THE NEWARK ROCKS 35 



The relation of the conglomerates to the shales is also sig- 

 nificant. They do not form a single horizon which may be used 

 in interpreting the structure. Instead, they grade either into 

 argillaceous shales, or black argillites, or arkose sandstones. 

 Time and again the pebbly layers were seen to appear in the 

 shales and to increase in thickness and numbers until they became 

 massive conglomerates. This is true both of the calcareous and 

 quartzite conglomerates, and probably also for the gneiss con- 

 glomerate, but owing to the glacial deposits its relation to the 

 shales could not be determined. 



The trap rocks. — The trap rocks of the Newark beds in New 

 Jersey and New York have been described more or less in detail 

 by several geologists, 1 and it has been demonstrated that over- 

 flow sheets, intrusive sills, plugs, and dikes occur. Owing to its 

 superior hardness, the trap rock has better resisted erosion than 

 the sedimentary beds, and consequently forms more or less well- 

 marked elevations. In the case of narrow dikes, the elevation 

 is slight and readily overlooked, but the greater masses form 

 hills or ridges rising not infrequently 300, 400, or even 500 feet 

 above their surroundings. The structural relations of the trap 

 masses are among the most interesting questions connected with 

 the Newark series, but only general conclusions can be given 

 here. 2 The most important of the overflow sheets are the three 

 concentric ridges forming the Watchung Mountains, Fig. 3. 

 These sheets are to all appearances strictly conformable, both 

 to the underlying and to the overlying shales. Nowhere is there 

 any indication that the trap breaks across the sandstone or shale 

 layeis. Wherever the basal contact is exposed, and exposures 

 several hundred feet in extent are known, the trap is seen to fol- 

 low exactly the bedding plane of the shales. 



Moreover, the extensive metamorphism of the associated 

 sedimentary beds, a marked feature in the case of all the intru- 



1 Chiefly Cook, Russell, Davis, Darton, Iddings, Kummel. 



2 Detailed descriptions are given by N. H. Darton, U. S. Geological Survey, 

 Bulletin No. 67, and by the author in the Annual Report of the State Geologist of 

 New Jersey for 1897, pp. 58-100. 



