ORIGIN OF LAKE I5ASINS. 5 



deposits, which in some instances have assisted other agencies in pro- 

 ducing inequalities of the surface. 



Basins duo to aqucoiis ag-encics. — In this chiss of basins there are 

 twf) important subdivisions : a, basins (kie to the action of streams, and 

 //. l)asiiis (Uie to the action of waves and currents. In each sulxhvision, 

 but more especially in the first, there are basins formed by excavation 

 and basins due to deposition, or basins due to destructive and to construc- 

 tive agencies. Frequently the two processes have united in the formation 

 of a single depression. 



a. Basins formed hy streams. — The drainage of new land areas, es- 

 }»eeiallv in humid regions, soon obliterates the depression due to the 

 original inequalities of the surface, as already explained; but other basins 

 resulting from the action of the streams themselves are formed. 



\\'hen the topography of a young land area is yet immature, and more 

 especially when the elevation is considerable and the climate humid, the 

 even flow of the draining streams is apt to be interrupted by rapids and 

 water-falls, at the bases of which excavation is accelerated and depressions 

 formed. The deepening of such portions of stream-beds, results princi- 

 }»ally from the friction on their bottoms and sides, produced by sand and 

 stones moved by the swift currents. Some distance below falls and 

 rapids, the current usually slackens, and the waters deposit a i)ortion 

 of their load. A basin of this character is now being excavated below 

 Niagara falls, and other examples may be seen in tlie channels of many 

 mountain streams. Even on old land areas like the southern portion of 

 the Appalacliian region, where the streams are engaged in cutting down 

 synclinal tal)le-lands in \\liicli hard and soft strata alternate, small basins 

 of the character here referred to are of common occurrence. Should a 

 stream channel in which such inequalities have been produced be aban- 

 doned as a line of drainage, the basins would be transformed into lakes. 



The best example of. a lake basin of considerable size formed at the 

 base of a water-fall, that has come under the writer's notice, is iri tlie 

 Grand Coulee, near Coulee City, in the State of Washington. The 

 Columbia liver now skirts the nortliern and western l)orders of the vast 

 lava-covered region known as the Great Plain of the Columl)ia, or more 

 familiarly as the " Big Rend countr}-," but in Pleistocene times its present 

 course was ol)structed by glaciers which descended from ilic mountains 

 to the iiortli. and it was forced to cut across the Big \\v\u\ through a 

 series of dee[) carious in the lava. Its tempoi'arv course was thronn'h 



