228 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



though sometimes they show sympathy without showing evidence of 

 liquid connection. The sources of lava seem to be meager in general, 

 and the eruptive agencies seem to be controlled by narrowly local 

 conditions. There is an absence of evidence that the lavas in the 

 craters or in the necks of volcanoes are parts of great liquid masses 

 below, responsive to the common stresses of a large region. 



Thus geological evidence, when critically scrutinized, seems to be 

 distinctly adverse to the existence of even large reservoirs of molten 

 matter within the earth ; it points rather to the presence of scattered 

 spots, very small relatively, on the verge of liquefaction, which pass 

 by stages into the liquid form and are then forced out by the dif- 

 ferential stresses that abound in the earth body, or are embodied in 

 the liquid itself, each such local liquefying center commonly giving 

 forth driblets of lava and gas at intervals, none of which often rise 

 to more than an extremely minute fraction of the earth mass or even 

 of the subterranean mass contiguous to the volcano. 



A revised view of the nature and location of earth stresses seems 

 also to be required by what is now know of earth conditions. Under 

 the former dominance of the tenet of a molten globe it was natural 

 to assign to the stress differences of the earth a distinctly superficial 

 localization and limitation; they were thought to be affections of 

 " the crust " almost solely. Hydrostatic pressures were of course 

 recognized as affecting the deep interior, but these were obviously 

 balanced stresses, and were ineffective in deformation. The stresses 

 supposed to give rise to the great reliefs of the earth's surface were 

 thought to be very superficial. But the stresses imposed by known 

 deformative agencies are not all superficial, nor are their intensities 

 always greatest at the surface. According to Sir George Darwin, 

 the stress differences generated in the earth by the tidal forces of 

 the moon are from three to eight times as great at the center of the 

 earth as at the surface. So, also, according to the same authority, the 

 stresses engendered by changes in the rotation of the earth are from 

 three to eight times as great at the center as at the surface and are 

 graded between center and surface. The tidal stress differences are 

 relatively feeble but are perpetually reneAved in pulsatory fashion. 

 Those that arise from rotation belong to the highest order of com- 

 petency. The stress difference that would arise at the center of the 

 earth from a stoppage of the earth's rotation would, according to 

 Darwin, reach 32 tons per square inch. Changes of the rate of rota- 

 tion are almost inevitable when great diastrophic readjustments take 

 place. Such periods are to be regarded as critical times at which 

 great floods of lava should be poured forth from the interior if liquid 

 material were there in great volume ready to respond to the changes 

 of capacity which the deformations of the earth's sectors and the 

 changes in the spheroidal form would inevitably impose. 



