STRATIGRAPHIO CLASSIFICATION — DIASTROPHIC CRITERIA 439 



found wedged in between the limestones, suggesting that at intervals the 

 shore conglomerate spread entirely across the bay. The larger accumula- 

 tions of shore conglomerate are confined to the eastern side of the bay, 

 from which it is inferred that the land was both steeper and higher on 

 this side. Corroboration of this view is found in the fact that the sedi- 

 ments as well as the faunas inhabiting the bay are of types characterizing 

 the interior continental basins and not at all like those found to the east 

 of the Eome barrier. (See figure 17 E, page 450.) 



Suggestion concerning cause of inland migration of belt of active 

 folding. — The accompanying crude sketches, representing generalized 

 and vertically much exaggerated theoretic cross-sections of Appalachia 

 at important stages in its geologic history, will give a ^ clearer idea 

 of what is meant by inland migration of the belt of folding and of a 

 possible cause than can be gained from words alone. Section 1 shows 

 the supposed relief of Appalachia at the beginning of Cambrian deposi- 

 tion in the Appalachian Valley. Section 2 represents the same in middle 

 Ordovician time. The valley has been gently folded into several shallow 

 troughs, all of which have received varying amounts of marine sediment. 

 Appalachia has a median submerged area; its western part is nearly 

 peneplaned and in course of elevation without folding, its eastern part 

 considerably elevated and folded and limited on the east by an assumed 

 fault-scarp. In section 3, representing an early Silurian condition, the 

 whole region is above sealevel, the Appalachian Valley is receiving some 

 land deposit, the displacement at the marginal fault has been increased, 

 and the zone of active folding has moved farther inland. Section 4 

 shows the result attained at the beginning and to the close of the Potts- 

 ville. The throw of the fault at the edge of the continental shelf has 

 been greatly increased, the Atlantic thrust affects deeper levels, and the 

 deeper shear planes emerge farther inland, causing folding in the median 

 and western parts of Appalachia. The erosion of the latter resulted in 

 land deposits of great thickness in tlie Appalachian Valley. Peneplana- 

 tion of Appalachia continued through the remainder of Pennsylvanian 

 time and probably to the close of the Triassic. In the latter age com- 

 paratively slight elevation of the western part and relative depression of 

 the median area afforded conditions favoring deposition of the Newark 

 series in the latter. In section 5, representing a subsequent, middle to 

 late Mesozoic, stage, the process has advanced another step, the movement 

 on the new, deeper thrust planes, emerging still farther inland, now being 

 chiefly expended in the folding of the middle and southern Appalachian 

 Valley troughs. The overlying oblique planes ride on the lower planes 



