CONSTRUCTION OF THE EARTH. 



145 



very busy in making their pilgrimages to the mountain, 

 which they consider very sacred, and call " matchless." 

 The steepness of the sides of volcanoes depends on the 

 material of which they are formed, being steepest when 

 the material is cinders, and least so when it is lava. Most 

 frequently volcanic cones are of a mixed character, hav- 

 ing layers of lava and of consolidated cinders mingled to- 

 gether. This is represented in Fig. 74, which is intended 



Fig. 74. 



to be a section through some volcanic cones. The parts 

 made of short vertical lines represent the layers of lava, 

 and the long lines represent the layers of ashes and cin- 

 ders. Irregularities occur in the forms of volcanic cones 

 from various circumstances. For example, a wind pre- 

 vailing in one direction may make ejected cinders accu- 

 mulate on one side of a crater more than on another. 

 The edge of a crater may be thrown over, as in the case 

 of Vesuvius ( 195). Lava may burst out through a fis- 

 sure, and, solidifying, cause accumulation on one side of 

 the cone. 



229. Trap Rocks. Though these rocks have a decided 

 resemblance to volcanic rocks in their composition, the 

 eminences formed by them, some of which are lofty 

 enough to be called mountains, were constructed in a 

 very different manner. They are igneous, and yet not 

 volcanic. They were thrust up from below, a molten 

 mass pushing up the superincumbent strata before them, 

 and gradually became solid by cooling. They came up 

 through fissures in rocks, forming dikes, so called, in 

 these fissures. If the trap was not so hard as the rocks 



G 



