CONSTRUCTION OF THE EAKTH. , 143 



227. Valleys- Intimately connected with the subject 

 of the formation of mountains is that of the formation 

 of valleys. These are chiefly of three kinds : 1. Valleys 

 of undulation. These occur between the hills and 

 mountains that are raised by flexures of the earth's stra- 

 ta, as represented in Fig. 52. 2. Valleys of dislocation. 

 These are caused by fissures in strata, as represented by 

 Fig. 55. Where the strata are very thick, such valleys 

 are' colossal in size and bold in their features. 3. Val- 

 leys of denudation. These vary much, according as the 

 rocks acted upon by the water are soft or firm. In Fig. 

 72 is represented an example of valleys formed by de- 



rig. 72. 



nudation. The strata here are not all continuous^but 

 portions of some of them have been worn away by wa- 

 ter, leaving the hills with valleys between them. 



228. Volcanoes. These are constructed in a different 

 way from common mountains. They are made out of 

 the matters which are ejected from them, so that, as long 

 as a volcano is active, the work of construction is to some 

 extent going on. The solid matters ejected are chiefly 

 lava, cinders, and cinders moistened and forming in their 

 solidification what is called tufa. The tendency in the 

 construction is to the form of a cone, which is sometimes 

 very perfectly carried out, as seen in the grand volcano 

 of Japan, Fusiyama, the summit of which is represented 

 in Fig. 73 (p. 144). It stands 14,000 feet high. It has 

 in its top an oval opening, one diameter of which is 3300 

 feet, and another 1800 feet, and the depth is 1000 feet. 

 This volcano has long been inactive, the latest eruption 

 recorded having occurred in 1707. But two months in 

 the year, July and August, is the summit sufficiently free 

 from snow to permit the ascent, and then the natives are 



