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, 61). 
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 
monoclinal flexures ; for these flexures had, as has been shown, 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 ?) 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 
