32 CLAYS AND CLAY INDUSTRY. 



ing the roof may be too 1 great. Timbering is always necessary 

 in underground clay work in New Jersey. Where the clay is not 

 interstratified between dense water-tight beds, it is often neces- 

 sary to leave the upper and lower foot of clay to form a roof 

 and floor, as shown in the diagrams. (Figs. 22 and 23.) 



4. The workings should be free from water, both on account 

 of the cost of removing the same and because of the tendency g£ 

 wet ground to slide. 



5. The output is usually restricted, unless the workings under- 

 lie a large area, and can be worked by several shafts or drifts. 



In New Jersey, underground mining has been attempted in 

 the South Amboy district, but abandoned in most cases. No 

 drift mining is now carried on, but shaft mining is employed at 

 one or two points in the South Amboy district along Cheese- 

 quake creek, where the overburden is very heavy. 



Swface working. — This consists in digging the material from 

 open pits or cuts of variable size. Where the pit is small, it is 

 commonly the custom to use picks and shovels to dig the clay, 

 and, indeed, this method is necessary in those cases where the 

 clay is not of uniform quality from top to bottom, or when a 

 number of layers of different kinds, as terra-cotta, fire and 

 stoneware clay are present. It is then necessary to strip off 

 each one separately and place it in a separate storage pile. This 

 is notably the custom in the Woodbridge and Perth Amboy 

 districts of New Jersey, and the practice followed there may be 

 described in some detail. 



When a pit is to be opened, the top dirt, stripping or bearing, 

 as it is variously called, is first removed to some place, where 

 it will not have to be disturbed, in order to avoid the cost of a 

 second moving, but, after one pit has been started, it is often 

 customary to use the stripping from a new pit for rilling the old 

 one. 



The cost of removing the stripping will depend on its char- 

 acter, whether hard or soft, the distance to be moved, and the 

 possibility of its being used for any purpose, such as filling or 

 grading. The methods of removal employed will also affect 

 the expense. If the thickness of the overburden is considerable, 

 and a large quantity has to be removed, it is cheaper to dig it 



