Correspondence — Mr. Arthur Vaughan. 95 



To illustrate the great importance of the time factor, I will assume 

 the extreme case, that entire rupture is immediately produced all 

 round an isolated suboceanic area, in spite of the natural effect of 

 the viscous friction beneath. The mechanical problem is then 

 reduced to the following : An inner nucleus of highly heated 

 material is surrounded by a spherical shell of rock. From this 

 outer shell a spherical cap is cut out, reduced in area, and replaced, 

 so that it does not quite fit and is in consequence entirely separated 

 from the rest of the shell ; every particle of shell and cap being 

 subjected to the attraction of gravity tending towards the centre 

 of the sphere. 



The forces acting upon the non-fitting cap are simply its weight 

 and the pressure of the subjacent nucleus ; and it is to be noticed 

 that, in the actual case, the weight of the suboceanic crust is almost 

 certainly greater than that of the continental (see Pratt, '' Figure 

 of the Earth"). 



In the case of any portion of the rest of the shell the forces are : 

 weight, resistance of nucleus, together loith the supporting com- 

 pression at its boundaries, arising from the fact that the area forms 

 part of a continuous shell. 



Hence the pressure on the nucleus will be greater under the 

 suboceanic than under the continental crust. Thus, remembering 

 that there is practically no limit to the time factor, transfer of 

 material will take place slowly from beneath the oceanic to beneath 

 the continental regions, and this the more easily on account of the 

 high temperature at which the subjacent rocks exist. 



I believe that I have now answered Mr. Eeade's objections, which 

 seem to me somewhat hastily conceived and dogmatically expressed ; 

 but I must confess to being unable to understand how the last 

 sentence in his letter has any bearing on the points at issue. He 

 says, alluding to my theory : " It seems to me veri/ like the case 

 of a man trying to lift himself up by pulling at the chair he is 

 sitting upon." I cannot answer this objection, because I cannot 

 imagine which is the man, nor which the chair beneath him ; nor 

 can I see how one is attempting to pull the other. As Mr. Eeade 

 has never made any reply to my criticism of the theory which he 

 advocates, may I definitely ask him how he explains the under- 

 mentioned geological facts ? 



It is well known that, during many periods, great thickness of 

 sediment have continuously accumulated, every part of which must 

 have been laid down in shallow water ; and that contemporaneously 

 the area of denudation has been rising. 



Does it not follow that, if two miles (say) of rock has been 

 deposited continuously under shallow-water conditions, that the sea- 

 flow must have been depressed through that distance, and that, 

 consequently, an equivalent amount of subjacent rock has been 

 squeezed out ? I entirely fail to see how this is in accordance 

 with Mr. Reade's theory, that, as sediment accumulates, the area 

 of sedimentation rises ; for, considering the slow rate at which 

 deposition goes on, the elevational elffects (if any) due to accession 

 of heat must be contemporaneous with sedimentation. 



