CLAYS OF CRETACEOUS FORMATION. 175 



1 to 4 inches in diameter. Frequently the outer shell or periphery 

 is completely changed to ferric oxide, while the interior is still 

 unchanged sulphide of iron. Pyrite in smaller lumps and frag- 

 mentary pieces is also quite common, and diffused throughout the 

 clay of the whole bed as worked in some places." 1 



In the banks of J. R. Crossman (65, 66), J. R. Such (67) and 

 some of the old excavations on the Kearney tract, north of Burt 

 Creek, a black, lignitic clay, or alternating bed of clay and sand 

 occurs immediately above the fire clay proper and is included 

 with the fire clay on the map. Small pieces of amber occur near 

 the base of the black clay in some localities. The surface of the 

 fire clay beneath this black clay is often sharply undulatory, so 

 much so as to suggest some erosion of the fire clay previous to' the 

 deposition of the black clay. The interruption to> continuous de- 

 position was probably not long, and the erosion is no more than 

 could have been accomplished by shifting tidal currents. At most 

 exposures, however, the fire clay is overlain either by the quartz 

 sand (No. 3), which belongs to the Raritan series, or by the 

 much more recent Pensauken gravel or red glacial drift. In the 

 latter case the overlying Cretaceous sand, and often the upper por- 

 tion of the clay itself was removed in the long- period of erosion, 

 after the formation of the Cretaceous beds, and before the deposi- 

 tion of the Pensauken gravel or the still later glacial drift. But 

 even where the clay is overlain directly by the quartz sand (No-. 

 3), its upper surface is sharply irregular and the bed varies greatly 

 in thickness, indicating that there was at least a brief interruption 

 in sedimentation and some erosion of the clay, as a result of the 

 changed conditions and swifter moving currents, which began the 

 deposition of the sand. 



As may be inferred from what has been said, the South Amboy 

 fire-clay bed presents great variations in thickness. At Sayre & 

 Fisher's bank (273) variations of 15 feet in the height of the 

 surface in a horizontal distance of 30 feet have been observed, 

 with corresponding variations in thickness. In a few banks a. 

 thickness of 30 feet is sometimes found, but the average is much 

 less than this. In not a few localities the clay is absent entirely. 



1 Report on the Clay Deposits of New Jersey, 1878, p. 67. 



