LESSONS IN GEOLOGY. 329 



very liquid, while those of Vesuvius are viscous. The fluidity 

 modifies the flow of the lava. That from Kilauea flows like mol- 

 ten iron; at first white hot, with a temperature of more than 

 2,000 F., it soon becomes red, growing darker till it assumes a 

 black, cinder-like appearance. The viscous lavas of Vesuvius 

 break in rough brown slags, which grind and grate over each 

 other with a metallic sound. Occasionally the lava flows over the 

 edge of the crater, but it often breaks through the side of the 

 mountain, sometimes 2,000 or 3,000 feet below the surface of the 

 lava in the crater, forming fountains of molten rock. Lava that 

 is completely fused when cooled quickly forms volcanic glass 

 obsidian; when cooled slowly crystals are formed and stony lavas 

 result. In most lavas the crystallization is quite complete before 

 the mass becomes solid. 



A volcanic cone is, to a great extent, the work of the volcano, 

 and its composition and form depend on the materials thrown 

 out by eruptions. If the product is fluid lava, as in the case of the 

 Hawaiian volcanoes, the cone will be broad and flat. If the lava 

 is viscid the cone will be steeper and narrower. If the product is 

 chiefly ashes, scoriae and lava, the cone will be steep. The cones 

 formed by successive eruptions have somewhat the structure of 

 an exogenous tree. Sometimes the cone has been ruptured by 

 radiating fissures, and these filled with lava dikes. Secondary 

 cones are frequently formed inside the craters or on the sides of 

 the main cone; there are some 200 such cones on the slopes of 

 Mount Etna. 



Volcanoes are mostly located in the mountain regions along 

 the shores of the Pacific ocean, and many of the islands of that 

 ocean are volcanic. Some are located about the Mediterranean 

 Sea, in the West India islands and elsewhere. 



The causes of volcanic action are not understood, but many 

 theories have been advanced to account for the phenomena. It 

 is generally agreed that the expansive force of steam is the chief 

 force of eruption, but how the water reaches the molten rock is 

 not explained. The source of the volcanic product and of the 

 volcanic energy cannot be deep, as water could scarcely penetrate 



