SCENOGRAPHICAL GEOLOGY. 597 



polished by water-action. The former usually exhibit striations parallel 

 with one another, and which also show the direction taken by the cur- 

 rent. The ledge covered by them may feel rough to the hand, but to 

 the eye appears rounded in a general way. The latter are smooth to the 

 touch, unless the rock is very coarse in texture; but there is not the 

 general rounding of the mass, as seen in the other type of sculpture. 

 The surface may be covered by a multitude of minor irregularities. 

 This type of smoothness is best seen on boulders and ledges along the 

 beds of mountain torrents. 



The action of frost greatly assists in the work of disintegrating ledges. 

 In the colder months water penetrates the crevices of ledges, and then 

 freezes. As water expands in freezing, the effect is seen in the breaking 

 off of larger or smaller fragments of rock from the ledge. Certain situ- 

 ations are especially favorable for this type of action. One is upon the 

 summits of the higher mountains. The finest known example is upon 

 the summit of Mt. Washington, and, to illustrate it, a heliotype has been 

 taken. By referring to it, the reader will observe large, angular masses 

 of rock scattered about promiscuously, just as they fell off from still 

 larger masses. Some of these are permeated by cracks, which will by 

 and by enlarge, and again fracture the rock in the same way that has 

 been described. The process of freezing and crumbling will go on so 

 long as the particles are capable of division. The seams are originally 

 the jointed structure of the ledges, and ultimately the natural cleavage 

 planes of the constituent minerals. The visitor may search in vain over 

 Mt. Washington for any evidence of transported rock, save what may 

 have been brought by human agency ; and a few minutes' walk over the 

 fragments will prove what a difference there is in the distribution of 

 blocks of stone, by the action of frost and gravity combined, as compared 

 with the arrangement of water-worn stones in a river, or the scattered 

 boulders of drift origin. The house in the view is known as the Tip-top, 

 built of fragments similar to those by which it is surrounded. The view 

 was taken before so much of the summit had been covered by edifices 

 as is now apparent to the visitor. 



Fig. 10 will give a general idea of these blocks of stone, when viewed 

 from some distance below the summit. Though taken in the winter, 

 when the interspaces were filled with snow and ice, the effect is the 



