RELATION OF RADIOACTIVITY TO VULCANISM 751 



peratures involved, and we must therefore assume some special heat- 

 ing agent in the cooler portions of the earth 's crust. For by the nor- 

 mal temperature gradient 1,000° C. would be reached at not much 

 less than about twenty miles ^ below the surface. 



It would seem that the general chemical characters of lavas would 

 be fatal to this contention. If the material extruded from volcanoes 

 was chiefly derived from the melting of rock at the depth of from one 

 to two and one-half miles, or even considerably deeper, the reservoirs 

 would be largely within the zone of sediments, and lavas should fre- 

 quently have a composition derivable from the major sedimentary types 

 alone or mixed in various ways. But igneous rocks have a rather 

 definite range in chemical composition, and there are general and 

 important differences between igneous types and at least the major 

 sedimentary types. The chief reasons for these differences are easily 

 explained by the nature of the processes leading up to sedimentation. 

 The materials of the igneous rocks are worked over and sorted on Hnes 

 of mechanical and chemical resistance, etc. , and from them are leached 

 much or all of the more soluble materials, a large part of which passes 

 down and becomes a permanent constituent of the waters of the sea,- 

 a corresponding deficit appearing in the composition of the sediments. 



Petrologists have recognized for some years that lavas in general 

 cannot be considered as derived from molten sediments, and yet, if the 

 reservoirs were limited to the first few miles of the crust, a large pro- 

 portion of them should have such origin. The abundant evidence 

 gathered along this line demands that lavas be derived entirely from 

 below the .zone of sedimentary rocks. 



Volcanoes which arise from the deep sea cannot be considered 

 as affected one way or the other by this line of argument, for we know 

 nothing of the chemical nature of even the shallow layers of the sub- 

 oceanic crust. Furthermore, there are continental volcanoes that do 

 not occur in sediment-covered provinces. Of these probably the 

 most common are found on granitoid bedrocks. Many of the Tertiarv 

 volcanoes of the Sierra Nevada were of this type, the lavas over con- 

 siderable areas breaking through granitoid rocks, with only here and 



I According to Strutt's curve, based on a uniformly radioactive crust 45 miles deep 

 6verlying a non-radioactive interior, 1,000° C. would be reached at about 18 miles, 1,200° 

 at about 23 miles. 



