CLAY-BEARING FORMATIONS. 361 



weathering, in unprotected deposits, frequently extends to a depth 

 of 6 to 8 feet or even more. 



The overburden is either Pensauken gravel, a pebbly loam, or a 

 sandy loam, the two latter being utilized at some yards for mixing 

 with the clay. In some banks, as at Maple Shade (Reeve's), 

 certain layers occur, which are very fat, of high shrinkage, and 

 which are unsafe to use alone. Those which are highly glauco- 

 nitic are also undesirable. 



Samples were tested from nine different localities, the location 

 of which can be seen by reference to the following table of tests. 



Slaking. — The clays of this formation are not dense, but, on 

 the contrary, are often porous, and hence slake very fast to a 

 powdery mass. Few of the samples tested show much evidence 

 of weathering. Where weathering has occurred the clay is com- 

 monly stained with limonite and is very sandy. 



Water required. — In tempering the amount of water required 

 by weight ranged from 20 per cent. to> 34 per cent., with an aver- 

 age of 26.2 per cent. 



In two cases, viz., localities 143 (620) and 149 (647) the tests 

 given are for the green-brick mixture, which in the case of locality 

 143 consists of Clay Marl I with a small amount of surface loam 

 added and at locality 149 of a mixture of Clay Marl I and II with 

 some loam. The effect of this addition did not materially affect 

 the quantity of water required for mixing the material up to a 

 stiff, pasty mass. 



When the clays of Clay Marl I are compared with those of 

 Clay Marl II it is noticeable that on account of their greater 

 sandiness they require much less water. 



Air shrinkage. — In the samples tested this ranged from 6 per 

 cent, to 8.8 per cent., with an average of 7.1 per cent. The higher 

 shrinkage shown by several is apparently due to their finer grain, 

 but the one with the maximum air shrinkage of 8.8 per cent, does 

 not absorb the greatest quantity of water. The air shrinkages 

 given in the table are slightly greater than those obtained in 

 practice, because most of these clays are molded in a stiff-mud 

 machine with little or no water added. 



Tensile strength. — None of the samples of Clay Marl I, so far 

 as tested, run low in tensile strength, and some of them, indeed, 



