ABSAROKITE-SHOSHONITE-BANAKITE SERIES. 
In the region of the Yellowstone National Park there are 
igneous rocks of a peculiar type which are associated with the 
normal andesites and basalts of the region, but which differ 
from them mineralogically and chemically, and deserves a special 
classification. The study of these rocks has been carried on in 
connection with that of all of the igneous rocks of this region, 
and the present paper is an abstract of a chapter prepared for 
the report of work done in the Yellowstone National Park by 
the division of the U. S. Geological Survey under the charge 
of Mr. Arnold Hague. 
The rocks mentioned occur in a number of separate localities 
within this region, where it is apparent that their generic 
relations are with normal basalts and andesites, and in each 
locality the varieties having the peculiarities in question are 
genetically related to one another by differentiation. But all of 
these peculiar varieties in the region are not closely related to 
one another, for they are separate offshoots from distinct reser- 
voirs of magma, and were probably produced by similar pro- 
cesses of differentiation. For purposes of systematic descrip- 
tion they may be classified together in a series having certain 
chemical and mineralogical characteristics. They thus form 
classes of similar rocks (7. ¢., like phases of differentiation), that 
belong to separate, but similar, families of rocks (7. e., groups 
or series of genetically, hence generically, related differentiation 
products). 
For the most part they are basaltic-looking rocks, occurring 
as lava flows and dikes, and less often as part of the basic vol- 
canic breccia which constitutes the major portion of the volcanic 
mountains of the Absaroka Range. They are quite subordinate 
* Published by permission of the Director of the U. S. Geological Survey. 
935 
