520 T. C. CHAMBERLIN 



rather to steepen the seaward gradients of the coasts than to 

 flatten them; tend rather also to deepen the adjacent ocean than 

 to shallow it. Theoretically this would seem quite certainly true 

 if any tenable form of isostasy is effectual in diastrophism, and of 

 this there seems less ground for doubt as inquiry goes on. On the 

 observational side, the frequency of fore-deeps and of notable 

 depressions off the continental edge not definitely shaped as fore- 

 deeps seems to support this. It may be assumed, therefore, with 

 much probability, that from Cambrian time onward topset and 

 foreset action kept rebuilding the form of the circum-continental 

 terraces into slopes that lay within their own normal range of 

 variation — diastrophism aside — and that the mean ocean depth 

 bordering the continents was not less than the normal mean depth. 

 It may further be assumed with high probability that the mean 

 effect of diastrophism, when it intervened, was to increase the 

 border slope and the border depth, while it compacted, folded, and 

 shortened the previous terrace outgrowth. 



It appears, therefore, that if the upper edge of the landward 

 border of the oceanic basin can be fixed approximately at the epoch 

 from which the terrace growth is to be estimated, the slope of the 

 terrace front and the depth of the adjacent ocean may be assumed 

 to be of the present order, with some likelihood that this assumption 

 is in reality conservative. These considerations relieve very appre- 

 ciably the embarrassments that would otherwise affect the lower 

 and landward configurations of the circum-continental terraces 

 when we attempt to restore them, by interpretation, for any par- 

 ticular post-Proterozoic epoch. 



The surface area of the present continental shelf between the 

 shore line and the loo-fathom line is usually taken at 10,000,000 

 square miles, following Murray. The additions to be made to this 

 to give the full area of the built terrace when its true border lies 

 on the landward side of the present shore line, and the subtractions 

 that are to be made when it lies on the seaward side, can only be 

 roughly guessed at until the geological determinations of the coastal 

 terranes of the several continents have reached a more advanced 

 state, for any close estimate requires data not now available. None 

 the less, it is worth while to make rough guesses of their values on 



