No. 398.] REVIEWS OF RECENT LITERATURE. 157 
ranging from archzan to cretaceous, and is confined within ancient 
orogenic barriers on the north and south. Evidence of erosion 
during the piling up of the eruptive material is shown by canyon 
cross-sections, which give evidence of old depressions filled with 
silts and gravels; this indicates that considerable erosion took place 
at different times, with long interruptions in the local volcanic 
activity. Over one hundred and fifty species of plants have been 
identified from the tuffs and breccias, indicating a range in age from 
eocene to upper miocene. They are thus geologically older than 
the neocene rhyolites that form the greater portion of the Yellow- 
stone Park. 
The intrusive bodies occur as stocks and dikes ; an earlier group, 
the * Sunlight ” intrusives, is characterized by orthoclase and augite, 
and cuts the earlier breccias and basalts. The “Ishawooa” intru- 
sives have broken into the later basalts as well as the earlier and 
are more siliceous rocks, ranging from diorite and diorite porphyry 
to true granitic types. Dikes are associated with all the larger 
bodies, sometimes being offshoots from them, in other cases cutting 
them or cut by them, and presenting a marvelous variety of struc- 
tural and lithological types that afford material for the study of 
extensive gradation between coarse crystalline and glassy volcanic 
rocks. Many of the dikes of the Sunlight group are orthoclase 
basalts, collectively called by the name “absarokite,” and are 
related to coarser monzonite stocks, which range in composition 
from quartzose augite syenites to coarse gabbros and diorites. 
These older intrusive bodies occur in three principal masses that 
form the points of a triangle, and about two of them the dikes show 
remarkable radiation. The Ishawooa intrusives extend for a dis- 
tance of fifty miles into the Yellowstone Park, occurring as stocks, 
sheets, and dikes; they are usually conspicuous about the head 
waters of the eastern flowing streams, but do not form culminating 
summits, these being usually composed of the breccias or late basalts 
Which overlie the intrusive rock. Dikes here, too, are abundant, 
but radiation is not especially marked, the greater masses hav- 
ing rather an axial trend in a northwesterly direction, as though 
injected through a common fissure, rather than as forming inde- 
pendent intrusions. The breccias in contact with the greater intru- 
Sive bodies are indurated to a distance sometimes of more than one- 
half mile. Coarse granites and diorites occur in both the intrusive 
Series — a remarkable fact when we consider that the lavas invaded 
by them are of Tertiary age. 
