186 CLAYS AND CLAY INDUSTRY. 



Fire-clay bed. — A thick, persistent bed of high-grade fire clay 

 occurs at the base of the Woodbridge clay. It is usually a light- 

 blue or gray color, although parts of it are red mottled, due to a 

 larger percentage of iron oxide in these portions. The clay carries 

 more or less white quartz sand, which is generally more abundant 

 in the upper and lower portions of the bed. These are often 

 called the "top-sandy" and "bottom-sandy" clay, respectively. 

 Y\ nere not sandy, the clay is hard and brittle and of a high degree 

 of refractoriness. Some of the No. i fire clay contains as low as 

 0.5 per cent, of quartz sand, but the average is 5 per cent., while 

 some of the sandy portion runs over 50 per cent. 1 Some portions 

 of the bed contain considerable pyrite in the form of "sulphur" 

 balls or nodules which have to be carefully picked out and rejected 

 in mining. 



The best clay, the "fine clay," or "No. 1 fire clay," as it is va- 

 riously called, is commonly in the central portion of the bed, but 

 there are frequent exceptions to- this. The spotted or red clays 

 are an inferior grade, and most commonly, but not always, occur 

 below the "fine clay." The line of demarkation is never a sharp 

 one, and the two varieties are both parts of a single bed. In some 

 pits the spotted clay does not occur and the whole bed is blue or 

 bluish-white clay. In other pits the spotted clay overlies the blue, 

 in still other localities it passes into the blue horizontally. As is 

 shown in Chapter XVIII, there is considerable variation in the 

 quality of even the "No. 1" clay dug in various banks. 



The surface of the Woodbridge fire clay is in places strikingly 

 irregTilar or wavy, just as was the case with the South Amboy 

 fire clay. Many instances are known where the top of the clay 

 undulates from 5 to 15 feet within a few rods. Where the top of 

 the bed is exposed over considerable areas, this undulatory surface 

 is apparent ; "sometimes rising and falling quite gently, forming 

 ridges or dome-like knobs or elevations and irregularly shaped 

 depressions or hollows ; at others, marked by exceedingly irregular 

 'bunks/ as the miners call them, and sink-like holes that succeed 

 each other without any apparent order or system." 2 The irregu- 



1 Cook & Smock, loc. cit. p. 51 and 53. 

 ' Cook & Smock, loc. cit. p. 49. 



