WHITE ON THE LAEAMIE GROUP. 873 



oscillations of surface or sea-bottom, whicli caused local unconformity of 

 strata, but these are so limited in extent, so far as tbey are known, tbat, 

 at no great distance away from each, the strata, which evidently corre- 

 spond with the displaced ones, show no evidence of disturbance. An 

 example of such local unconformity exists in the Bitter Creek Series, 

 near its top, in the vicinity of Point of Eocks Station. 



Although the disturbances at or near the close of the Laramie period 

 "were greatest in the region of the western border of the Laramie Sea, 

 there were necessarily minor disturbances over a large part of the area 

 which it occupied, because it was no doubt a continuation of continental 

 elevation that narrowed the area of the Laramie Sea and fixed the 

 boundaries of the freshened waters that continued to cover a large part 

 of its former site. The evidence seems conclusive, however, that while 

 there was then at least a slight elevation of that part of the continent, 

 and a freshening of the remaining great body of land-locked waters, 

 sedimentation was not interrupted thereby over a large part of the area 

 occupied by those freshened waters. It is not claimed that the disturb- 

 ances of strata which marked the change from the Fox Hills Group to 

 the Laramie approached in extent or degree those which occurred at or 

 near the close of the Laramie Group, although there was a radical- 

 change in at least the molluscan fauna in both cases ; but the facts seem 

 to prove that we have in these western strata, including the great fresh- 

 water deposits, an unbroken geological record, extending at least from 

 the earlier Mesozoic far into Tertiary time. The apparent paleontolog- 

 icfil breaks in that record are regarded as only faunal displacements and 

 restrictions which were caused by radical changes of environment that 

 were consequent upon the different physical changes Avhich took place 

 in the progress of the evolution of the continent. 



The already accumulated geological facts show that the general con- 

 tinental elevation was continued after the Laramie period, much in the 

 same manner that it progressed up to that time (for the Eocky Mount 

 ains were not yet elevated); still inclosing large bodies of water, but 

 which were no longer salt. The surface of the Laramie Sea was doubt- 

 less only slightly, if at all, elevated above the level of the great open 

 sea ; but the elevation of its former bed was no doubt considerably in- 

 creased during its successive occupancy in part by the Wasatch, Green 

 Ei ver, and Bridger Lakes. There must, however, have been a subsidence 

 of the bottom of each of these great bodies of fresh water during their 

 existence, which permitted the accumulation of the immense thickness 

 of their strata which now remain, besides that which has been removed 

 by erosion. Free drainage of overflow into the open sea must also have 

 been maintained during these later epochs, which kept their waters fresh, 

 but which evidently did not exist during the Laramie period; but the 

 present discussions are necessarily confined mainly to the last-named 

 period. 



In the foregoing discussion of the palcontological characteristics of the 



