AKID CYCLE IN A MOUNTAINOUS REGION 591 



rock floors only thinly veneered by debris and soil — a fact clearly indi- 

 cating that they also are as truly areas of d(3gradation as are the mountain 

 areas. 



As already stated, the commonly accepted inferences regarding erosion 

 in the arid region are based chiefly upon the observations made in the 

 loftier mountains, especially the Wasatch, the Sierra Nevada, and the 

 southern Eockies. In these ranges the climatic conditions down nearly 

 to their very baCses are not so much those of tj^pical aridity as they are 

 those of a normally wet climate. These ranges may be properly consid- 

 ered as elevated belts of the moist land extending into the arid country. 

 With the true desert ranges the dissection by water is not so marked. 



On the theory that each intermont plain is an independent basin of 

 initial deformation,*^ the resulting contripetal drainage system tends to 

 make for it a separate baselevel of its own. By aggradation of a higher 

 basin and the headwater erosion of intermittent, consequent, or subse- 

 quent streams from an adjoining lower basin, coalescence of basins is sup- 

 posed to go on until there is finally a more or less perfectly adjusted 

 drainage everywhere throughout the region. This is, of course, the neces- 

 sary deduction for a normal moist country. In a region of mountainous 

 aspect, as has been postulated for the Great Basin changing suddently to 

 a desert country, this sequence of events might be expected were it not 

 for the fact that very few of the intermont plains, or basins, are actually 

 connected directly with one another by means of waterways. Moreover, 

 it is very questionable whether in the stage having local centripetal drain- 

 age systems, the arroyos — for such all the drainageways really are — 

 which are as discontinuous at their lower as at their upper ends, could 

 ever accomplish a coalescence of contiguous drainage basins. I know of 

 but few instances in all of the vast arid country in which such an inter- 

 pretation could possibly be entertained. 



As a matter of fact, the intermont basins have probably gone through 

 evolutionary stages directly the opposite from those suggested. The en- 

 tire region was no doubt more or less perfectly drained at the commence- 

 ment of the arid period, and before epeirogenic upraising took place. Since 

 that time deflation has separated the larger tracts into the smaller basins 

 with no outlets. The Spanish title, '^IdoIsous," for these basins thus ac- 

 quires a technical or rather genetic and physiographic meaning of which 

 the early explorers knew nothing. To be sure, the rims of many basins are 

 cut by canyons which, at first glance, might appear to be capable of event- 

 ually producing a coalescence of neighboring basins. Such outlets seldom, 

 if ever, drain an intermont plain as a whole. They usually merely cut off 



Journal of Geology, vol. xiil, 1905, p. 382, 



