738 DYNAMICAL GEOLOGY. 



on slow solidification, often take a parallel position, so it might have 

 been in the cooling crust. This appears the more probable, when it 

 is considered with what extreme slowness the thickening of the crust 

 has gone on, and the immeasurable length of time it has occupied. 



6. Formation of Continents and Oceanic Basins. — The earth's crust 

 rises over large areas into plateaus or continents, leaving, between, a 

 depressed area, of much larger extent, occupied by the ocean ; and 

 the depression has rather abrupt sides against the plateaus (p. 11). 

 These plateaus show by their position thus sustaining the infer- 

 ences from geological history (p. 160) — that they were the parts of 

 the crust which first stiffened, in the gradual cooling of the exterior, 

 and that the oceanic basins are due to a subsequent consolidation of 

 the areas they occupy, the attending contraction carrying them below 

 the level of the previously solidified continental areas. 



The crust over the first solidifying areas, — now the continents, — after attaining a 

 thickness that would enable it to overcome, by its gravity, the cohesion in the liquid 

 rock beneath, would have sunk in masses, and then have been remelted by the heat 

 beneath; and this remelting would have cooled somewhat the liquid layer. So, this 

 process of crusting and sinking, with an overflow from either side, remelting and cool- 

 ing, would have gone forward until the masses could sink without much remelting, 

 to bring up at the level where the density of the liquid layer was that of the solid 

 rock, if this liquid layer had not become so stiffly viscid by the cooling as to offer too 

 great resistance to their reaching quite to this level. The sinking rock-masses may 

 have had their density somewhat increased, by the pressure to which they were sub- 

 jected on descending: but, whatever density they acquired, this density would deter- 

 mine the limit to which, setting aside resistance from viscidity, — the}' would have 

 sunk. It may be that portions went down until they came in contact with the nucleal 

 solid mass. As the crust sank, the liquid material adjoining would have continued to 

 flow over the solidifying area, and to 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 had 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 process 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 ratio of 

 8 per cent, less volume for the solid than for the liquid), 5,000 feet; or, if the layer 

 consolidated were thirty-six miles thick, 15,000 feet; that is, supposing the continental 

 part to have undergone no contraction during the time. As such contraction would 

 have been in progress, from the continued cooling, the above 5,000 feet is not the actual 

 depth which 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 

 from 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 the steps in the process, and illustrates 

 how it is that great depressed areas would have been an inevitable result, and why 

 they should have had comparatively abrupt sides, or a basin-like character. The 



