328 PRACTICAL LESSONS IN SCIENCE. 



usually an opening somewhere on the line of such a fissure, around 

 which a somewhat conical hill has been built up from the ma- 

 terials thrown out. 



In a volcanic eruption gases and vapors are the first materials 

 to appear; they continue during the eruption of other material, 

 and often for centuries after all other evidence of subterranean 

 action has ceased. Aqueous vapor is probably most abundant; 

 it is sometimes given off in such quantities as to result in heavy 

 rainfall. Among the gases the most abundant are hydrochloric 

 acid, sulphuretted hydrogen, sulphurous acid, carbon dioxide, 

 free hydrogen and others. The typical volcanic material is mol- 

 ten rock called lava, which includes material varying considerably 

 in composition and specific gravity. The lighter varieties contain 

 a larger per cent, of silica, and are acid lavas, as trachyte and 

 pumice. Others are basic lavas containing less silica and more 

 iron and pyroxene, as basalt and leudte lava. Others again are 

 intermediate between these groups. Lavas also vary in structure ; 

 some are wholly crystalline, as some liparites ; some show a glassy 

 or stony ground mass, with imbedded crystals, which is, perhaps, 

 the most common structure. Again, others are pure volcanic 

 glass, as obsidian. Then some, as basalt, are dense and compact, 

 while others, as pumice, are vesicular. 



Great quantities, a fine ash-like dust and coarser particles, 

 called sand, usually accompany eruptions. The dust and sand 

 are lava pulverized by escaping vapors and gases. 



The quantity of dust formed and thrown out is sometimes 

 enormous. Dust from volcanoes in Iceland has fallen abundantly 

 in Sweden and Holland, and fell in such quantity at a distance of 

 600 miles as to destroy crops. 



Large blocks of lava are frequently ejected. Water is some- 

 times a volcanic product, which, mingling with the dust and 

 sand, forms a mud stream, and mud itself is a product of some 

 volcanoes. A good idea of the flow of lava, and of the peculiar 

 forms it often assumes, may be obtained by watching the flow of 

 slag at a blast furnace. 



Lavas differ in their fluidity ; those of Kilauea in Hawaii, are 



