406 E, O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



continent, the middle and northern parts at such times being tilted so 

 that the boreal sea extended southward beyond Chicago and occasionally 

 as far as northern Tennessee. Possibly the invasions incident to such 

 broad tilting were slightly accentuated, according to the theory of Schu- 

 chert (Bull. Geol. Soc. America, vol 20, page 505), by the return to the 

 poles of waters previously piled up at the equator during periods of 

 active earth shrinkage. However, assuming essential permanence in 

 location of the poles, it is inconceivable that such returns could by them- 

 selves produce the condition of advance in the north and retreat in the 

 southern States, the latter, according to theory, being in latitudes of 

 negligible change in water level. But granting that for some unknown 

 cause they were competent, then the exclusion of the Gulf waters would 

 have continued to the next period of great shrinkage — a prohibition that 

 we have no reason to believe obtained. Hence, even these extensive re- 

 versals of sea invasions are regarded as unquestionably indicative of dif- 

 ferential vertical movements. (See figure 8, page 407.) 



These north-south tiltings of the continent are relatively modest phases 

 of diastrophic movements of a high order of magnitude that are expressed 

 in a more striking and indubitable manner by marked orogenic phe- 

 nomena along the southern borders of the northern tier of continents. 

 The crustal folds which have resulted from this series of movements have 

 a general east and west strike and are especially well developed in areas 

 adjacent to the Mediterranean and Indian seas in the eastern hemisphere, 

 and to the Gulf of Mexico in the western. The available data suggest a 

 tendency of the outer part of the crust to move away from the poles, with 

 sinking in the polar regions and corresponding heaping of the crust equa- 

 torially. This process long continued might be expected to result finally 

 in great depressions of the lithosphere about the poles and in nearly 

 compensating elevations within the median east-west zones of orogenic 

 activity. For two reasons chiefly the present conditions fall short of the 

 expectations. First, the elevated areas have been subject to consequent 

 acceleration of erosion, the disparity in relief being thereby reduced ; and, 

 second, local loading of the areas in which mountain-building was in 

 progress (the loading being occasioned by folding and consequent heap- 

 ing of the superficial crust) unsettled the isostatic equilibrium, and thus 

 aided erosion in bringing the high parts back to former levels. Sedi- 

 mentary loading of adjacent submerged areas assisted by tending to drag 

 the high lands down with them. Finally, with sinking going on, say, in 

 lands bordering the Gulf of Mexico, deep-seated, northward flowage of 

 rock matter doubtless took place, presumably resulting in refilling of the 

 polar depressions from below and in gentle, upward movement of the 



