

602 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



Of the 17 submergences, 10 are marked ones, having inundated either 

 of the two denned areas more than 20 per cent. In the Trenton the sub- 

 mergence covered about 60 per cent of North America. It is also seen 

 that of these 10 decided submergences, 9 occurred during the Paleozoic, 

 while the tenth took place during the close of the Mesozoic. The 7 

 smaller submergences inundated the areas from 2 to 19 per cent. The 

 first of all these floods (Georgic) is classed in the so-called secondary 

 inundations, and records the beginning of the Paleozoic. However, it is 

 the most marked of the minor submergences, and 'is followed by 8 primary 

 and successive inundations. Another minor flood (Saint Louis) then 

 appears, which is succeeded by the last of the greater Paleozoic submer- 

 gences. It may therefore be said that all the Paleozoic inundations 

 except one (Saint Louis) are of the major type. Further, the Paleozoic 

 submergences attain their maxima in the Ordovicic, each subsequent flood 

 being of smaller extent. These facts seemingly furnish decisive indica- 

 tions that during the Paleozoic' the oceanic areas had not yet attained 

 their present abyssal depths, and that outside the eastern border region 

 the North American continent was a low, featureless land-mass through- 

 out this era. The slightest secular or aggradational change anywhere on 

 the globe then affected the oceans more than at any later time, so that 

 they readily flowed widely over the lands, thus developing the very shal- 

 low continental seas. In a general way, it may be said that the present 

 broader relations of the oceanic areas to the lands have been fixed since 

 the Appalachian revolution of the Atlantic realm. 



It is becoming more and more apparent that the geologic chronology 

 for the greater divisions must be based on criteria additional to those now 

 in use. In chronology, dependence will always have to be placed pri- 

 marily on the fossil content of the sediments. In correlating the more 

 or less similar developments of the various faunal provinces, however, 

 some physical basis is needed underlying the faunal likeness or dissimi- 

 larity. Such a principle apparently exists in the submergences or the 

 positive changes of the strand-line, for when one of these attains its max- 

 imum of spread, the widest distribution of similar faunas and identical 

 species would naturally be expected. Conversely, the maximum of emer- 

 gence must, mark the absence of faunas in most land areas, followed for 

 a time by more or less dissimilar faunas in all the provinces. 



As long ago as 1883, Suess concluded that the geologic time table 

 would eventually be based on diastrophism and the faunal changes caused 

 by these movements in their physical environment. He states : "It is the 

 physical causes of faunal transformations which will, when once they are 



