562 W. M. DAVIS — DATES OP TOPOGRAPHIC FORMS. 



Of the crystalline portions of the southern Appalachians, I can say little 

 from my own observations ; but, judging from the topographic maps of the 

 Blue ridge in Virginia, it is likely that observation on the ground would 

 discover the peneplain and the hills rising over it without difficulty. Its 

 presumable occurrence in Virginia may be indicated by returning to New 

 Jersey, where it is so well defined, and tracing it southward along the crys- 

 talline belt. Two divisions of this belt are to be recognized, separated more 

 or less continuously by the Triassic formation, which lies in a structural de- 

 pression between them. It is the northwestern of the two divisions that 

 makes the highlands of the lower Hudson and of New Jersey. Following 

 this into Pennsylvania, it is known as South mountain, by reason of its lying 

 opposite to Kittatinny or North mountain, with the great limestone valley 

 between. South mountain narrows as it is followed southwestward, and 

 at Reading, where the Schuylkill crosses toward the southeast, the plateau 

 ends, because the crystallines here dip under the old Cretaceous baselevel, 

 and indeed below the present base-level as well ; and the Triassic belt laps 

 over to the limestones of the great valley. Beyond the Susquehanna the 

 crystallines rise again, just as they sauk at Reading, once more bearing the 

 name of South mountain. The evenness of the upland surface is well 

 marked here at several places.* It continues across Maryland, and enters 

 Virginia to form the Blue ridge, already mentioned. The comparative even- 

 ness of the upland at Harper's Ferry, where the plateau is crossed by the 

 Potomac, is illustrated in plate V, Bulletin 52, of the United States Geo- 

 logical Survey. 



The southeastern division of the crystalline belt begins at Trenton as a 

 narrow wedge between the Triassic and the Cretaceous formations. It corre- 

 sponds to, and is presumably continuous with, the crystallines of Manhattan 

 island (New York city), which come southward from Connecticut and Massa- 

 chusetts, the two divisions of the crystalline belt separating on the Hudson 

 where the Triassic belt of New Jersey begins. The interruption of this 

 division between New York city and Trenton is due to the early Cretaceous 

 depression of this part of the peneplain and its consequent burial under the 

 Cretaceous cover, which, though in part worn off* again, still remains in suf- 

 ficient extent to conceal the intermediate part of the crystalline division in 

 question. From Trenton it may be easily traced as it widens to Philadelphia 

 and beyond. At Philadelphia, or rather just northwest of that city, the 

 upland level of the plateau is made all the more distinct by the narrow and 

 deep trenches cut into it by the Schuylkill and the Wissahickon. The gorges 

 of the Neshaminy, Brandywine and other creeks are of like character. The 

 same is true where the Susquehanna cuts this part of the plateau. A view 

 of the lower part of this gorge is given in the Seventh Annual Report of the 



*See contoured maps by Second Geol. Surv. Penna., atlases to reports D3 and D5. 



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