164 CLAYS AND CLAY INDUSTRY. 



far as known, the lowest layers are always derived from the 

 subjacent formation. At Brinkman's clay pit, Piscataway (96), 

 there is a gradual transition from the undecomposed Triassic 

 red shale to a red and white plastic clay, which is undoubtedly 

 Cretaceous. Since, however, the Triassic beds dip to the north- 

 west, and the nearly horizontal Cretaceous beds lie upon their 

 beveled edges, we know that there was a long period of erosion, 

 accompanied by great crustal movements after the formation of 

 the shale and before the deposition of the Cretaceous. Never- 

 theless, it is here impossible to draw a. sharp line between them. 

 The apparent transition is undoubtedly due to a partial re-work- 

 ing of the residuary red clay, which mantled the Triassic at the 

 beginning of Cretaceous time. At other localities there is a 

 distinct alternation of sediments derived from the shale, and 

 from more distant sources. 1 In general, wherever the under- 

 lying rock is red shale, the lowest bed of the Raritan is a very 

 sticky red clay, evidently derived in part, if not wholly, from 

 it. Where the Raritan rests upon a micaceous schist or gneiss, as 

 is the case at Delair, near Camden, borings have shown that the 

 lowest beds of the Raritan are mica sands, such as would be 

 derived from the subjacent formation. 



Conditions of formation. — It has been generally considered, 

 that the Raritan formation was accumulated under broad estu- 

 arine conditions. The rapid alternation of layers, the horizontal 

 variation in character of the beds, and their abrupt changes in 

 thickness, have been interpreted to mean shifting currents and. 

 great variations in conditions within comparatively narrow limits. 

 The cross-bedded structure of many of the sand beds, the billowy, 

 eroded upper surface of some clay layers, the bits of lignite, and 

 even trunks of trees and great masses, of leaves, all indicate 

 shallow water and proximity to a shore line, as well as shifting 

 currents. The beds of extremely line clay, however, indicate 

 that still waters must have prevailed a portion of the time, even 

 although the shore was not many miles distant. The few fossil 

 shells which have been found are of brackish water, rather than 

 marine types. In all these respects the Raritan beds stand in 



1 Cook and Smock, loc. cit., pp. 169-170. 



