HEAT— VOLCANOES. 685 



(4) Origin of the forms of volcanic cones ; (5) Subordinate volcanic 

 phenomena ; (6) Source of volcanoes. 



1. General nature of volcanoes, and their geographical 

 distribution, 



1. Volcanoes. — Volcanoes are mountains or hills, of a more or 

 less conical shape, in a state of igneous action, and consequently 

 emitting vapors, and occasionally melted rock or lava, with showers 

 of fragments or cinders, from a central opening called the crater. 

 They are conduits of fire, opening outward from within or beneath 

 the earth's crust. An extinct volcano is a volcanic mountain that 

 has ceased to be active, — the body with the fire out. 



The lavas flow out either over the edge or lip of the crater, or, 

 more commonly, through fissures in the sides or about the base of 

 the mountain. The cinders are thrown upward from the vent or 

 crater to a great height, as a jet of sparks or fiery masses, and fall 

 around in cooled particles or fragments, which are only granulated 

 lava : they may build up a conical elevation around the vent, or be 

 carried to a distance by the winds. 



When rain or moisture from any source descends with the cin- 

 ders, the mass forms tufa, — a stratified, somewhat earthy, granular, 

 and rather soft rock, of gray, yellowish-brown, and brownish colors. 



2. Geographical distribution. — Volcanoes occur (1) over the 

 border regions of the continents, — that is, the regions between the 

 oceans and the summit of the border range of mountains, as be- 

 tween the Pacific and the summit of the Kocky Mountains ; (2) in 

 the continental islands, or those near sea-coasts ; (3) in oceanic 

 islands, nearly all of which, excepting a few of very large size and 

 the coral islands, are throughout volcanic, — and the coral islands 

 have probably a volcanic basis. (4.) Volcanoes are mostly confined 

 to the borders of the larger ocean, the Pacific and the vicinity of 

 the seas separating the northern from the southern continents, 

 namely, the West Indies between North and South America, — the 

 Mediterranean between Europe and Africa, — the Eed Sea between 

 Asia and Africa, — the East Indies between Asia and Australia. 

 There are but few about the Atlantic, excepting those of the islands ; 

 and over the interior of continents, remote from the regions men- 

 tioned, they are almost unknown. 



(5.) Volcanoes are very commonly in linear series or groups. 



1. Borders of the Pacific. — The Pacific is almost completely belted with vol- 

 canic mountains. They occur in Fuegia, the southern extremity of the Andes; 

 in Patagonia; 32 in Chili, — that of Aconcagua 23,000 feet high ; 7 or 8 in Bolivia 



