SCENOGRAPHICAL GEOLOGY. 59 1 



fore, for stating the causes and methods of sculpture in general terms 

 only. 



The special forms assumed by our mountains are mostly those peculiar 

 to the crystalline schists, subsequently modified by glacial action. The 

 most readily distinguishable are the following: First, conical mountains 

 having some resemblance to volcanic summits, but composed of the 

 earlier eruptive rocks, like granite, sienite, and protogene. Second, long 

 reaches of rounded ranges composed of schistose formations. To this 

 class most of our ridges belong. Third, isolated more or less conical 

 masses of the same class of rocks. Fourth, deep, narrow valleys of ero- 

 sion, akin to gorges and canons. Fifth, broad, sloping valleys. Sixth, 

 plains formed by transportation of drift. Seventh, terraced valleys. 

 Eighth, limestone hummocks. Every one of these typical forms has 

 been modified by the drift agency. 



AGENTS OF EROSION. 



The agents of erosion should be briefly mentioned. They are mainly 

 atmospheric disintegration, rain, rivers, glaciers, icebergs, ocean, land- 

 slides, and the great northern Drift. Each of these agents has left 

 behind its particular mark, by which the work performed may be easily 

 identified. Some of them have operated with greater intensity in the 

 by-gone periods of geological time than at present ; others are supposed 

 to have been more energetic in their action in the more recent epochs. 



Atmospheric disintegration has been the most powerful of these 

 agents acting throughout all the periods, yet it is of comparatively little 

 consequence at the present day. I refer especially to the penetration of 

 the ledges by carbonic acid, introduced partly through rain-water, and 

 partly acting upon the surface by its envelopment of the ledges. Before 

 the Carboniferous period, when a large share of the work of disintegra- 

 tion in New Hampshire had been accomplished, the atmosphere con- 

 tained a much larger proportion of this reagent than it does now, and, 

 of course, its action upon the surface must have been more manifest. I 

 refer to the decomposition of feldspathic rocks more particularly, a 

 reaction that has been alluded to previously (p. 550), resulting in beds 

 of white kaoline clays and gravelly heaps for the residua, while the sal- 

 eratus flows off in the streams. Consequently this decomposition is 



