148 ANNALS NEW YORK ACADEMY OF SCIENCES 



rocks. The dip, which is fairly constant, averages about 15 degrees 

 northwest, which is normal for the whole system in this vicinity.^ 



The Lockatong formation is thickest in the middle portion of the 

 belt, as appears very plainly when the whole belt is mapped together 

 (PL VII and Fig. 1, p. 149). Exposures along the Delaware Eiver*^ fur- 

 ther prove that it steadily thickens for some distance westward. Sections 

 of the formation are shown by the river at an average distance of eight 

 miles apart, east and west, on account of the repetition of the beds due 

 to the great Flemington and Hopewell faults, which together have a 

 throw of approximately 17,000 feet. West of Phoenixville, the Locka- 

 tong rapidly narrows and finally terminates in a thin edge, its horizon 

 being taken by a heav}^ conglomerate, apparently of Brunswick age. 

 l^ortheast of Princeton it narrows considerably; northward of this point, 

 it is hidden beneath a covering of later sediments of Cretaceous and 

 Pleistocene age. Its total failure to re-appear twenty miles farther 

 north, where only the softest of red shales are exposed, has led to the 

 belief that its northward termination is perhaps much like the southern 

 one west of Phoenixville. From these observations, it appears that the 

 Lockatong is a deposit of a decidedly lens-like character. A comparison 

 of the area of the Stockton formation with that of the Lockatong, 

 throughout the extent of the latter, shows that the Stockton varies with 

 the Lockatong, widening and narrowing with it. 



The Lockatong consists of a thick series of exceedingly fine grained 

 and close textured rocks. The sediments were so thoroughly sorted that 

 scarcely a single coarse textured layer is to be observed among them in 

 the field. The rocks as they now exist appear as massive, fine grained 

 argillites and shales, the former, on account of jointing, often forming 

 '^flagstones" or more massive blocks. The "slates" are often friable, 

 having a cleavage which is too uneven to afford good roofing slate. The 

 color of the shales and of the argillites may be gray, reddish brown, 

 black, or olive green. Red and gray colors often alternate on a large 

 scale. Impure limestone layers one or two inches thick occasionally 

 appear. 



The bedding of the Lockatong argillites and shales is very uniform, 

 although a slight irregularity is sometimes present in the bedding of 

 shaly layers. No cross-bedding appears. Eipple marks and mud-cracks 

 occur, the latter sometimes abundantly, in the upper and lower portions 

 of the series. 



^ A detailed description of this series is given in the Trenton Folio, No. 167, United 

 States Geological Survey, p. 7. 

 ^ Ibid., Geological map. 



