ORIGIN OF VOLCANIC MINERALS. 177 



which elevates mountain chains, is one of the conditions of the linear 

 form of volcanic activity. 



Origin of the Eruptive Power of Volcanoes. We next have 

 to inquire into the source of the explosive and eruptive power of 

 volcanoes, for the phenomena do not permit a belief that, the 

 materials are merely extravasated through a crack by the uplifting 

 force of lateral pressure. It has already been seen that volcanoes 

 exist in positions where superincumbent pressure on the mass below 

 must have been relieved by the formation of cracks penetrating from 

 above, and the researches of Mr. Hopkins and Professor James 

 Thomson taught us that, if a mass is greatly heated and kept in the 

 solid state by pressure, it will become liquid as that pressure is 

 removed. But it will not become eruptive ; and the first sound 

 explanation of the eruption was due to Mr. Poulett Scrope, who 

 attributed it to the influence of water. This conclusion is almost 

 inevitable, considering what enormous masses of steam are discharged 

 from volcanic vents during eruptions, and how the vapour given off 

 from Stromboli forms a constant cloud or mist above the island, 

 while it may even give rise in Polar regions to showers of snow. 1 

 It is well known that in a closed vessel water may be made white 

 hot without being converted into vapour; and if we suppose the 

 water from the sea to penetrate down such fissures in the neighbour- 

 hood of volcanoes as have been suggested, then, heated beneath the 

 surface by contact with rocks at a high temperature, it would escape 

 by the path where the pressure was least, flashing into steam with 

 explosive energy as the pressure disappeared. 



Influence of Water on kind of Rock Ejected. We have already 

 seen how thoroughly some granitic rocks have their crystals saturated 

 with water, which was included and caught Up at a high temperature ; 

 and when this is borne in mind, it will be understood that heated rock 

 has the power of dissolving vast quantities of water which produce 

 many changes in its substance. The researches of Daubre"e show that 

 when common glass is raised to a temperature of 400 C., in the presence 

 of its own volume of water, it swells up and changes into a mass of 

 crystals of wollastonite ; while the alkali is separated, and the excess 

 of silica crystallises in the form of quartz. When the glass thus acted 

 on contains oxide of iron, the wollastonite is replaced by diopside. 

 Similarly, it was found that the volcanic glass obsidian, when thus 

 treated, produced crystals of felspar, and was changed into a rock 

 like trachyte. 2 And when kaolin was heated with a soluble alkaline 

 silicate and cooled, the mass was converted into crystalline felspar, 

 and quartz. It is thus seen that water plays a very important part 

 in the formation of volcanic minerals, as well as in the actual 

 eruption. 



Mr. Scrope inferred that the presence of water would render the 

 particles of mineral matter easily movable upon each other, and that 

 the rise of lava in a volcanic vent is occasioned by the expansion of 



1 Scrope's "Volcanoes," 2d edition, 1862, p. 38. 



2 Sterry Hunt, " Chemical and Geological Essays," p. 6. 

 VOL. I. M 



