262 KEPOKT UNITED STATES GEOLOGICAL SURVEY. 



Laramie sea did not materially interrupt the continuity of sedimentation 

 witliin at least a very large part of its area, although the effects of 

 those physical changes were such as to cause a total change in at least 

 the molluscan fauna. The wide geographical distribution and great ver- 

 tical range of many of the molluscan species of the Laramie Group, and 

 the great uniformity of its lithological characters, show that the period 

 was one of comparative quiet within the region which was occupied by 

 its waters. There were, however, some comparatively slight oscillations 

 of surface or sea-bottom which caused local unconformity of strata, but 

 these cases are so limited in extent, so far as they are known, that at no 

 great distance away from each the strata, which evidently correspond 

 with the displaced ones, show no evidence of disturbance. An esami)le 

 of such an oscillation is illustrated by the unconformity among Laramie 

 strata in Bitter Creek Valley, which has already been discussed. 



While there is evidence that this general quiet was preserv^ed, not 

 only through the Laramie period, but that it was continued into the 

 Tertiary epochs which immediately followed, it is true that at or near 

 the close of the Laramie period in the region which now embraces a part 

 of Bear Eiver Valley, and there covered in part by the western border 

 of the Laramie sea, there was an extensive displacement of the Laramie 

 and older strata, which brought the subsequently-formed Tertiary de- 

 posits unconformably upon them. These facts have been briefly dis- 

 cussed on preceding pages, but that region, with, its imi)ortant geolog- 

 ical and paleontological features, I have yet only slightly investigated. 

 It has been shown on preceding pages that notwithstanding these and 

 doubtless other disturbances which occurred elsewhere at several local- 

 ities, in the great Green Eiver Basin and in the valleys of White and 

 Yampa Elvers the strata show satisfactory evidence that there was con- 

 tinuous sedimentation from the close of the Laramie period to the begin- 

 ning of, and during the Wasatch epoch. Besides this, the continuity of 

 sedimentation from the epoch of the Wasatch to that of the Bridger Group 

 inclusive is a fact that, so far as I am aware, is disputed by no one. Ad- 

 mitting these facts, together with the conclusions that have been drawn 

 on preceding pages, we have in these Western strata an unbroken geo- 

 logical record, extending, at least, from earlier Mesozoic far into Tertiary 

 time ; the apparent paleontological breaks in that record being really 

 only faunal displacements, which were caused by radical changes of en- 

 vironment, notably the removal or variation of the saltness of the waters 

 that were consequent upon the different i)hysical changes which took 

 place in the progress of the evolution of the contineut. 



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 whicli 

 were no longer salt. The elevation of the Laramie sea was doubtless, at 

 most, only slightly above that of the great open sea, but the elevation 

 of its former bed was no doubt considerably increased during its suc- 

 cessive occupancy in part by the Wasatch, Green Eiver, and Bridger 

 lakes. There must, however, have been a gradual subsidence of the 

 bottom of each of these great bodies of Iresh water, which permitted the 

 accumulation of the immense thickness of their strata which now re- 

 main, 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, wliich kept their waters fresh, but which evidently 

 did not exist during the Laramie period; but it is not my purpose to dis- 

 cuss these questions in this report. It is proper, however, to i3resent very 



