FRAGMENTATION BARRELS 295 



much as 1,000,000 square miles and attain a volume of 12,000,000 

 cubic miles. Even though these quantities are vast, they are not at 

 all unreasonable in proportion to the areas concerned. There is 

 reason to believe that the thicknesses beneath the Lake Superior Basin 

 are as great, though not the area of the intrusions, and the volume 

 of extrusions within that limited region has been estimated at 24,000 

 cubic miles. Even so, this amount is probably only a fraction of the 

 volume of the associated intrusions. The ratio of the volume of the 

 Lake Superior basic rocks of Keweenawan time, to those of the 

 North Atlantic in the Tertiary era is therefore no more than in pro- 

 portion to the areas involved. 



To produce subsidence in central Asia through basic intrusion, 

 several conditions would have to be met, as shown in Figure 1. First, 

 the material would have to come from below the level of isostatic 

 compensation in order to make the crust above that level heavier, as 

 a whole, than it was before. The crust, or lithosphere, within which 

 isostatic compensation exists is a shell of strength. In order, there- 

 fore, that isostatic adjustment shall take place, it must be underlain 

 by a thick shell of weakness, the asthenosphere. This sphere is con- 

 ceived of as highly rigid under vibratory stresses, but yielding read- 

 ily under prolonged ones, probably by a process of recrystallization 

 as in glacial floAv. It is presumably then very close to the temper- 

 ature of fusion, as this is the physical condition which permits in 

 this shell the coexistence of high rigidity and low permanent strength. 

 Basic magmas originating in its deeper part could, therefore, by aid of 

 the smelting, drilling, and disruptive pressures of their included gases 

 and the notably decreased density attending the fluid state, force 

 their way upward into a colder and stronger shell of lower specific 

 gravity than the solid phase of the intrusive. 



The decreased density of the fluid condition over the solid phase 

 of the identical magma is about the same as the difference in density 

 between solid granite and solid gabbro. As long, therefore, as the 

 intrusive was in a fluid state, there would be little or no tendency 

 for net subsidence of the crust, a condition which probably still exists 

 beneath Iceland, as shown by its geysers and active volcanoes. The 

 crystallization of the magma and the dissipation of the excess heat 

 of the thick intrusive masses would require a time comparable to the 

 length of a geologic period. During this time it would slowly be- 

 come denser; but, in proportion as this downward drag made itself 

 felt, the crust, which was temporarily weakened by the intrusions, 

 would heal and grow stronger. The subsidence, on the other hand, 

 would require a slow lateral displacement of material in the astheno- 

 sphere. In consequence, the breaking down would be shown on 

 the surface by great fractures and downfaultings. 



