10 MEMOIK.S UF THE NATIONAL ACADEMY OF SWENCE8. 



thereafter no more teudeucy to one side tbau the other. Detlective effects of rotation are there 

 fore not to be songht in regions of alluvial deposition. 



It may be remarked also that the tendency of a stream toward one bank or the other by reason 

 of curvature and rotation is often overjjowered by an opposite tendency due to obstructions. 

 These include resisting members of the eroded terraue, and alluvial dams dei)osited at one bank 

 or the other by tributaries. 



A general curvature in the course of the valley through which the stream flows has the same 

 tendency as does the curvature of a short bend, only in a less degree; and this tendency must in 

 many instances nullify or conceal the results of rotation. 



Visible examples of the work of rotation are therefore to be sought esjiecially in streams 

 which, with courses in the main direct, are slowly deepening their valleys by the excavation of 

 homogeneous material. The best locality of which I have knowledge is one to which attention was 

 called by Mr. Elias Lewis, in the American Journal of Science for February, 1877, and which has 

 recently been visited at my reiinest by Mr. L 0. Russell. The south side of Long Island is a plain 

 of remarkable evenness, descending with gentle inclination from the morainic ridge of tiie interior 

 to the .Vtlantic ocean. It is crossedby a great number of small streams, which have excavated 

 shallow valleys in the homogeneous modified drift of the plain. Each of these little valleys is 

 limited on the west or right side by a bluff from 10 to 20 feet high, while its gentle slope on the 

 left side merges imperceptibly with the general plain. The stream in each case follows closely the 

 bluff at the right. There seems to be no room for reasonable doubt that these peculiar features 

 are, as believed by -Mr. Lewis, the result of terrestrial rotation. As the streams carve their val- 

 leys deeper they are induced by rotation to excavate their right banks more than their left, gradu- 

 ally shifting their positions to the right, and maintaining s'ream cliffs on that side only. 



