Appalachian Geosyncline. 103 



something of the magnitude of the work and the degree of 

 completion of the later Mesozoic cycles. Thus it becomes 

 easier to accept the results arrived at in regard to the erosion 

 accomplished in the earlier periods. The discussion adds force 

 to the general conclusion regarding the competency of erosion 

 in a single geologic period to go far toward leveling mountain 

 structures. The Newark sedimentation was brought to a close 

 near the beginning of the Jurassic by a crustal fracturing on a 

 large scale and a tilting of the fractured blocks ; in the Con- 

 necticut area the tilting was to the east, in the New York- 

 Virginia area to the west. The Newark strata dip at an aver- 

 age inclination of 15 to 20 degrees away from an axis between 

 the two areas. It is not known to what extent the tilting ex- 

 tended beyond the present areas of Newark rocks. Presum- 

 ably between the two graben the tilting disappeared, but in its 

 place was a considerable upwarp, implied by the tilted struc- 

 tures of the two sides. The upwarp, however, was not 

 measured simply by the amount of the tilt ; since the fault 

 movements neutralized more or less the geanticlinal structure. 

 Nevertheless the movement was of a mountainous order since 

 individual crust blocks are many miles wide. 



By the close of the Jurassic, erosion had again reduced the 

 mountains to a hilly country, planing across the entire thick- 

 ness of the Newark sediments, across their included traps, 

 across the resistant floor below. A tilt of 15° to the previously 

 level floor gives an elevation of 1367 feet per mile ; 20 degrees 

 gives 1806 feet. Areas, miles in width, of granite-gneiss 

 sloping at these inclinations were planed across in Jurassic time 

 after the Newark sediments had been removed. Then a mar- 

 ginal downwarping of the continent began and the Potomac 

 sediments of the early Comanche began to be laid down on this 

 new floor prepared by Jurassic erosion. The floor was marked 

 by hills one or two hundred feet in height, but this was a 

 minor relief in comparison with that developed by block fault- 

 ing and tilting which marked the beginning of the Jurassic 

 period. This relief in the floor of the Potomac sediments is of 

 such an order as may have been due to minor fluctations in the 

 baselevel, late in the cycle of erosion. 



The Post-Jurassic Erosion Cycles. 



In discussing the post-Triassic erosion of the Appalachians, 

 ■commonly no distinction has been drawn between Jurassic, 

 Comanche, and Cretaceous cycles; it being assumed that erosion 

 throughout that time was essentially with respect to one base- 

 level and that the residuals which have survived above the 

 Cretaceous baselevel to the present had previously resisted the 

 erosion of all earlier Mesozoic times. Such a view would lead 



