THE ORIGIN OE IGNEOUS ROCKS. 
135 
CHEMICAL EVIDENCE OF CONSANGUINITY. 
The consanguinity of the igneous rocks of one district is 
also proven by their chemical characteristics. Here also the 
same line of argument applies as that followed in discussing 
their mineralogical characters. 
A study of the rocks of one group, like that of Electric 
peak and Sepulchre mountain * or that of Crandall basin,f 
convinces one of the gradual transition in chemical com¬ 
position throughout the series of rocks, as well as of the 
variability of the molecular proportions. The chemical in¬ 
dividuality of the group of eruptive rocks in the region of 
Christiania has been pointed out by Brogger in the work 
already cited in the historical part of this paper. 
At Electric peak and Sepulchre mountain, as already 
described in another place, there is a series of rocks which 
pass by gradual transitions from basic diorite to quartz- 
mica-diorite and granite, and by gradations of crystalliza¬ 
tion into porphyrites of variable composition; and, finally, 
into andesites, basalt, and dacite. In the region of Crandall 
basin the series of rocks ranges from gabbro to diorite, quartz- 
mica-diorite, and aplite, and into porphyrites corresponding 
to these,- and also into basalts, andesites, and unusual rocks 
of the lampropliyric kinds, with their feldspathic comple¬ 
ments, which also occur in a somewhat different form as lava 
flows of alkali-feldspar-basalts, leucite-basalts, and trachytic 
tuffs. The petrography of these rocks will be found in a 
future publication of the U. S. Geological Survey. 
The granular rocks of Crandall basin and the volcanic 
rocks associated with them form a natural geological group. 
The unusual lampropliyric rocks and the corresponding sur¬ 
face flows are partly connected with this center of eruption 
*Iddings (J. P.) The eruptive rocks of Electric peak and Sepulchre 
mountain, Yellowstone National Park. Twelfth Annual Report U. S. 
Geol. Surv. 8°. Washington, 1892, pp. 569-664; also Bull. Phil. Soc. 
Washington. 8°. Washington, vol. 11, pp. 191-220. 
t Iddings (J. P.) The dissected volcano of Crandall basin, to appear as 
a publication of the U. S. Geol. Surv. 
