SCENOGRAPHICAL GEOLOGY. 593 



taries. These narrow gorges may be continuous for hundreds of miles. 

 The surface of the country is dry, parched, mostly without vegetation, 

 both for the want of rain and the settling down of all the water flowing 

 through the country from the moist regions higher up to the bottoms of 

 the canons, hundreds and sometimes thousands of feet below the general 

 level. The edges of the ravines are sharp, as no tributary rills wash 

 away the projecting angles. This type of river erosion is represented 

 in the well-watered districts by short gorges, where rivers fall over prec- 

 ipices, and gradually eat their way up the channels. But the edges of 

 the gorges below the cataracts are being gradually rounded, and pass 

 insensibly into the other type of river-sculpture. 



The other type is best expressed by the general term of valley. The 

 constant flowing of small streams down the banks of rivers removes the 

 angles of the square edges of the plateau, and there result gradual slopes 

 from the water's edge to the dividing ridge between different hydro- 

 graphic basins. The valleys/ are broad or narrow in proportion to the 

 amount of rain flowing down the banks, not forgetting that the original 

 direction may have been given to the water by the formation of synclinal 

 basins. 



These two types of river action are very marked ; and the geologist, 

 by this feature, can at first glance determine whether a newly-visited 

 country is a dry or rainless one, and, to some extent, whether the rains 

 are abundant or limited. 



On applying these criteria to New Hampshire, we find many examples 

 of interest. One immediately recalls the flumes at Dixville, Lincoln, 

 and Nancy's bridge, as similar to the canons. These, however, owe 

 their perpendicularity to the nature of the rocks. Along the river-beds 

 are easily decomposing dikes, which are quickly worn away by the water. 

 Then the granite bordering the dikes is permeated by joints parallel to 

 the stream. The action of water freezing in the seams has pushed out 

 the first layer of rock on each side, and thus the flumes are quite wide, 

 with vertical walls. 



Limited gorges are quite numerous. They are to be explained either 

 by the presence of softer rocks in the beds of the streams, or by the fall- 

 ing of water over precipices. Many of them will be noticed hereafter. 



There is a gradation from the narrow valleys, where water runs more 

 vol. i. y7 



