410 Jones — Review of Chamberlin's Groundwork 



to give rise to such a degree of gaseous volcanism as is 

 actually manifested. ' ?38 



The first statement quoted needs some qualification, 

 for the amount of gas set free is not only a question of 

 heat but also of pressure. Very evidently under the 

 pressure conditions existing at no great depth very 

 material quantities of gas would remain entrapped. 



The second statement quoted also requires some dis- 

 cussion for it is doubtful if the major manifestations of 

 volcanism require any such great stores of gaseous 

 material as are implied. By far the larger part of vol- 

 canism has been expressed in fissure eruptions and deep- 

 seated intrusions. The mechanisms of both of these do 

 not call into use any mechanical force from gaseous 

 expansion. In fact, the greater lava floods are very often 

 free of vesicularity. Volcanic eruptivity of the central 

 vent type is not conspicuous in the igneous record of 

 pre-Tertiary time. Only in late-Tertiary and recent 

 times have the dying phases of an era of great vol- 

 canism expressed themselves in vent eruptivity on 

 what may possibly be called a worldwide scale. 

 Such is to be expected under the conception of a 

 slowly thickening crust. Furthermore, volcanic vent 

 eruptions call into use only localized concentrations of 

 the juvenile gases in their underlying diatremes or 

 cupolas. 



The broad facts of the occurrence of igneous rock types 

 become extremely difficult of explanation under the con- 

 ception of an earth composed throughout of material 

 similar to the visible rocks from which magmas are 

 derived by deep-seated selective liquefaction. Chamber- 



lin states 39 " if magmas consist merely of partial 



solutions of heterogeneous mixtures, they would quite 

 certainly become highly diversified in the making. The 

 primary problem would then lie in the generation of the 

 magmas; in the ascent of magmas rather than their 

 descent. While differentiation in the process of solidi- 

 fication would still remain a factor, it would be a second- 

 ary matter, in the sense that it was necessarily condi- 

 tioned by the previous generative process* Each 



38 Bull. Geol. Soe. Am., vol. 32, p. 207, 1921. 



39 Op. eit., p. 208. 



*° 'Die italics are mine — W. F. J. 



