30 CLAYS AND CLAY INDUSTRY. 



In some districts, the clay is underlain by a stratum of wet sand, 

 which should not be penetrated. In rare cases an underlying sand 

 bed is dry and may even serve for drainage purposes, as at Fish 

 House and City Line station, Camden. If the clay deposit lies 

 below the level of the surrounding country, drainage will be more 

 difficult than where the bed outcrops on a hillside, although in 

 the latter case trouble may be and often is caused by springs. 



Some banks contain several different grades of clay, and it 

 then remains to see whether they are all of marketable character, 

 or if not, whether the expense of separating the worthless clay 

 will overbalance the profit derived from the salable earth. 



Transportation facilities are not to be overlooked, either for the 

 raw clay, or for the product, where the factory is located at the 

 pit or bank. Long haulages with teams are costly, and steam 

 haulage is far more economical when the output warrants it. But 

 even with the establishment of favorable conditions in every case, 

 the successful marketing of the product is sometimes a long and 

 tedious task, for many manufacturers hesitate to experiment with 

 new clays. 



METHODS OF MINING. 



The methods of mining employed are slightly different for 

 clays and shales, the latter on account of their greater hardness 

 requiring stronger machinery. All of what follows regarding 

 mining methods will apply to clay, the mining of shale, being 

 referred to in a separate paragraph at the end of the section. 

 Clays may be mined by underground methods or by open pits. 



i. Underground zvorkings. — This method may be resorted to 

 when the clay bed is covered by such a great thickness of over- 

 burden that its removal would be too costly. If the bed sought 

 outcrops on the side of a hill, a tunnel or drift is driven in along 

 the clay bed, as shown in Fig. 22, but in case no outcrop is 

 accessible it is necessary to sink a vertical shaft (Fig. 23) until 

 the bed of clay is reached, and from this levels or tunnels may 

 be driven along the clay bed. 



Underground methods are desirable, however, only under cer- 

 tain conditions, which may be enumerated herewith : 



