574 W. M. DAVIS — DATES OF TOPOGKAPHIC FORMS. 



that the reduction of these Appalachian lowlands so nearly to a baseleveled 

 peneplain in the Tertiary cycle cannot possibly be ascribed to marine action. 

 They are manifestly the product of subaerial action. 



The great valley is drained by longitudinal streams that join the few 

 transverse master rivers and then cross the crystalline belt. The degrada- 

 tion of the great valley, as has been already explained in describing the 

 Hudson outlet, has been accomplished only so fast as the trenching of the 

 crystalline belt allowed. The number of rivers that cross the crystallines is 

 small, but they are of large size — the Hudson, Delaware, Schuylkill, Sus- 

 quehanna, Potomac, James and Staunton. One of the smallest of these, the 

 Schuylkill, is located where the northwestern division of the belt dips below 

 even the present baselevel, thus affording opportunity for a small stream to 

 survive the transverse passage. In contrast with this, it is noticeable that 

 where the crystallines increase in width and height no rivers cross them. 

 New England is not traversed by any cross-rivers, nor are the Carolina 

 mountains. On approaching the latter the great valley is drained toward 

 the northwest, instead of toward the southeast. 



The Triassic belts only repeat the lesson of the great valley, being well 

 reduced to the Tertiary baselevel, except where possessing harder beds, as 

 has been stated for J^ew England. The curvature of the trap-ridges, conse- 

 quent on the faint dishing of the lava-sheets, is a well-marked peculiarity of 

 this belt, appearing in Massachusetts and Connecticut, where the curves are 

 convex toward the west ; and in New Jersey and Pennsylvania, with curves 

 convex toward the southeast. It may be noted that the Rogers geological 

 map of Pennsylvania (1856) by no means does justice to the systematic de- 

 velopment of this feature, so far as the belt between the Delaware and the 

 Schuylkill is concerned. The large-scale contoured maps of a portion of the 

 Triassic area northward from Philadelphia, surveyed by the w^ater depart- 

 ment of that city,* show the curvilinear form of the ridges with great dis- 

 tinctness. 



Coming next to the southeastern division of the crystalline belt, it is pe- 

 culiar in the Philadelphia district in being traversed from the low back 

 country to the coastal plain by streams of relatively small size, as well as by 

 the master rivers of the region. The explanation of this feature by super- 

 imposition, as given by McGee and already referred to, appears to be the 

 only one that deserves consideration ; and the drainage of the still buried 

 portion of this belt, between Trenton and New York, may be taken as repre- 

 senting now the earlier stage of the transverse drainage that once character- 

 ized the remainder. The several streams already named, the Neshaminy, 

 the Wissahickon, the Brandyvvine and others, all cut picturesque gorges in 



* Reprinted by the Second Geological Survey of Pennsylvania (atlas to Report C7), and in the 

 Quakertovvn, Pa., topoj^raphic sheet of the Geological Survey maps. 



