CLAY-BEARING FORMATIONS. 359 



fully stained with limonite. They do not slake more rapidly, 

 however, than the unweathered ones, and in fact most of the 

 samples, with the exception of the sandy ones, slake rather 

 slowly. All show a few small mica scales. 



Water required. — In the samples tested the percentage of 

 water necessary to produce a pasty mass ranged from 22.8 per 

 cent, up to 38.5 per cent., with an average of 29 per cent. One 

 brick mixture of clay and loam required but 21.8 per cent of 

 water. 



Air shrinkage. — The air shrinkage with a few exceptions is 

 rather high, ranging from 3 per cent. to> 11 per cent., with a num- 

 ber showing shrinkage of 7 per cent, or over. This high air 

 shrinkage is sometimes accompanied by a warping of the clay in 

 air drying, and the addition of sandy material is often necessary 

 to avoid it. 



Tensile strength. — The samples of Clay Marl II show con- 

 siderable variation in their tensile strength, ranging from an 

 average of 126 pounds for the lowest sample, to 286 pounds per 

 square inch for the highest. The one with the highest strength, 

 from 1 mile south of Collingswood (Loc. 144) is used in the 

 manufacture of brick and tile. Its air shrinkage at the factory 

 is lower than that given in the table, because it is molded much 

 drier than was the sample in the laboratory. None of the clays 

 of this formation are, however, deficient in tensile strength. 



Behavior in burning. — With the exception of the sample from 

 \Yz miles south of Haddonfield (Loc. 145) all burn to a light 

 brick-red at cone 05, but they did not all burn steel-hard, only 

 those from 1 mile west of Woodbury (Loc. 157) and north 

 of Matawan (Loc. 231) doing so. Most of them become 

 steel-hard at cone 1, and two, viz., those already mentioned 

 from Loc. 231 and Loc. 157 vitrify at cone 3, but the others 

 did not vitrify below cone 5. When heated above cone 1, 

 most of them lose the bright red color, and become grayish or 

 brownish in tone. Since glauconite grains are wanting in Clay 

 Marl II, none of the samples show the fused specks of this min- 

 eral, so noticeable in Clay Marl I. They also> lack the numerous, 

 large mica scales found in the clays of the preceding member. 



