THE OROGENIC EPOCHS IN NORTH AMERICA 653 



There is a conception familiar to present-day geologists, that 

 the great mountain systems coincide with geosynclinal depressions 

 in which sediments have previously accumulated to unusual 

 thicknesses. The Appalachians have served as a classic example. 

 In the opinion of some, the trough was deepened by lateral pressure 

 and the deposition of the sediments thus invited. The extreme 

 isostasists, on the other hand, view the trough as a result of the 

 loading of the strip by the deposition of thick sedimentary beds. 

 Wilhs urged the importance of the initial dips developed in these 

 geosynchnes in predetermining the locus of the folded belt when the 

 compressive force became efTective. It is prudent, however, not to 

 assume that there is a causal relationship between belts of thick 

 sediments and subsequent mountain folding, merely because one 

 preceded the other. Much of the thickness of the Appalachian 

 sediments was directly due to the ruggedness of the land-mass of 

 Appalachia, from time to time in the Paleozoic era; and it should 

 be recalled that at the close of the Permian not only the Appa- 

 lachian geosyncline, but a still broader region to the east, was 

 intensely deformed. The observed relations may be stated in 

 another way, namely, that sediments accumulate rapidly along 

 mountainous coasts, and that coasts in turn are liable to repeated 

 crumplings, for reasons not here discussed. Hence the two phe- 

 nomena are generally associated. 



In each great portion of the continent there have been successive 

 roughly parallel crumplings, separated by long periods of quiescence, 

 and, in some cases at least, eventually terminated by a cessation of 

 activity which has endured down to present times without pre- 

 monition of change. In the Lake Superior region, three, and 

 probably many more orogenies, were passed before the Cambrian, 

 but in all subsequent time there have been none. In the Atlantic 

 mountain system three or more crumpHngs before the Triassic 

 have been followed by a stabihty prolonged until the present, and 

 with no suggestions of an end. In the western Cordilleras the 

 orogenic activities seem to have slumbered until the Jurassic, but 

 since then the region has been subjected to repeated compressions 

 and is the one in which future disturbances are most likely to take 

 place. 



