750 GEORGE DAVIS LOUDERBACK 



tions from Strutt's measurements are to the effect that the earth is 

 either not appreciably coohng or is doing so with excessive slowness. 

 Post-Tertiary foldings can hardly be referred to such a cause, and 

 probably early Tertiary and even Mesozoic movements should be 

 excluded. On account of the fundamental similarity of the geological 

 history of the steps leading to the formation of the systems of folds 

 produced at different periods, it would seem reasonable to assume 

 very similar causes and to believe that, at least since the opening of the 

 Paleozoic,^ contraction due to cooling is as unnecessary to explain the 

 earlier as the later foldings. 



If radioactivity may be accepted as a sufficient cause for the pres- 

 ent thermal gradient, we must also conclude that no major contrac- 

 tional movements under the influence of self-gravitation have taken 

 place in recent geological time of such magnitude as to give rise to any 

 great addition of heat of mechanical origin. 



Leaving now the general aspect and turning to the particular sub- 

 ject of volcanoes, we may accept radioactivity as the cause of the 

 major part or all of the general internal heat, at least in late geologic 

 time, and still have all of the special problems of vulcanism to face 

 that have confronted those who assume other origins for the tempera- 

 ture gradient, and the same explanations might do equally well irre- 

 spective of the type of general hypothesis. 



But Major Dutton^ has put forward a more particular hypothesis 

 to the effect that volcanoes are caused by local excessive radioactivity 

 which melts small areas of rock near the surface and allows the ex- 

 pansive force of "water vapor to Hft its covering and force a way to the 

 surface." The rest of this communication is devoted to a discussion 

 of the points involved in this very interesting and ingenious hypothesis. 



The first point is the shallowness of the reservoir. Button esti- 

 mates that "most of the volcanic eruptions originate at depths between 

 one mile and two and one-half miles." The bearing of this point on 

 the local-radiation hypothesis of volcanoes is that at such shallow 

 depths the general earth heat cannot be invoked to produce the tem- 



1 The post-Archean (pre-Paleozoic) foldings are sufficiently different — especially in 

 their incomparably wide distribution and almost universal association with abundant 

 plutonic intrusive masses — to invite a special treatment. 



2 Loc. cit. 



