STRUCTURES COMMON TO ALL ROCKS. 247 



of interior heat information. If the mountain is rough- 

 hewed by the latter, it is shaped and chiseled by the 

 former. The great swell of the crust, which is only seen 

 from a distance, is due to igneous agency ; but all the 

 scenery, which so charms us when we are among moun- 

 tains, is due wholly to erosion. Moreover, there is a 

 peculiar charm in the study of the latter, because it is 

 more easily understood. The cause of mountain-origin 

 is obscure, and the folded structure of mountains is 

 hidden, and can only be unraveled by the skillful geol- 

 ogist ; but the forms of mountain-sculpture may be 

 studied by all, and their study gives great additional 

 charm to mountain-travel. 



Resulting Forms. The forms produced by erosion 

 are infinitely various, depending upon the kind of rock 

 and upon the amount and style of folding. They are, 

 therefore, of great interest also as revealing interior struc- 

 ture. We can only touch very lightly on a few of the 

 most common and characteristic forms. 



1. Horizontal Strata. These, when sufficiently hard, 

 give rise to table forms, the top of the table being deter- 

 mined by a hard stratum of some kind, as sandstone, or by 

 a lava-flow. In the latter case, however, we have this 

 form, whatever be the position of the underlying strata 

 (see Fig. 6, page 24). Good examples of this form are 



FIG. 151. Table-mountains. 



seen in Illinois and in Tennessee (Fig. 151), and" espe- 

 cially in the mesas of the Plateau region (Fig. 7, page 25). 

 If, on the contrary, the horizonal strata are soft, and 

 yield easily to erosion, they are worn into the most fan- 

 tastic forms conical, castellated, pinnacled such as are 

 found in the "Bad Lands" of the West, which are pro- 



