EXCAVATING THE RAW MATERIALS. 375 



In the cases where a wet clay or shale is quarried some distance 

 away from the mill the saving in transportation charges, due to dry- 

 ing at the quarry , would, of course, be considerable. 



Mining. 



The term "mining" will be used, in distinction from "quarrying", 

 to cover methods of obtaining any kind of raw material by underground 

 workings, through shafts or tunnels. Mining is rarely employed 

 in excavating materials of such low value per ton as the raw materials 

 for Portland-cement manufacture. Occasionally, however, when a thin 

 bed of limestone or shale is being worked, its dip will carry it under 

 such a thickness of other strata as to make mining cheaper than 

 stripping and quarrying for that particular case. 



Mining is considerably more expensive work than quarrying, but 

 there are a few advantages about it that serve to counterbalance the 

 greater cost per ton of raw material. A mine can be worked steadily 

 and economically in all kinds of weather, while an open cut or quarry 

 is commonly in a more or less unworkable condition for about three 

 months of the year. Material won by mining is, moreover, always dry 

 and clean. 



Dredging. 



The term "dredging" will be here used to cover all methods of 

 excavating soft, wet, raw materials. The fact that the materials are 

 wet implies that the deposit occurs in a basin or depression, and this 

 in turn implies that the mill is probably located at a higher elevation 

 than the deposit of raw material, thus necessitating uphill transporta- 

 tion to the mill. 



The only raw material for Portland-cement manufacture that is 

 extensively worked by dredging in the United States is marl. Occa- 

 sionally the clay used is obtained from deposits overlain by more or 

 less water; but this is rarely done except where the marl and clay are 

 interbedded or associated in the same deposit. 



A marl deposit, in addition to containing much water diffused through- 

 out its mass, is usually covered by a more or less considerable depth 

 of water. This will frequently require the partial draining of the basin 

 in order to get tracks laid near enough to be of service. 



In dredging marl the excavator is frequently mounted on a barge 

 which floats in a channel resulting from previous investigation. Occa- 

 sionally, in deposits which either were originally covered by very little 



