THE TRIASSIC PERIOD. 19 



low. On physical grounds, therefore, there would be ample justifica- 

 tion for referring the Newark series to the earlier, rather than to the 



Fig. 324. — Diagram showing structure of the Triassic beds and their relation to older 

 terranes, southeast of Harper's Ferry, in Virginia. Pre = Cacoctin schist, Protero- 

 zoic; €. Cambrian; Tn, Newark series, Triassic. (Keith, U. S. Geol. Surv.) 



later part of the time-interval between the Permian and the Creta- 

 ceous. 



Physiography of the Newark of New England and New Jersey. 



The trap ridges of New England and New Jersey illustrate so clearly several 

 fundamental principles of physiography and structural geology that a few points 

 in their history are here sketched. 



Subsequent to its peneplanation in pre-Cretaceous or early Cretaceous time, 

 the area covered by Triassic beds was elevated, and a new cycle of erosion inau- 

 gurated. In this post-Cretaceous cycle of erosion, most of the sedimentary beds 

 were degraded readily, while the trap, being notably more resistant, withstood 

 erosion more effectively and came to stand out in conspicuous ridges. Many 

 of the prominent ridges in the Connecticut valley, including such elevations 

 as the Holyoke Range, Toket, Pond, Lamentation, and Farmington Mountains, 

 are simply the outcropping edges of trap sheets isolated by the removal of 

 the less resistant shale and sandstone. The Watchung Mountains of New Jerse3', 

 and the Palisade Ridge along the lower Hudson, as well as many other elevations 

 of that and adjoining States, owe their origin to a similar sequence of events. 

 In New Jersey, the Lockatong formation, as well as the trap sheets, is a ridge- 

 maker. 



That deformation other than tilting affected the Triassic system is shown 

 by numerous phenomena. Among these is the curvature of some of the trap 

 ridges, such as Cushetunk Mountain (Fig. 321), the rock of which is an extrusive 

 sheet of diabase. Since the general direction of dip of the Newark series in New 

 Jersey is to the northwest, the curvature means a syncline, the axis of which 

 is northwest and southeast. This folding probably accompanied the first de- 

 formation to which the Newark series was subject, rather than that which fol- 

 lowed the Cretaceous base-leveling, for the curved crest is approximately level. 

 The curvature of First and Second Mountains (Fig. 321) is probably to be explained 

 in the same way. The trap ridges of the Connecticut valley show similar phe- 

 nomena. 



The trap outcrops of the Connecticut valley illustrate the manner in which 

 faulted strata of unequal hardness may come to express themselves topographic-. 

 ally, and their study throws light both on structural and physiographic problems. 



