CLAY-BEARING FORMATIONS. 345 



It therefore seems to be desirable to discuss their physical prop- 

 erties separately. 



First group of Glacial and Colluvial Clays. 



Texture. — The clays of this group range from coarsely gritty 

 clays, to fine-grained laminated ones. They are often more 

 markedly stratified than those of the second group. 



Slaking. — They vary in their slaking qualities, but those which 

 have been formed in ponds and are thinly laminated, break up 

 first along the thin laminae of sand and then slake completely. 



Air shrinkage. — The clays tested varied from 3.5 per cent, 

 to 8 per cent, in their air shrinkage with an average of 6.3 per 

 cent. The lowest was a gritty clay loam from Flemington ( Loc. 

 276), and the two highest were fine-grained clays from Murray 

 Hill and Schooley's mountain, respectively. 



Water for tempering. — The percentage required ranged from 

 20.5 to 35.4, with an average of 26.4 per cent. Owing to the 

 abundance of fine grit, many of the Pleistocene and post-Pleisto- 

 cene clays absorb as much water as some of the more plastic ones. 

 Thus a loamy clay from Flemington took 23.9 per cent, of water 

 although its air shrinkage was only 3.5 per cent., and one from 

 Washington, derived by hillside wash from disintegrating gneiss, 

 with a shrinkage of 6.3 per cent., required 26.2 per cent, water. 



Tensile strength. — None of the samples tested except one (Loc. 

 292) showed a low strength, and one from Somerville (Loc. 234) 

 had a high strength. These two represented the extremes of 65 

 pounds and 297 pounds per square inch, respectively. 



Burning. — Many of the clays included in this section burn 

 steel-hard at a lower temperature than the majority of clays 

 found in the other formations with the exception of the Alloway 

 •clays. Those from localities 234, 276, 280, 290, 291 and 292 

 all burned steel-hard at cone 05. Most of them burn to a good 

 red color at this cone. The fire shrinkage at cone 05 is usually 

 low, although one from Murray Hill (Loc. 290) had a fire shrink- 

 age of 11 per cent, at this cone. On account of their low fire 

 shrinkage none of the samples tested burned dense at cone 05, 



