STRUCTURES COMMON TO ALL ROCKS. 251 



high Sierra region, a remarkable dome-structure is very 

 characteristic of the scenery. This is determined by a 

 huge concentric structure of the granite, as in Fig. 157. 



PIG. 157. Ideal section showing dome-structure. Dotted line above shows original 



surface. 



This, however, must not be confounded with arched strata. 

 These great domes are still scaling off concentrically. 



6. Outbursts of Igneous Rocks through Dikes 



often give rise to prominent ridges on account of their 

 superior hardness. Examples are found in the trap ridges 

 of the Connecticut Valley and in many other places. 



7. The Nature of the Erosive Agent. The scenic 

 forms of mountains are also largely determined by the na- 

 ture of the erosive agent. Simple water tends, by erosion, 

 to form rounded summits and ridges, and narrow V-shaped 

 gorges. Ice, on the contrary, tends to make pinnacled 

 summits (aiguilles) and comb-like ridges, and broad, 

 meadow-like valleys. 



