156 EDITORIAL 



earth do not altogether hold if the earth was built up by gradual 

 accretions and was solid at all stages. 



If the earth was once molten, it is a fairly sure inference that 

 all the deeper portions were affected by essentially the same spe- 

 cific gravities at the same distances from the center. Only at and 

 near the surface is it probable that there was much differentiation of 

 specific gravity in the original liquid spheroid, if indeed much even 

 there. If, therefore, the geodetic determinations were to show that 

 the oceanic surface is nearly spheroidal, and the pendulum observa- 

 tions were to show that the crust is in approximate isostatic equi- 

 librium in a general way, neglecting local inequalities, it is a firm 

 inference that there must be a rather marked difference in the spe- 

 cific gravities of the sub-oceanic and of the continental portions of 

 the crust respectively, as Professor Schwarz has indicated, for the 

 sub-oceanic crust, plus the water that is on it, must counterbalance 

 the continental protrusions. The amount of this difference in 

 specific gravity must depend on the depth to which the differentiation 

 0} rock extends. For example, if the continental protrusions be 

 taken to average three miles in height above the oceanic bottoms, 

 and the lower limit of differentiation of specific gravities be assumed 

 to be reached at a depth of six miles below the average surface, 

 the specific gravity of the differentiated portion of the sub-oceanic 

 crust, three miles thick, must be enough, with the aid of the over- 

 lying water, to counterbalance the six miles of the differentiated 

 continental shell, from which it is obvious that a very high specific 

 gravity for the sub-oceanic rock is required. If the differentiated 

 portion of the crust were no thicker under the continental surfaces 

 than under the oceanic, the differences of specific gravity would be 

 still greater. If the limit of differentiation be taken at the greater 

 depth of nine miles below the average surface, six miles of rock and 

 three miles of water on the oceanic side must counterbalance nine 

 miles of rock on the continental side, in which case a less, but still 

 a pronounced, difference in specific gravity is required to meet the 

 conditions of the case. For any such moderate depth of differentia- 

 tion as would probably arise in the development of a crust upon a 

 molten sphere, a marked difference between the specific gravities of 

 the sub-oceanic and of the continental rocks seems to be implied, 



