330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



northern parts at such times being tilted so that the boreal sea ex- 

 tended southward beyond Chicago and occasionally as far as northern 

 Tennessee. 



Strictly, these widely operating movements hardly fall under the 

 category of epeirogenic movements. On the other hand, they are 

 not truly orogenic, if that term is to be confined to movements 

 originating in shrinkage of the centrosphere. Apparently they indi- 

 cate a combination of causes, perhaps beginning or ending with the 

 play of orogenic factors that built mountains in the submarginal 

 areas, whereas the warping and deformation of the more stable 

 interior areas was mainly occasioned by the necessity of isostatic 

 readjustments to stresses incident to the greater deformations of the 

 orogenic movements. 



Then there were many relatively local changes in the strand line 

 of continental seas that may be explained only by assuming corre- 

 spondingly local differential, vertical movements of the lithosphere. 

 I do not refer to movements connected with volcanism. On the con- 

 trary, the best examples of the kind in mind are found in areas but 

 rarely or not at all directly affected by volcanism. These differential 

 movements indicate actual elevation of one area while another near 

 by was sinking. Moreover, in the next recorded age the directions 

 of ensuing movements at the two places often were reversed. The 

 phenomenon might be likened to a gently convex platform supported 

 in the middle and tilted alternately to the east and west and at other 

 times to the north and south. The condition is recognized by the 

 alternate presence and absence of sediments of particular ages on 

 opposite sides of the tilting platform. (See fig. 2.) 



Comparative studies of the Paleozoic deposits in the Appalachian 

 Valley region, from eastern Pennsylvania on the north and central 

 Alabama on the south, have brought out over a hundred clearly de- 

 fined examples of such oscillations. They are manifested by the 

 restricted distribution or local deposition of many overlapping forma- 

 tions, having maximum thicknesses of from 200 to over 2,000 feet. 

 In many cases these formations are wholly or mainly confined to one 

 or more narrow, troughlike, longitudinal divisions of the Appalachian 

 geosyncline and commonly to one or another of three divisions of 

 the geosyncline that are more or less effectively separated from each 

 other by low transverse axes. The most northerly of these broad 

 axes passes across the valley between Carlisle and Lebanon, Pennsyl- 

 vania. It is known as the Harrisburg axis. The next, to the south, 

 intersects the valley of Virginia between Staunton and Harrison- 

 burg. The third, or Wytheville axis, passes across southwestern 

 Virginia, which is to-day the highest and narrowest part of the great 

 valley. The fourth axis crosses in a more northerly direction than 



