CLAYS OF MIDDLESEX COUNTY. 437 



Layers j and 12 are practically the same bed under different 

 names and with perhaps somewhat different physical properties ; 

 so, too, layers f and 8 are not so different as the names given them 

 by the miners would imply, and layers d and 5 are readily recog- 

 nized in the field to be the same bed, although put to different 

 uses, as indicated by the names. Differences in texture are more 

 striking in the case of layers b and c, as compared to layer 4 ; of 

 e compared to 6 and 7; of g and h compared to 9 and i to 10. 

 Nothing corresponding to layers 2 and 3 is found at all in the 

 other bank, owing to pre-Glacial erosion, which removed all clay 

 beds higher than b. 



The two sections given above will also serve to indicate some- 

 thing of the number of grades of clay that are recognized in the 

 Middlesex district. The more important of these are No. 1 and 

 No. 2 fire clays, retort clay, ball clay, wad clay, pipe clay, brick 

 clay, hollow -brick clay, etc., but neither the use nor the physical 

 properties of the material are always exactly indicated by the 

 name. Sometimes the same clay may be employed for several 

 different purposes, or again the same clay may be designated by 

 different names in adjoining pits. 



In the present description of Middlesex county it is chiefly the 

 economic aspect of the clays that is discussed, and no attempt is 

 made to describe all the localities where clay is dug, or to give 

 sections of all the clay banks, for the latter vary from season to 

 season, and nothing would be gained in publishing these details. 

 It is suggested that the stratigraphic discussion of the Raritan 

 clays (Chap. VIII) be read first, to insure a clearer understand- 

 ing of their mode of occurrence in the field. 



A systematic economic description of the Raritan clays in Mid- 

 dlesex county is attended with more or less difficulty, because, 

 owing to the many different sections shown in the pits, it is diffi- 

 cult to discuss them collectively, and, while a great many notes 

 were made in the field, it does not seem advisable to publish all 

 of these details. More or less difficulty is also met with in dis- 

 cussing the uses of the clays from this area. One type of clay is 

 sometimes used for three or four different purposes, or several 

 clays of widely different physical character may all be employed 

 for making the same kind of ware, being used not alone, but as 



