ABRASION BY GLACIERS, RIVERS, AND WAVES TSI 7, 
stream for removal. In each case the character of the bottom bears 
evidence to the ineffectiveness of abrasion. 
This fact in regard to the form of the stream-bed has been noticed 
by the writer in the streams cutting the Ohio shale and Cincinnati 
limestone (see Fig. 2) in Ohio, in the sandy shales near Ithaca, in 
the Berea sandstone 
of Ohio, in the Tri- 
assic sandstone along 
the lower Westfield 
River and the Con- 
necticut River, and 
in the crystalline 
rocks along the upper - 
Westfield River. 
Views showing 
stream-beds, notably 
the collection in 
_Tarr’s New Physical 
Geography, give evi- 
dence in the same 
Fic. 2.—Stream in thin-bedded Cincinnati limestone, 
in which the characteristic irregular surface of the 
stream-bed, the result of plucking rather than abrasion, 
direction. is shown. Near Camden, O. 
In qualification of 
what has just been said in reference to stream-abrasion, two things 
may be mentioned. First, reference should be made to pothole 
action. It is abrasion, and where numerous potholes are forming 
and connect, they may decidedly aid downward erosion. This 
action, however, is believed to be exceptional; the great majority 
of streams are without it. Secondly, and forming a really important 
exception, in certain cases streams are flowing over rock-beds which 
are thoroughly smoothed, and appear to have been deepened by wear 
of stream-swept detritus. In these cases it is believed that it will be 
found that the rock is nearly jointless. ‘This is especially the case™ 
with crystalline rocks, particularly the more massive granites. This 
process of wear by abrasion is most common in swift streams in 
mountainous areas, but even here it is exceptional, and in the con- 
sideration of stream-erosion generally it is insignificant. 
In conclusion, the joint-controlled form of the rock sides and 
