91 ON EROSIONS OF THE EARTH'S SURFACE. 



the difference is, for example, between chalk and Silurian limestone, especially 

 when a small proportion of silex enters into the composition of the latter. Certain 

 kinds of syenite disintegrate with great ease, compared with common trap : yet 

 both are composed of feldspar and hornblende. 



4. Rocks, essentially alike, may yet be decomposed with very different degrees of 

 facility, on account of the presence in some of tbem of such minerals as carbonate 

 or sulphuret of iron or manganese in some form. It is surprising sometimes to see 

 to what depth the whole character of the rock will be changed, and how it will be 

 disaggregated, so that aqueous agency can easily denude its surface. 



Detail of Facts. 



Guided by the preceding principles, I have made a collection of examples, which 

 I suppose to be cases of erosion mainly by rivers. In some of them, however, 

 other agencies have been largely concerned, perhaps more largely than the rivers. 

 I have not confined myself to examples founded on personal observation, and of 

 course, in those cases which I have not seen, I feel less confidence than in the 

 others ; for careful examination is sometimes necessary to decide certainly whether 

 river agency has produced the gorges. Yet by observing the characters of those 

 erosions, which personal examination refers with great confidence to river action, 

 we can with great probability refer other cases to the same cause, which we have 

 only seen described by travellers or geographers. 



The first example below, is not one of the most satisfactory ; yet as it is the 

 case which first called my attention to the subject, I shall describe it with more 

 detail than usual. 



I cannot doubt that a more extensive examination than my time will allow of 

 the works of travellers and geographers would enable me easily to double the fol- 

 lowing list. Still, as travellers usually describe such scenery only in general 

 terms, the geologist can but seldom decide certainly what cases are examples of 

 erosion by rivers. 



So far as it is in my power, I shall describe these erosions under the head of the 

 different rocks in which they exist. 



1. Erosions in the Hypozoic or older Crystalline Eoc7cs, such as Gneiss, Mica Slate, 



Talcose Slate, &c. 



a. In BucMancl, on Deerfield River, a little west of Shelburne Falls. 



A ridge of gneiss and hornblende slate lies west of the village of Shelburne Falls, 

 through which Deerfield river has cut a passage. On the road from that village to 

 Charlemont, where it crosses this ridge, we meet with pot-holes in the ledges of 

 gneiss ; and, indeed, the road occupies an old bed of the river. These pot-holes 

 are 80 feet above the present bed of the stream, and the terrace materials rise to 

 that height on the north side of the river. This proves that the stream was once 

 dammed up to that height, else the pebbles and sand could not have been sorted 



