VOLCANOES. 93 



It is highly probable that the fusion and subsequent cooling of 

 granite, or gneiss, or even of the purer varieties of mixed sandstones 

 and clays, would make a trachytic lava ; while the fusion and cooling of 

 impure slates and shales and limestones would produce basaltic lava. 



Gas, Smoke, and Flame. — The gases emitted by volcanoes are princi- 

 pally steam, sulphurous vapor (S and S0 2 ), hydrochloric acid, and car- 

 bonic acid. By far the most abundant of these is steam. In violent, 

 explosive eruptions, which eject principally cinders and ashes, it is prob- 

 able that water, mostly in the form of steam, is one of the most abun- 

 dant of all the ejected materials. In quiet lava-eruptions, like those 

 of the Hawaiian volcanoes, the quantity of steam and gases is small. 

 It is worthy of notice, in connection with the position of volcanoes 

 near the sea, that the gases ejected are such as might be formed from 

 sea- water and from limestone. The so-called smoke and flame of vol- 

 canoes have no connection with combustion. The condensed vapors 

 and the ashes suspended in the air, often in such quantities as to make 

 midnight-darkness at high noon, form the smoke ; and the red glare 

 of the same, reflecting the light from the incandescent lava beneath, 

 forms the apparent flame. 



All volcanic ejections, except the gases, accumulate about the crater, 

 and continue to increase with every successive eruption, forming a sort 

 of stratified deposit. Sometimes the cone is made up of successive 

 layers of lava, as in Hawaiian volcanoes ; sometimes it is made up of 

 successive layers of cinders or tufa ; sometimes of alternate layers of 

 lava and tufa. Stratified materials of this kind, however, can not 

 be confounded with those produced by the action of water. In the 

 former case the stratification is not the result of the sorting of the' ma- 

 terials. 



Kinds of Volcanic Cones. — Volcanic cones and craters have been 

 divided into two kinds — viz., cones of elevation and cones of eruption. 

 A cone of elevation is formed by interior forces lifting the crust of the 

 earth at a particular point until the latter breaks and forms a crater, 

 through which eruptions take place. It is an earth-blister which 

 swells and breaks at the top. A cone of eruption, on the other hand, is 

 formed by the accumulation around a crater of its own ejection. There 

 has been much discussion among physical geologists as to whether 

 existing volcanic cones are formed mostly by the one method or the 

 other. We will not enter into this discussion. It seems probable, 

 however, that most cones are principally cones of eruption, although 

 their height and size have been increased somewhat also by elevating 

 forces. 



Mode of Formation of a Volcanic Cone. — A volcano commences — 1. 

 As a simple opening in the earth's crust, in most cases with little or no 

 elevation. Through this openiug or crater are ejected, from time to 



