CLAY-BEARING FORMATIONS. 351 



Prospecting for Cohansey clays is attended with considerable 

 difficulty, partly because of the heavy pine and brush growth 

 covering most of the regions in which they are found, and partly 

 because the clays are often overlain by a considerable bed of sand, 

 so that it is completely hidden. The flatness of the region and 

 absence of many rivers which would provide clay outcrops along 

 their banks, further interfere with the search. The absence of 

 outcrops along a river bluff would not necessarily mean that there 

 was no clay deposit in that vicinity for the reason that the Co- 

 hansey clay beds are rather basin-shaped, and do< not underlie 

 large areas in unbroken extent. The river might, therefore, have 

 cut its channel but a few feet beyond the limit of the deposit, and 

 thus give no indication of its presence. In searching for Cohan- 

 sey clays, therefore, the necessity of careful and detailed pros- 

 pecting with an auger cannot be too strongly emphasized. Dug 

 wells and records of artesian wells where available may also be 

 of value. 



ALLOWAY CLAY. 



This is one of the largest individual deposits . found in New 

 Jersey (PI. XIII), and yet one of the least worked, although it is 

 commonly found outcropping on the hill slopes, under conditions 

 that would facilitate its extraction or favor the erection of a plant. 

 The overburden in many cases is thin, and the beds where exam- 

 ined were usually found to be quite free from water. The clay is 

 commonly a tough, dense, plastic material, with little coarse sand, 

 and some fine grit, mica grains are comparatively rare, and the 

 pebbles when present are commonly quartz. Most of the Alloway 

 clays are so fine-grained that but few mineral particles are recog- 

 nizable with a hand lens, and quartz or rarely small flakes of mus- 

 covite are the only ones distinguished. Even sand grains one- 

 eighth of an inch in diameter do not always show except on 

 mechanical analysis, because they form such a small proportion 

 of the clay. Many of the clays are stained or mottled. 



Owing to the fact that the Alloway clay is but little worked, 

 a number of localities, 13 in all, were sampled. Nearly all the 



