REVIEWS 327 
rather to the later part of the period, and doubtless owe their origin to 
the great gulf which entered the United States from the northwest at 
that time. Another period of emergence without distortion of the 
strata followed the exclusion of this gulf, and no surely marine forma- 
tions were deposited in the region until after the Dakota epoch of the 
Cretaceous. 
The Lakota, Minnewaste. and Fuson formations were formerly not 
separated from the Dakota group, but they are now segregated on the 
testimony of the plant remains contained in the Lakota sandstone. 
The local unconformities and beds of coal in the Lakota indicate a 
condition of slight crustal oscillations near sea level. Marine fossils 
have not been detected in any of the Lower Cretaceous formations 
nor in the Dakota. The land was probably low, and frequently inun- 
dated by lakes, or brought to a marshy condition. 
By the beginning of the Benton stage the great interior sea of the 
Cretaceous had spread over the plains, and in it sediments continued 
to be deposited until near the close of the period. The prevalence of 
fine clastic sediments was interrupted by temporary depositions of chalk 
and limestone in clear open waters. The retreat of the interior sea 
beginning in the Fox Hills stage was probably completed during the 
Laramie, a large part of the latter formation being non-marine. 
Although the Laramie beds do not occur in the Black Hills, they are 
now upturned like the older beds upon the west side of the uplift, and 
it is probable that they originally covered the whole of it. 
The Black Hills dome was probably uplifted very early in the 
Eocene. During that epoch it was truncated and fashioned into its 
present general form. That many of the larger valleys date from that 
time is proved by the existence of Oligocene deposits even in their 
deeper portions. 
In the Oligocene epoch a great lake is supposed to have partially 
surrounded the Black Hills and spread far up on their sides, leaving the 
Dakota hog-back ridges isolated as off-shore islands. In support of 
the view that the beds are lacustrine the author cites the horizontal 
continuity of the beds, the fine assortment of the material, and the 
existence of thin beds of limestone. Although much of the formation 
has been removed by subsequent erosion, its former extent is roughly 
indicated by outlines and by a superposition of drainage which allowed 
many Eocene valleys to drain northward through Neocene canons cut 
across the old divides. After the Oligocene no extensive sediments 
