ORIGIN OF LAKH liASINS. 6 



ordinary course of topographical development are transient features. 

 In humid regions they are drained more quickly than where the rainfall 

 is small. They are fresh or saline according as they overflow or are 

 without outlet. 



On old land areas where the streams have reached maturity or old 

 age, the inequalities of the surface due to the accidents of original depo- 

 sition are removed, and lakes of the class here considered are al)sent. 

 This is shown in a striking manner by contrasting Florida with the 

 adjacent Appalachian region. In the former, lakes are alamdant. and 

 their surroundings give abundant evidence of recent origin : in the latter, 

 the topographic forms as well as the terranes from ^^■llit■h they have been 

 carved, bear the stamp of antiquity. 



Lands that have been subjected to intense glaciation, or have re- 

 ceived a covering of glacial deposit, are essentially new land area, and 

 bear evidence of topographic youth ; but the lakes characteristic of such 

 rejuvenated lands will be considered in advance in connection with other 

 results of glacial action. 



Basins due to atinosplieric ag-eiicies. — The weathering of rock 

 surfaces progresses unevenly, on account of varying hardness and the 

 varying degree to whicli they yield to chemical changes. This is 

 noticeable i^articularly on granitic areas, as granite is especially prone to 

 disintegration, and produces uneven surfaces when weathered. The 

 tendency to decay unequally, as weathering progresses, probably exists 

 in all rocks ; and it is to be expected that hills and hollows would result 

 for the action of the atmosphere on any variety of deposit, especially if 

 marked variations occur in its texture and composition. This tendency is 

 most easily detected when the bedding is nearly horizontal, and large 

 sheets of nearly level strata are exi)osed to the sky. 



The products of weathering are removed by water in solution anil in 

 suspension, and are blown away by the wind. When removed by water, 

 tlie, formation of basins is checked by the cutting of outlets. When 

 carried away by the wind, depressions known as " wind-erosion l)asins *' 

 are left.^ These are basins of excavation or true rock basins, and in tliis 

 respect reseml)le de[)ressioiis eroded l)y glaciers. Some observers have 

 concluded that many of the rock l)asins commonly ;iscril)ed to glacial 



1 Xuniernus examples of sliallow. saucer-sliai)cil dcpressioiis in sliale, due to the action 

 of the wind on areas bare of vetretation, in tlie southeastern part of t^olorado, have recently 

 been (h^scribed by (i. K. (iilbert. Jour, of Geol., vol. :], 18!»5, pp. 47-40. 



