I 58 THE AMERICAN NATURALIST. [VoL. XXXIV. 
Mr. Hague concludes that variation in coarseness of crystallization 
‘is not dependent upon pressure, but is far more affected by the rate 
of cooling. He agrees with Professor Iddings in the belief that, for 
the same stock and its complex of dikes, variations in composition and 
structure are due to varying conditions of crystallization from a 
single molten magma. Hague, however, differs from Iddings in his 
conception of the significance of the apparent radiation of dikes 
about the diorite and monzonite stocks of the Crandall quadrangle ; 
Iddings believes this radiation to indicate that the central mass rep- 
resented the root of a great conical volcano now eroded away, which 
he reconstructed to a height of ten thousand feet above the pla- 
teau at its present level. Mr. Hague fails to find any evidence of 
the building up of great volcanic piles, but compares the region to 
Iceland, where there are many centers of eruption and fissures 
through which the lava breaks forth, old sources of eruption in time 
becoming obliterated by fresh flows from newer vents. 
The geologic maps of the Absaroka Folio show in very striking 
fashion the horizontal character of the flows and breccias where the 
digitate drainage intersects adjacent formations. The most con- 
spicuous features shown by the map are the regular trend of the 
main divide on the southwest, parallel to the Ishawooa intrusive 
bodies, and the wonderful abruptness of the eastern gorges in con- 
trast with the gentle slope of the streams which flow down to the 
high basin of Yellowstone Lake on the west. A conspicuous feature 
of the physiography is the remarkable curve from west to east of the 
head of North Fork Stinkingwater River, its head-water trend con- 
forming exactly to the trend of Sunlight Creek, across the high 
divide formed by intrusive bodies in the Sunlight mining region. 
The Absaroka folio is illustrated by reproductions from photographs 
showing the deep canyon of Clark Fork cut in archzan granite, 
Index Peak composed of breccias overlying palzozoic limestone 
above archzean rocks, other pictures of dikes and breccias, and a 
view of Sunlight Glacier, one of the small remnants of the great 
glaciers that formerly played an important part in the erosion of the 
plateau. 
The geological history of the region is briefly as follows. Palæ- 
ozoic and mesozoic sediments were deposited throughout a long 
period to a thickness of many thousand feet, and at the close of 
Laramie time, mountain-building took place, producing uplift and 
deformation that was contemporaneous in all the ranges of the 
northern Rocky Mountains. This post-Laramie movement produced 
