GLIMATAL CONDITIONS AND CHANGES. 
223 
most instructive one is De Motte Park, on the Kaibah. A considerable 
number of others are still distinguishable. These indicate that as the Colo- 
rado began to sink into the Carboniferous strata, some cause dried up their 
very fountains, and they ceased to flow. No explanation seems at all 
adequate except the advent of an arid climate. If, then, we could fix the 
period at which this change of climate occurred, we should have strong 
presumptive reasons for selecting the same period as the one in which the 
present Grand Canon originated. 
We know that the Miocene climate of the West was moist and sub- 
tropical. This is indicated by the great extent of fresh-water lakes in some 
portions of the West, their abundant vegetable remains, and the exuberance 
of land life But the remnants of Pliocene time are usually of a difi’erent 
character. In the Great Basin we have many proofs of the arid character 
of that age, and it is eqiially evident throughout the Plateau Country that 
the Pliocene climate was in the main very much like the present. We can- 
not, it is true, correlate with precision any definite boundary between Mio- 
cene and Pliocene; but, with no unreasonable latitude, I think we may 
still say that the Miocene climate of the Plateaus was a moist climate, while 
the Pliocene was arid, and that the transition from one climate to the other 
occurred near the close of the former age or near the beginning of the latter. 
At the epoch when the cutting of the present Grand Canon began, no 
doubt the district at large presented a very dilferent aspect from the mod- 
ern one. While the greater part of the denudation of the Mesozoic had 
been accomplished, there Avere some important remnants still left which 
have been nearly or quite demolished in still more recent times. The 
basalts of the Uinkaret and Sheavwits have preserved some extensive Per- 
mian outliers, and even these must have shrunken greatly by the waste of 
erosion during the long period occupied in the excavation of the Grand 
Carton. Although the basalts which cap Mounts Logan and Trumbull are 
certainly very ancient, and are older than the faults — or at least older than 
a great part of the faulting movements — there is no assurance that they are 
as old as the origin of the present carton. Still I do not doubt that they go 
back nearly as far, and they are certainly much more ancient than the inner 
gorge at the Toroweap. At the time of their outpour large masses of Per- 
