160 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



teau faults the initial, stages." He goes on to say that the Basin range 

 province was disturbed at the close of the Jurassic and of the Eocene 

 periods ; while the faults in the plateau province are of Tertiary date. 

 In the border land between the two provinces, the later disturbances 

 not merely run parallel to the Jurassic upheavals, but in places actually 

 coincide with them (a, pp. 58, 59, Gl). 



The Effect of the Flexures. — During the earlier elevation and denu- 

 dation of the Basin ranges, in post-Jurassic times, much waste seems to 

 have been carried from them towards the east and northeast. While 

 the Cretaceous strata of Utah and Arizona are marine deposits, the 

 Tertiaries are continental ; and from this it may be inferred that the 

 movements of the Cretaceous-Tertiary interval (such as the Water- 

 pocket flexure) formed enclosed basins, from which the sea was excluded, 

 and into which the waste was gathered as lacustrine or fluviatile depos- 

 its. The great volume' of the successive Tertiary formations implies 

 that the highlands of the southwest long maintained a considerable 

 altitude. It is possible that their height was intermittently renewed 

 by movements which gave. rise, in the Grand canyon district, to the 

 monocliual flexures ; for these flexures had, as has been s"hown, prevail- 

 ingly an uplift on the west or southwest. It is postulated that the 

 initial effect of a series of flexures would be to form a flight of very 

 broad steps, such as certainly must have been the case with two well- 

 defined members of the series, the east Kaibab and the Echo flexures ; 

 but as the surface of each " tread " may have departed from a level 

 attitude, it is not safe to assert that the height of the top of the 

 flight (southwest) above the bottom (northeast) was equal to the sum of 

 all the " rises." It is, however, here assumed that the top was higher 

 than the bottom, and that the flexures were effective re-enforcements 

 of the mountain-making upheavals in maintaining the Basin range 

 province at such an altitude that it could shed waste abundantly to the 

 northeast. In this connection, it should be noted that the Aubrey cliffs 

 on the south of the plateaus are determined by a north (northeast 1 ?) dip- 

 ping flexure (Gilbert, a, p. 46, section), and that the east-dipping strata 

 which are now found close along the fault lines by which the Basin 

 ranges are separated from the plateaus, show some of the strongest dips 

 of the region. 



Through all the time during which the mountains of the Basin range 

 province stood higher than the plateau area, the lines of river-flow may be 

 assumed to have followed the direction in which the mountain waste was 

 so abundantly transported ; namely, east and northeast ; and through 



