SEDIMENTATION. 199 



teristics of which are not yet fully known. Geologists will understand that 

 the thicknesses given refer only to the beds which remain to be studied. 

 There may have been much greater sedimentation ; beds may have been 

 formed during periods of sedimentation which were carried away during 

 periods of dry land conditions, and especially during the last great period 

 of degradation not yet ended, and of which sediments there are no residuary 

 fragments attesting to their former existence. 



On both sides of the range, but especially to the north, we know that 

 the progress of sedimentation was interrupted by periods of dry land con- 

 ditions. These dry land conditions prevailed over a large area during the 

 epoch separating the Lower Green River from the Bridger period, but a 

 part of this interval of time at least was occupied in sedimentation over a 

 portion of the great lacustrine area. Again, a period of dry land con- 

 ditions prevailed between the deposition of the Bridger and the Brown's 

 Park Groups. The latter was deposited over areas in the region of uplift 

 beyond the reaches of the antecedent lakes represented by the four lower 

 Cenozoic groups. How far the Brown's Park Lake extended over the region 

 which had previously been occupied by the waters in which the earlier 

 Tertiary sediments were deposited we do not know. Wherever the overlap 

 of the Brown's Park beds on the Lower Cenozoic groups has been studied 

 the former terminate in escarpments, and no evidence of shore line has been 

 seen. The Bishop Mountain Conglomerate which has been found to lie uncon- 

 formably on all the other geological formations of this region, except the 

 Brown's Park, and possibly on this latter also, is neither a marine nor 

 lacustrine deposit, but is believed to be a subaerial gravel. It is possible 

 many geologists would ascribe it to the action of ice, but in any case it need 

 not be considered in our account of sedimentation. 



Such were the general changes from emergence to submergence through- 

 out the region of downthrow, but there was a narrow belt of country between 

 the general region of uplift and the general region of downthrow which 

 was subject to more frequent changes than the greater ones described above. 



We find, first, that there are many overlaps, i. e., that later beds extend 

 beyond the limits of earlier beds. To accomplish this result it is manifest 

 that the waters of the lakes must have risen or the land subsided. Again, 



