OF WESTEBN NORTH AMERICA. 



21 



exception of important upper Miocene ('^Peace Creek" and '^Archer" 

 formations) and possibly mid-Pliocene deposits in Florida, the coun- 

 try east of the Great Plains remains unknown until the lower Pleisto- 

 cene. 



These facts, which are often overlooked by paleontologists, have a 

 very important bearing on theories as to the source or origin of the 

 new forms of mammals which suddenly appear from time to time. 



RESEMBLANCES AND CONTRASTS BETWEEN MOUNTAIN 

 AND PLAINS REGIONS. 



Resemblances. — Opening with a moderately warm and humid but 

 far from tropical climate, with mild winters, the common physio- 

 graphic and climatic history of both the Mountain and western 

 Plains regions was that of progressive elevation, slowly progressing 

 aridity, gradual soil denudation and deforestation, progressively 

 sharper definition of the winter and summer seasons, concluding 

 with destruction of most of the larger forms of life during the lower 

 Pleistocene glacial epoch. 



Contrasts. — The geologic history of the two regions presents some 

 strong contrasts. 



First, with some exceptions, the Tertiary deposits of the Mountain 

 Region are in clearly defined basins drained by the same great river 

 systems which drain them to-day, while those of the Plains Region 

 are widely scattered over broad areas, with frequent changes in the 

 river courses, the present river courses being comparatively modern. 



Second, it follows that in the Mountain Region, from the basal 

 Eocene to the summit of the upper Oligocene or John Day formation, 

 there was little or no working over of the older Tertiary rocks into 

 newer deposits, but there exist a number of continuous local deposi- 

 tions. Erosion of these depositions has been retarded fortunately 

 in the John Day basin of Oregon by heavy cappings of lava, in the 

 Bridger basin by a dense Pleistocene (?) conglomerate, and in the 

 Washakie basin by a fine conglomerate. Broad expanses of these 

 historic strata have thus been preserved in their original purity and 

 continuity for the geologist and paleontologist. 



Third, by contrast, in the Plains Region the original very extensive 

 Oligocene strata were in part worked over to form Miocene strata, 

 and these in turn were in part eroded to form Pliocene strata; again, 

 all three contributed to the Pleistocene strata; and finally all four 

 are now contributing to the alluvium of the Great Plains. Thus in 

 the Plains Region we find Miocene river deposits laid in old Oligocene 

 channels, and Pliocene deposits embedded in Miocene channels, as 

 well illustrated in Gidley's sections" in the Llano Estacado of Texas 



aGidley, J. W., The fresh-water Tertiary of northwestern Texas. American Museum expeditions 

 of 1899-1901: Bull. Am. Mus. Nat. Hist., vol. 19, 1903, pp. 617-633. 



