GENERAL DI^CJUSSION 278 



have pierced through solid rock or massive beds for the greater part of 

 their course. In the cases of the Zolfo, Sciarra, and Torreone vents the 

 conduits must reach almost, if not quite, up to the surface. The Central, 

 Scarp, and Fumarole groups of vents, as has been mentioned above, oc- 

 cupy a portion of the crater terrace which has been especially liable to 

 profound explosive action, resulting in the occasional formation in this 

 part of the terrace of a deep and extensive funnel, which is subsequently 

 filled up with loose and little-compacted material. Thro-ugh such an 

 overburden the lava and its contained gases coming up from a fixed vent 

 ijelow would naturally tend to shift position somewhat from time to time 

 and become at the surface a diffuse group of vents rather than a single 

 one. 



2. From the persistence in location it may be reasonably assumed that 

 tiie lengths of the conduits, and consequently the depth of the top of the 

 magma reservoir below the crater floor, must be very considerable — pre- 

 sumably of the order of hundreds or, more probably, thousands of meters. 

 For, as has been stated, were the conduit of an area commensurate with 

 that of the crater and with its top near the terrace, the volcanic activity 

 would most presumably, in the course of a century and a half, have 

 broken out in vents at various points on the crater terrace, instead of 

 showing the fixity of position which we have observed. The method by 

 which this fixity of position is maintained will be discussed later. 



3. Both the persistence in location and the relatively small size of the 

 vents, which vary in diameter from about 10 to 60 meters (the diameter 

 of the visible orifice being almost certainly greater than that of the con- 

 duit below), as well as the persistence in small size, indicate that there 

 has been during their existence but little melting, erosion, or assimilation 

 of their walls by the lava and gases which have passed up through them. 

 This lack of action on the walls of the conduit (after its formation) may 

 be ascribed to the fact that the ascending lava, owing to the presence of 

 gases and water vapor in solution, would have a lower fusion temperature 

 than that of the already solidified basalt of the conduit walls, which is of 

 about the same chemical composition, but without the gases and water 

 vapor. Interreactions between the gases would tend to maintain the tem- 

 perature and liquid condition of the lava column, but would not neces- 

 sarily or probably raise its temperature above that of the fusion tempera- 

 ture or temperature-interval of the surrounding solid rock.^^ 



In this connection it may be mentioned that the features of the vents 

 just noted and the explanation here suggested are opposed to the idea ad- 

 vocated by some,^^ that a large part, if not all, of the water vapor present 



*5 Cf. Sosman and Merwin : .Jour. Wash. Acad. Sci., vol. iii, 1913, p. 380. 

 « Cf. J. P. Iddings : The problem of volcanism, 1914, pp. 1G9 ff. 



