754 GEORGE DAVIS LOUDERBACK 



quences might be disastrous beyond all precedent." He also doubts 

 the possibility of a magma at 5 or 6 miles erupting at all. It seems, as 

 stated above, that many Tertiary volcanoes received their material 

 from beneath that depth of sediment. With present volcanoes it is 

 impossible to say from what depth their magmas arise. One might 

 be inclined to suspect, for example, that Mauna Loa and its com- 

 panions, arising in a regular curve from the fiat floor of the sea, 

 received their molten rock from below the level of the ocean bottom, 

 and, if so, as the mass rises 30,000 feet above this base, a lift of over 

 6 miles, followed by extrusion, would seem possible without any ter- 

 rific outbreak. But Major Button explains this by considering that 

 while the lavas were piling up the whole mass has been rising, ' and 

 the reservoir is in the protruded mass and considerably above the sea- 

 floor. 



The chief argument put forward for the shallowness of lava reser- 

 voirs is that volcanic earthquakes always have shallow foci. This 

 is probably true. But whether they be due, as Button supposes, to 

 the "fracturing or sudden yielding of the rock masses imediately above 

 the lava reservoir," or whether they be due to gas or steam explosions, 

 as commonly believed, we would expect them to be shallow. For the 

 conditions which would make explosions possible (such as sudden 

 relief of pressure by fracturing of solid obstructions and the conse- 

 quent explosive expansion of water vapor, etc.) would probably only 

 be found near the surface, however deep the magma originated, and 

 fracturing could only take place in that superficial zone suggestively 

 named the zone of fracture. The occurrence of such phenomena at 

 slight depths seems therefore to have no bearing on the depth of the 

 lava reservoir. 



While the above considerations are believed to be incompatible 

 with the particular form of volcanic theory under discussion, and with 

 any theory postulating such shallow reservoirs, and while a pushing- 

 down of the reservoirs toward the 15 or 20-mile limit makes less urgent 

 and soon unnecessary the demand for a local special heat supply, the 

 possibility of local radioactivity as an explanation still remains. But 

 there are some considerations which appear to make improbable any 

 theory of local strong radioactivity. 



^ See U. S. Geological Survey, Fourth Annual Report, p. 195 (1884). 



