100 PROCEEDINGS OF THE AMERICAN ACADEMY. 



h3^potliesis. On any other hypothesis it is hard to explain the fact 

 that the pipes of moderately large cones are about as large as those of 

 the very greatest cones. In all cases there seems to be a limital size 

 and that is controlled by the available heat supply along the axis of 

 the vent. The size is small because the (indirect) fusing power of ema- 

 nating gas must be strictly limited. Moreover, the cylindrical shape 

 of each typical pipe is a solutional or fluxing form. (Figure 10.) 



Explosive types : Magmatic and Pkreatic. 



The foregoing genetic statement for the Hawaiian vents has been 

 sketched in terms of a quite general process and it is necessary to 

 glance at the relation of the hypothesis to the explosive type of 

 central eruption. 



Volatile matter occurring in the rocks of the contact-shell about any 

 intrusive magma must show increased tension. If the intrusion is 

 large and near enough to the earth's surface, this tension may lead to 

 explosion in the roof of the igneous body. In case no incandescent 

 matter is extruded, the explosion is not volcanic according to our defi- 

 nition of that term (p. 48). Following Suess, it may be called pkreatic. 

 A similar explosion may happen as a result of the slow conduction of 

 heat from the conduit of a long dormant volcanic cone. Such a cone 

 is normally porous. Rain-water, snow-water, or sea-water is trapped 

 in the vesicular flows and loose tuffs, as these are in turn buried during 

 the original growth of the cone. The circulation of vadose water is also 

 facilitated by this special porosity. 



The suggestion of Suess that the remarkable explosion at the famous 

 Kieskessel was of phreatic origin has been supported by the detailed 

 studies of Branco and others. ^^ Purely geological studies had indicated 

 the presence of a large laccolithic mass beneath the great Ries depres- 

 sion. That conclusion has been brilliantly supported by the magnetic 

 studies of Haussmann in the region. The local disturbances of the 

 needle in dip and azimuth can be explained, according to Haussmann, 

 only by the assumption of one or more large subterranean bodies of 

 basic rock. 46 In the Rieskessel itself the upper surface of the basic 

 rock is calculated to be no more than two kilometers deep. Outside 

 the depression, its average depth was estimated at five kilometers. 

 Since the visible floor of the Ries is the granite of the " Grundgebirge," 



" W. Branco, Abhand. kon. preuss. Akad. Wiss. Berlin, 1902, p. 14. 



** K. Haussmann, Abhand. kon. preusa. Akad. Wiss. Berlin, 1904, Abt. IV, 

 p. 137. Sauer has suggested that the liparite of the Kieskessel tuffs is due to 

 the melting of the intruded granite by the basic magma. See W. Branco and 

 F. Fraas, Abhand. kon. preuss. Akad. Wiss. Berlin, 1901, p. 54. 



