TRANSGRESSIONS 509 



lution. It was undoubtedly this revolution, with its long-continued 

 oceanic subsidence, that slowly withdrew the continental Coloradoan sea 

 from the land. The principal Mesozoic transgressions are believed to be 

 due to the same cause as those of the Paleozoic, namely, displacements of 

 the seas and oceans by the land wash. 



It has been said that the wearing away of the high lands and the trans- 

 portation of this material into the sea results in the progressive inunda- 

 tion of the lands. Chamberlin and Salisbury 133 state: "It is estimated 

 that the cutting away of the present continents and the deposition of the 

 material in the ocean-basins would raise the sealevel about 650 feet (R. D. 

 George)." Displacement of the oceanic level to this extent would inun- 

 date the entire medial region of present North America and produce a 

 transgression not greatly unlike that of the Middle Siluric. This would 

 bring into existence another large Mississippian sea in wide connection 

 with the Saint Lawrence sea. The Arctic waters would have a broad 

 sweep across the Canadian shield through Hudson bay, with another pro- 

 longation extending south from lakes Manitoba and Winnipegosis to some 

 distance along the Eed Eiver valley, and there probably would be com- 

 munication with the Mississippian sea across the Great Lakes. The faunas 

 of this northern sea would not be sharply differentiated from those of the 

 southern waters, because of the wide-open connection of both northern 

 seas with the North Atlantic. There would be still other marginal over- 

 laps that need not be described, for it is readily seen that in this way 

 another great transgression would result without any secular changes in 

 the North American continent. (The basis for this estimate is the Belief 

 Map of Canada and the United States, published by the Geological Survey 

 of Canada, 1900.) 



Whether the continental materials were transported to the oceans or 

 were unloaded into the continental seas would make no difference ; the 

 effect of displacement of the strand-line would be the same. In the latter 

 areas the isostatic adjustment (due to detrital loading and warping) 

 would cause some portion of the sea bottom to rise sufficiently to displace 

 an amount of water equal to the mass of the displaced lithosphere. 



During critical periods the continental margins are apt to be decidedly 

 raised, and this elevation is thought to exceed the accumulated stresses 

 produced during times of long quiescence. Dana 134 states : "After a 

 mountain birth there has commonly succeeded a time of relaxed lateral 

 pressure; and then occurred adjustments, largely by gravitation, in the 

 moved masses or faulted blocks making chiefly down-throw displacements, 



133 Chamberlin and Salisbury : Geology, vol. 1, 1904, p. 520. 



134 Dana : Manual of Geology, 1895, p. 386. 



