35 



raw materials for cement manufacture. If marl, which is usu- 

 ally worked by dredging, be excluded from consideration, it 

 is probably within saife limits to say that 95 per cent, of the 

 raw materials used at American Portland cement plants are 

 obtained by quarrying. If marls be included, the percentages 

 excavated by different method's would probably be about as 

 follows : Quarrying, 88 per cent. ; dredging, 10 per cent. ; 

 mining, 2 per cent. 



In the majority of limestone quarries the material is blasted 

 out and loaded by hand on to cars or carts. In a few lime- 

 stone quarries a. steam shovel is employed to do the loading, 

 and in shale quarries this use of steam shovels is more fre- 

 quent. In certain clay and shale pits, where the materials are 

 of suitable character, the steam shovel does all the work, both 

 excavating and loading the ra,w materials. 



The rock is usually shipped to the mill as quarried without 

 any treatment except sledging it to convenient size for load- 

 ing. At a few quarries, hbwever, a crushing plant is installed 

 at the quarry, and the rock is sent as crushed stone to the mill. 

 A few plants also havfe installed their driers at the quarry, and 

 dry the stone before shipping it to the mill. Except the sav- 

 ing of mill space thus attained, this practice seems to have little 

 to commend it. 



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 tun- 

 nels. Mining is, of course, rarely employed in excavating ma- 

 terials of sucht low value per ton as the raw materials for Port- 

 land 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 counter- 

 balance 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 mo>re or less un- 

 workable condition for about three months of the year. Ma- 

 terial 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 



