36 Introduction 



point. The oceans then begin slowly to subside, drawing away the con- 

 tinental seas from the lands and establishing new ranges of mountains or 

 rejuvenating the old ones along the continental borders. Further, as the 

 earth's diameter periodically lessens, the planet attains a somewhat quicker 

 daily revolution, the days are thus progressively shortened, and the oceanic 

 waters flow toward the equatorial zone during the period of shrinking, 

 being drawn away mainly from the polar regions. It has been computed by 

 Barrell " that a general shrinkage of the earth to the extent of one mile 

 in mean radius will result in an increase of equatorial over polar radius 

 of about 95 feet; a shrinkage of ten miles will result in a relative increase 



of about 950 feet The equatorial bulging for each mile of radial 



shrinkage may therefore be spoken of as from 90 to 100 feet. In latitude 

 35° the water level will suffer no change. At the poles it will sink some 

 60 feet, and at the equator it will rise about 35 feet." ^ 



These cycles of land submergence and emergence are world wide and 

 most of them are thought to be recorded more or less definitely in most 

 continents. That there are exceptions is well known from the geology of 

 Africa. This continent has almost no marine Paleozoic record ; only the 

 Oriskany equivalent is present in a limited region along the southeast 

 coast. In all lands where there is a marine record, however, it is thought 

 that the formations will naturally arrange themselves into diastrophie 

 cycles, and that these cycles represent the systems of rocks or the periods of 

 time of geology." 



It has long been known that the Devonian succession and faunas vary 

 greatly in different regions of North America. The succession along the 

 Appalachian Mountains, from New York to central Virginia, is har- 

 monious, but when these formations and faunas are contrasted with those 

 of Ohio and Indiana, and especially with those of Missouri and Iowa, great 

 differences are found to exist. The Appalachian region and that of 

 Missouri-Iowa have almost no species in common. These marine differ- 

 ences are due to isolation and different oceanic connections, the western 



' Bull. Geol. Soc. America, vol. xx, 1910, pp. 507-508. 



' Those wishing to study this subject in greater detail are referred to a 

 memoir by the writer entitled: Paleogeography of North America, Bull. Geol. 

 Soc. America, vol. xx, 1910, pp. 427-606, pis. xlvi-ci. 



