6oo 



PHYSICAL GEOGRAPHY. 



The origin of the conical form may be due to three causes : First, to 

 the original shape of the materials. Our granite mountains have been 

 erupted from below, and, when the entire mass is limited in amount, it 

 would naturally be conical, the pasty substance tending to flow centrif- 

 ugally from the vent. Secondly, the tendency of denudation is to wear 

 away the summits of mountains. The forces act powerfully upon the 

 exposed ledges at the tops, and the fragments will seek the bases of the 

 hills, thus tending to the cone in form. Thirdly, the more energetic 

 action of denudation in granite mountains is caused by a sort of con- 

 centric structure in cliffs. A good example occurs in Benton, near the 

 summit of the Boston, Concord & Montreal Railroad, as one looks north- 

 erly upon the steep side of Owl's Head. Another may be seen in the 



Fig. 72. WELCH MOUNTAIN, FROM CAMPTON. 



heliotype of Eagle cliff in Volume II, where the vertical wall of Profile 

 mountain, on the left-hand side, shows the concentric, nearly vertical, 

 slabs of granite peeling off and falling to the base of the cliff. The 

 steep sides of Carrigain notch (see sketch) have been produced in the 

 same way. The right-hand slope in Fig. 9 shows the base of a cliff 

 opposite the Willey house, where similar action has taken place. A 

 heliotype shows the same thing upon both sides of the Notch, in the 



