OSCILLATORY CHARACTER OF CONTINENTAL SEAS 351 



other criteria argues for long durance of such conditions ; and the longer 

 baseleveling operations were in play the greater the lack of positive infor- 

 mation concerning the ill-recorded interval. In this connection it is sig- 

 nificant to note that, with the doubtful exception of the Saint Peter and 

 Sjdvania sandstones, the sand of which was cemented by lime infiltration 

 as it was covered by the advancing sea, and possibly certain Pennsylvanian 

 sediments, there are no strictly land deposits of pre-Pleistocene age in the 

 median (Mississippi Valley-Hudson Bay) depression of the North Amer- 

 ican continent. In this great flat area land deposits could never have 

 been very thick, but what there was baseleveling agencies removed before 

 the several accumulations could be consolidated and preserved beneath 

 succeeding marine deposits. They passed away just as the Pleistocene 

 deposits are being removed from the same area today. 



Preservation of land deposits was favored only in areas of considerable 

 downwarp, and in these only while the area was subject to merely broad 

 folding and not to close folding or high elevation with consequent erosion 

 prior to subsequent submergence. It is primarily to this circumstance 

 that we owe the preservation of the late Ordovician-early Silurian and the 

 Devonian land deposits in the middle Appalachian and Allegheny re- 

 gions; also of the Jura-Triassic deposits in the broad depressions of 

 Appalachia and Taconia, which latter areas had passed the stage of close 

 folding and after the Jurassic moved inland bodily. (See page 435.) The 

 Mesozoic and Tertiary land deposits of the Eocky Mountains area were 

 similarly preserved because they lie mostly in broad depressions unfavor- 

 able to extensive or complete removal. The much greater inland extent 

 of the western areas of development and preservation of land deposits 

 than in the eastern part of the continent is connected with the principle 

 of "suboceanic spread." On account of the much greater width and the 

 somewhat greater depth of the Pacific than the Atlantic the forces devel- 

 oped beneath the former produced folding much farther inland than did 

 those originating beneath the Atlantic. 



Though we may gain an obscure idea of the conditions prevailing dur- 

 ing some of the Paleozoic emergent stages, of the most of them we know 

 and probably can learn little or nothing of a definite nature. Concerning 

 the pre-Cambrian or earliest Cambrian the discovery of facetted pebbles 

 by Willis in China makes it probable that glaciation prevailed locally at 

 that time. Then we are justified in inferring considerable local elevation 

 (chiefly in the marginal lands), active erosion and vulcanism as begin- 

 ning in the closing stages of the Ordovician and continuing into the early 

 Silurian. Elevation and consequent erosion is indicated also toward the 



