102 .GRAND CANON DISTRICT. 



lias, no doubt, occurred to many geologists, but, so far as known to me, 

 it had not before received such definite treatment nor been so fully and 

 justly emphasized. It may be explained as follows. 



Whenever a smooth country lies at an altitude but little above the 

 level of the sea, erosion proceeds at a rate so slow as to be merely nomi- 

 nal. The rivers cannot corrade their channels. Their declivities are 

 very small, the velocities of their waters very feeble, and their trans- 

 porting power is so much reduced that they can do no more than urge 

 along the detritus brought into their troughs from highlands around 

 their margins. Their transporting power is just equal to the load they 

 have to cany, and there is no surplus left to wear away their bottoms. 

 All that erosion can now do is to slowly carry off the soil formed 

 on the slopes of mounds, banks, and hillocks, which faintly diversify 

 the broad surrounding expanse. The erosion is at its base-level or very 

 nearly so. An extreme case is the State of Florida. All regions are 

 tending to base-levels of erosion, and if the time be long enough each 

 region will, in its turn, approach nearer and nearer, and at last sensibly 

 reach it. The approach, however, consists in an infinite series of ap- 

 proximations like the approach of a hyperbola to tangency with its 

 asymptote. Thus far, however, there is the implied assumption that 

 the region undergoes no change of altitude with reference to sea-level ; 

 that it is neither elevated nor depressed by subterranean forces. Many 

 regions do remain without such vertical movements through a long 

 succession of geological periods. But the greater portion of the exist- 

 ing land of the globe, so far as known, has been subject to repeated 

 throes of elevation or depression. Such a change, if of notable amount, 

 at length destroys the pre-existing relation of a region to its base-level of 

 erosion. If it is depressed it becomes immediately an area of deposi- 

 tion. If it is elevated new energy is imparted to the agents and ma- 

 chinery of erosion. The declivities of the streams are increased, giving 

 an excess of transporting power which sweeps the channels clear of 

 debris; corrasion begins; new topographical features are literally 

 carved out of the land in high relief; long rapid slopes or cliffs are 

 generated and vigorously attacked by the destroying agents ; and the 

 degradation of the country proceeds with energy. 



It is not necessary that a base-level of erosion should lie at extremely 

 low altitudes. Thus a large interior basin drained by a trunk river, 

 across the lower portions of which a barrier is slowly rising, is a case 

 in point. For a time the river is tasked to cut down its barrier as rap- 

 idly as it rises. This occasions slackwater in the courses above the 

 barrier and stops corrasion, producing ultimately a local base-level. 

 Another case is the Great Basin of Nevada. It has no outlet, because 

 its streams sink in the sand or evaporate from Salinas. Its valley bot- 

 toms are rather below base-level than above it. The general result of 

 causes tending to bring a region to an approximate base-level of erosion 

 is the obliteration of its inequalities. 



