SUBTERRANEAN HEAT. 83 



in Japan several well-marked waves had reached the coast 

 of California. 



The destruction occasioned by earthquakes is obvious 

 enough, but even the gradual sinking and rising of the 

 land contributes much to the wear and tear of the coast, 

 since by this means different parts and fresh surfaces are 

 brought within reach of the waves. The straits between 

 islands, for instance, are worn deeper and deeper as the land 

 gradually rises, until, when it has been lifted quite beyond 

 their reach, the straits become mountain passes, connecting 

 one valley with another.. 



It has been said that whatever the condition of the 

 interior of the earth, we have positive evidence that at 

 some spots not very far beneath the surface, the heat is 

 great enough to melt rock, for many a subterranean dis- 

 turbance, which begins only with the quaking of the earth, 

 ends with the pouring forth of liquid matter. 



A crack is made in some weak place, through which 

 large volumes of steam and other vapours are forced up, 

 with showers of red-hot ashes and streams of molten rock. 



The fall of these materials round the mouth of the hole 

 forms a cone-shaped mound, called a volcano, which usually 

 has a funnel-shaped opening, or crater. The pipe, or chim- 

 ney, which leads down into the interior of the earth, has 

 a hard stone lining, formed by the melted rock or lava, 

 which cements the loose ashes and cinders into a compact 

 mass wherever it comes in contact with them. 



It seems probable that much of the force by which 

 volcanic matter is driven to the surface, and shot up into 

 the air, is due to the conversion into steam of water which 



