364 CLAYS AND CLAY INDUSTRY. 



Structurally the Raritan clays present a great series of beds of 

 varying" character, the majority oi which seldom retain their 

 thickness for any great distance but may thicken and thin at 

 frequent intervals. The most persistent beds are the black 

 laminated sands and clays found above the Woodbridge fire clay 

 and SO' extensively developed along the Raritan river. 



In texture the Raritan clays are mostly fine-grained, this being 

 especially true of the fire clays found in the Woodbridge, Perth 

 Amboy, South Amboy and Piscataway districts, and while they 

 often feel very plastic and sticky when wet, still their tensile 

 strength is low. Closely associated with these fine-grained ones, 

 we find the very coarse, impure, sandy and micaceous laminated 

 clays referred to above. The Raritan clays found to the south- 

 westward along the Delaware river from Burlington to Camden 

 are also* coarsely gritty in most cases, but are usually of Nov 

 2 grade. 



Slaking qualities. — The slaking quality of the Raritan clay 

 depends to a large extent on its degree of porosity and coarse- 

 ness, the sandy open-textured ones slaking rather fast. 



Tensile strength. — The average tensile strength is not high and 

 rarely exceeds ioo pounds. The only high ones are those from 

 Kinkora (Loc. 112) and near Monmouth Junction (Loc. 271). 



Burning qualities. — Leaving out the black, sandy, laminated 

 clays of the district along the Raritan, and found to a smaller 

 extent around Woodbridge, and the brick clays near Cliffwood and 

 of the Delaware river district, it is seen that practically all the 

 materials burn buff, or even yellowish white at cone 8. These 

 same clays may, if dense burning, become steel-hard at cone 1 or 

 3, but usually do not reach this condition until they have been 

 heated to cone 5 or in some cases to cone 8. Comparing a number 

 of fire-clay samples from the Middlesex district, which have been 

 burned to cone 5, it is seen that the fire shrinkage may vary 

 from 1 per cent, up to 13 per cent., and the absorption from 

 0.05 per cent, to' 19 per cent., while at cone 8 the fire shrinkage 

 ranged from 3 per cent, to 12 per cent., and the absorption from 

 2 per cent, up to 15 per cent. 



With such a variation in the characters of the raw materials, it 

 is possible for the miner of clay to meet many different demands 



