36 



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 up-hill transportation to the mill. 



The only raw material for Portland cement manufacture that 

 is extensively worked by dredging, in the United States, is marl. 

 Occasionally 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 

 throughout its mass, is usually covered by a more or less consid- 

 erable 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 investi- 

 gation. Occasionally, in deposits which either were originally 

 covered by very little water or have been drained, the shovel is 

 mounted on a car, running on tracks laid along the edge of the 

 deposit. 



The material brought up by the diredge may be transported 

 to the mill in two different ways, the choice depending largely 

 upon the manufacturing processes in use at the plant, At 

 plants using dome or chamber kilns, or where the marl is to 

 be dried before sending to the kiln, the excavated marl is usu- 

 ally loaded by the shovel on cars, and hauled to the mill by horse 

 or steam power. At normal marl plants, using a very wet 

 mixture, it is probable that the second method of transporta- 

 tion is moire economical. This consists of dumping the marl 

 from the excavator into tanks, adding sufficient water to make 

 it flow readily, and) pumping the fluid mixture to the mill in 

 pipes. 



Cost of raw materials at mill. The most natural way, per- 

 haps, to express the cost of the raw material delivered at the 

 mill would be to state it as being so many cents uper ton or 

 cubic yard of raw material ; and this is the method followed by 

 quarrymen or miners in general.. To the cement manufacturer, 

 however, such an estimate is not so suitable as one based on the 

 cost of raw materials per ton or barrel of finished cement. 



In the case of hard and comparatively dry limestones or shales, 

 it mav be considered that the raw material loses 33 1-3 per cent. 



