RELATION OF RADIOACTIVITY TO VULCANISM 753 



The zone of rock fiowage, the characters^of which have been so 

 well set forth by Van Hise, is probably not completely established 

 until we get considerably below the depths where Button would place 

 lava reservoirs. One indication of this is that earthquakes have their 

 foci at those and apparently often at considerably greater depths. 

 Earthquake foci must be limited to the zone of fracture, for when 

 with increasing "hydrostatic" rock pressure and temperature the 

 strength of the harder rocks is well passed, we can hardly imagine a 

 dislocation or other diastrophic movement that would be accompanied 

 by seismic jars. 



The depth to which certain sediments can be buried and then 

 deformed without metamorphism — several miles, as shown by the 

 thick Cretaceous formations of California and Oregon — indicates that 

 the zone of chemical plasticity, as we may call it — the zone where read- 

 justments take place by molecular (or atomic) interchange and 

 recrystallization without rupture and yet without melting — is con- 

 siderably below the limit set for volcanic reservoirs. But sediments 

 and intercalated lavas have not uncommonly descended into this zone 

 of chemical plasticity (as, for example, the Paleozoic-Mesozoic Bed- 

 rock complex of the Sierra Nevada, and other "regionally metamor- 

 phosed" formations), and indeed many thousands of feet below the 

 upper limits of this zone, representing a burial beneath the surface 

 of probably from 5 to 10, or possibly more, miles, and while there they 

 are sometimes invaded by intrusive rocks from still farther down — 

 rocks of a batholitic character, rising from a region wherein all of the 

 necessary heat may easily be derived from the general interior supply. 

 For, according to the curve presented by Strutt, 500° C. would be 

 reached at about 8 miles, 600° at about 11 miles; and these tempera- 

 tures are quite probably sufficient for granitic aqueo-igneous fusion. 

 But, after these beds have been brought up again and exposed by 

 deep erosion, we see no evidences of local reservoirs formed within 

 them, though they may be cut through to their bases by various, often 

 basic, dikes which may also cut down to an undeterminable depth 

 through the granites. 



Button suggests that, if an eruption occurred from a depth of 

 5 or 6 miles, "the temperature of the lava would be very high — 

 probably a white heat — and the mass would be very great. Its conse- 



