318 



PHYSICAL GEOLOGY 



be seen from young cones, such as Lassen Peak, California, which was 

 active in 1914-1915, to those which have been worn down to their 

 roots. Mt. Shasta, California, 14,350 feet high (Fig. 316), is a good 



Fig. 316. — Mt. Shasta, California, a partly denuded volcanic cone. 



example of a volcano which has suffered much erosion, but Mt. Hood, 

 Oregon, is still more worn, the sides being deeply trenched by ravines 

 and only a part of the wall of the crater being left. 



Distribution and Number of Volcanoes 



Number of Volcanoes. — It is impossible to determine accurately 

 the number of active volcanoes, since some that appear to be extinct 

 may be merely dormant, and others that have recently been active 

 and from which steam is still rising, may have been in eruption for 

 the last time. It is, moreover, sometimes difficult to distinguish 

 between independent and subsidiary vents. It is safe to- say that 

 there are approximately 325 active volcanoes, of which one third are 

 on the continents. 



Distribution. — A glance at a map of the world in which the vol- 

 canoes are conspicuously indicated (Fig. 317) shows some striking 

 features of their distribution. It is seen that they are not scattered 



