CLAYS OF CRETACEOUS FORMATION. 193 



Excavations and borings indicate that this clay bed, at least its 

 lower part, rests in hollows in the shale, and that it is more or less 

 discontinuous. This was clearly shown a number of years ago> in 

 exposures near Silver lake, Piscataway, 1 where the clay, with a 

 maximum thickness of 18 feet, was pinched out within a few yards 

 to the south between the underlying shale and overlying drift, by 

 the ascent of the shale. The same thing is indicated by borings in 

 the vicinity of Bonhamtown. At the trolley barns a well pene- 

 trated 65 feet of gravel and then struck red shale; at the road 

 fork, a few rods east and at an elevation 10 feet lower, a fire clay 

 was reached beneath 40 feet of gravel, 2 and a third boring in the 

 bottom^ of the gravel pit just north of these localities revealed a 

 white clay beneath the gravel at about the same elevation. 



The top of the clay bed beneath the fire sand is, also, very un- 

 even, undulating sharply within comparatively narrow limits. 

 This was well shown (1901) at Bloomfield's and Pfeiffer's banks 

 (92, 90). It is evident that with the changed conditions, which 

 brought about the deposition of the coarse fire sand on the clay, 

 the surface of the latter was somewhat eroded by the estuary 

 currents. 



Owing to these variations in the top and bottom of the clay, its 

 thickness is extremely variable. Locally, the fire sand seems to 

 rest upon the shale and so cut out the clay entirely. More com- 

 monly, however, the clay is present in thicknesses varying from a 

 few feet up to' 35 feet. The latter thickness is said to occur at 

 Bloomfield's bank (92), 15 to< 20 feet of it being merchantable 

 clay. 



The fire clay. — The upper portion of this clay bed is a fire 

 clay — the Raritan fire clay of Cook's report. It is a drab or light- 

 blue clay, sometimes red mottled and sometimes almost black. It 

 is usually quite sandy, much more so than the better portions of 

 the Woodbridge fire clay, specimens, showing from, 29 per cent, 

 to 50 per cent, of fine-grained quartz sand. 3 At present it is dug 

 by C. A. Bloomfield (92), John .Pfeiffer (90), C. A. Edgar (89), 



1 Cook & Smock, Report on the Clays of N. J., p. 171. 



2 Cook & Smock, loc. cit. p. 162. 



3 Cook & Smock, loc. cit. p. 45. 



13 CL G 



