CONTINENTAL PROBLEMS OF GEOLOGY. 167 



What caused the continental plateau? — The i)r()blein of tlie origin of 

 tlie continents remains almost untouched. Those who have pro- 

 ]>ounded theories for tlie formation of mountain ranges have sometimes 

 included <'ontinents also, but as a rule without adequate adaptation to' 

 the special conditions of the continental problem. So far as 1 ani 

 aware, the ••.ubject has been seriously attacked only by our second 

 ju'csident, Prof. Dana. He postulates a globe with solid nucleus and 

 molten exterior, and ])ostulates, further, local differences of condition, 

 in consequence of which the formation of solid crust on the liquid en- 

 veloi)e was for a long period confined to certain districts. In those 

 districts successive crusts were formed, which sunk through the liquid 

 envelope to the solid nucleus and by their accumulation built up the 

 continental masses. The remaining areas were afterward consolidated, 

 and subsequent cooling shrunk the ocean beds more than it shrunk the 

 continental masses, because their initial tem])eratures (at the beginning 

 of that process) were higher.* That the philosoj)hic mind may find 

 satisfaction in this explanation, it appears necessary to go behind the 

 second postulate and discover what were the conditions which deter- 

 mined congelation in certain districts long betbre it l)egan in others. 



Can it be shown that the localization of congelati(m, having been 

 initiated by an otherwise unimportant inequality, would be periiet- 

 uated by any of those cumulative processes which are of such im])or- 

 tance in various departments of idiysics? Andean it be shown that 

 siu'h a process of continent-buildiug" would segregate in the continental 

 tract certain kinds of matter and thus institute the conditions essential 

 to isostatic eqnilibrium "? To the first of these questions no answer is 

 ap]»arent, but I incline to the opinion that the second maybe answered 

 in the affirmative. If we assume the liquid envelope to consist of va- 

 rious molten rocks arranged in the order of their densities and if we 

 assume, further, that their order of densities in the li(iuid condition 

 corresponds to their ordei- ol densities in the solid condition, then the 

 successive crusts whose heaping built up the continents would all be 

 formed from the lightest material, and the isostatic condition would be 

 satistied. 



It was the fashion of the last generation of physical geographers to 

 study the forms of continents as delimited by coasts, seeking analogies 

 of continental forms with one anotiier and also with various geometric 

 figures, especially the triangle. The geiu^rali zations resulting from 

 these studies have not yielded valuable ideas, and the modern student 

 is apt to smile at the effVu't of his ])redecess()r to dis(M)ver the ideal geo- 

 metric ligure where th<^ unbiased eye sees only irregularity. Ibit barren 

 as were those studies i am not satisfied that their method was faulty, 

 and as a i)hysiograplier 1 have such a})pr(H',iation of the ideas that 

 sometimes grow from studies of form that 1 have attem])ted to apply 

 the old method to the new conception of the continental ])latcau. Con- 



*. lames I). Daua: Matnial of (iroldi/n, 2d edition, Now Yoilv, 1S74, p. 738. 



