SCENOGRAPHICAL GEOLOGY. 589 



liable to give way, and precipitate calamity upon the valleys beneath. 

 Rivers plunge clown precipices, giving rise to cataracts and cascades. 

 Vegetation assumes character according to the degree of elevation. 

 Even the pure skies, the gathering of mists and clouds, depend upon 

 the presence of the elevated ridges. Hence I think the position well 

 established that geological agencies have produced all the charms of 

 landscape; and, were we so disposed, we should be amply justified in 

 describing minutely the special causes of change that have fashioned 

 every foot of surface. Those who would thoroughly understand the 

 features of our scenery are invited to peruse the various geological de- 

 tails of this report. They will be necessary, and more besides, if one 

 would describe our mountains with the pains which Ruskin takes to set 

 forth the causes that have moulded the Alps into their present form. 

 Many may imagine it to be of little consequence whether Mt. Washing- 

 ton be an anticlinal or synclinal axis, whether it be composed of granite 

 or slates ; but the decision of these scientific questions is essential to the 

 proper delineation of its scenographical structure. The artist, who repre- 

 sents a mountain correctly upon canvas, has discovered the fundamental 

 type of its structure, whether he uses geological phrases or not, other- 

 wise his painting will not be recognizable. 



It is a well known fact that many surface configurations are due to a 

 peculiarity of rock formation. Conical hills suggest a volcanic origin ; 

 and if on examination they prove to be composed of scoriae or lava, the 

 evidence is plain of igneous eruption. A prairie is not merely an ex- 

 panse of thick loam and deep soil; it is underlaid by horizontal layers 

 of rock, which give evenness to the surface as truly as to a table-land. 

 Chalk hills, not common in the eastern half of our continent, assume 

 rounded and graceful undulations in consequence of the easily-moulded 

 character of the mass. Similar shapes characterize limestone hills. 

 Ranges like that of Holyoke in Massachusetts and Connecticut, pre- 

 cipitous on one side and sloping on the other, assume this form in con- 

 sequence of the situation of the hard trap-rock of the mountain. It 

 dips easterly, so as to expose the tough edge of an inclined sheet high 

 up in the air, and this covering protects the underlying friable material 

 from denudation. The Table mountains of the Sierra Nevada slopes 

 are the remnants of horizontal igneous overflows which have never been 



