266 
ELDRIDGE. 
in a degree speculative, but the inferences are: first, that soon 
after the completion of the Niobrara period elevation began, 
and so much of the hill as is above the altitude indicated 
in the section by the height upon its flanks reached by 
the upper layers of the Niobrara was then, sooner or later, 
brought within the erosive power of waves or currents, and 
the sediments last laid down, being now brought into a 
favorable position, and still in a condition sufficiently soft 
to permit their being easily broken down and comminuted, 
were removed by the transporting powers of the waters 
washing them ; secondly, that the conditions of sedimenta¬ 
tion in the immediate seas were the same as those in all 
mediterranean or large inland seas or along the margins of 
the continents at the present day—that is, comparatively 
deep and quiet water at a distance somewhat remote from 
the nearest coast line, which permitted the quiet settling of 
the sediments which go to make up the clays of the Mon¬ 
tana group, and which correspond to those under which the 
blue mud of sub-continental areas is now being deposited. 
The apparent complete removal over the space originally 
covered by them of the materials resulting from the break¬ 
ing down of the early Cretaceous strata is somewhat strik¬ 
ing, but it may readily be accounted for in the nature of the 
formations removed, and in the action of waves and currents 
and the long time through which the higher parts of the 
elevation were probably subjected to them. Furthermore, 
it cannot be positively asserted that the line of non-con¬ 
formity is as clear of debris as represented, since on the 
steeper flanks of the arch it is rare that the beds above this 
line can be traced to actual contact with those below; still 
further, it is to be remembered that nothing whatever is 
known of the conditions on other profiles of this ancient hill. 
During the deposition of the beds of the Montana group 
gradual subsidence of the area at a generally uniform rate 
must have taken place, the sedimentation, with two excep¬ 
tions, being that of quiet and deep water. The exceptions 
noted—the sandy zone midway the Fort Pierre and the more 
