370 Prof. J. D. Dana on some Results of 



this density would determine the limit to which (setting aside 

 resistance from viscidity) they would sink. It may be that 

 portions went down until they came into contact with the nu- 

 cleal solid mass. As the crust sank, the liquid material ad- 

 joining would continue to flow over the solidifying area and 

 add to the solidifying material. 



Finally, a layer of crust-rock miles in thickness would have 

 been made over the great continental areas. Throughout the 

 other portions of the sphere, the surface, whether all liquid or 

 in incipient solidification, would have the level of that of the 

 continental areas. For the sake of the illustration, suppose 

 them to have been all liquid and the continental crust twelve 

 miles thick, and the oceanic areas to go through the same pro- 

 cess of solidification as had been completed over the continental 

 areas ; when, finally, the material of the oceanic regions had 

 solidified down to the same plane with that of the continental 

 (that is, to the twelve-mile limit), the oceanic crust thus formed 

 would have become depressed in the consolidation (on the above 

 ratio of 8 per cent, less volume for the liquid than for the solid) 

 5000 feet, or, if the layer consolidated was thirty-six miles thick, 

 15,000 feet — that is, supposing the continental part to undergo 

 no contraction during the time. As such contraction would 

 have been in progress from the continued cooling, the above 

 5000 feet is not the actual depth the basin would under the 

 supposed circumstances have acquired ; and yet, since the change 

 of volume in the cooling of solid rock is small, it is not very 

 wide of the fact. 



The case here supposed is partly hypothetical, because the 

 condition over the oceanic areas when the solidified crust of the 

 continental areas was completed may have been that of incipient 

 solidification, so that some of the contraction had already taken 

 place. But, apart from this, it represents correctly, as it appears 

 to me, the steps in the process, and illustrates how it is that 

 great depressed areas would be an inevitable result, and 

 why they should have comparatively abrupt sides or a basin- 

 like character. The present mean depth of the oceanic areas 

 below the mean level of the continental plateaux is probably 

 about 16,000 feet. The thickness of the layer of liquid rock 

 required to make a depression of 16,000 feet by its consolida- 

 tion would be about 38J miles. But as contraction has gone on 

 through time over both continental and oceanic areas, this is 

 the mean excess of depression for the oceanic area. What part 

 of this- excess existed when the oceanic depression was first made, 

 there are no facts for satisfactorily deciding. If the coral-island 

 subsidence was due in any considerable part to radial contrac- 

 tion beneath the Central-Pacific crust itself, it is probable that 

 the excess has increased even in Csenozoic time. 



