192 CLAYS AND CLAY INDUSTRY. 



Its thickness varies from: 15 to over 35 feet. North of the 

 Raritan river its average thickness is between 15 and 20 feet, but 

 south of the river it is known to exceed 35 feet at several localities. 



THE RARITAN FIRE AND TERRA-COTTA (POTTER's) CEAY. 



In the Report upon the Clays of New Jersey, 1878, the terms 

 "Raritan potter's clay" and "Raritan fire clay" were used by Dr. 

 Cook to indicate the two- lowest subdivisions of the Plastic clay 

 series. Inasmuch as the whole series is now known as the Raritan 

 formation, these names are not now so appropriate, and yet, since 

 they have been used in the earlier clay report, and are to some 

 extent used locally by the clay workers, it seems best to retain 

 them, in spite of the possibility of confusion. 



Inasmuch, however, as the evidence is not conclusive that the 

 fire clay and the potter's clay form coextensive distinct beds, and 

 since it is not easy to separate the two* in mapping, they are here 

 considered together. 



This clay underlies the fire sand described above. Wherever 

 its. base has been seen it rests upon the red shale, into' which it 

 apparently grades without a break. This is the case at Brinck- 

 man's pit (96), on the Mill brook, and at the abandoned pits of 

 Calvin Pardee (237) a little to the east. The apparent transition 

 from, the Triassic shale, which dips 15 degrees to 20 degrees 

 northwest, to the Cretaceous clay, which dips very gently south- 

 east, is interpreted to mean, not that the clay was deposited im- 

 mediately after the shale, but that the tilted and eroded edges of 

 the shale were, at the beginning of Cretaceous time, covered by a 

 residuary clay formed from the decay of the shale itself, and that, 

 as the land was . submerged prior to the deposition of the Cre- 

 taceous beds, this clay layer was in many places gently re-worked 

 in its upper portion and re-deposited. With the gradual influx of 

 other material, the deposit passed upward into the more typical 

 Cretaceous clay, which was evidently not derived from the red 

 shale. So gradual is this transition that even where the section 

 is. freshly exposed, it is frequently impossible to say within sev- 

 eral feet, where the Triassic shale ends and the Cretaceous clay 

 begins. 



