 
