FRESH-WATER MARLS. 343 



in steady practice. The following data may throw some light on what 

 sort of results, are really obtained when a marl deposit is worked con- 

 tinuously. 



First, as to water percentages: At a Michigan plant which takes 

 its marl from under about 10 feet of water, an average of ten con- 

 secutive analyses gave the following results. 



Silica (SiO 2 ) 0.07 



Alumina (A1 2 3 ) j Q lg 



Iron oxide (Fe 2 O 3 ) / 



Lime carbonate (CaCO 3 ) 39.80 ( = CaO 22.29%) 



Organic matter . 59 



Water 59.36 



The marl in question is, it will be noted, very pure, being low in 

 both clay and organic matter. The point to be noted is the high per- 

 centage of water, each 100 Ibs. of material dredged containing approxi- 

 mately only 40 Ibs. of lime carbonate, with 60 Ibs. of water. It must 

 be borne in mind that this marl was not pumped to the plant, so that 

 this large percentage of water represents only what was unavoidably 

 taken up with the marl during dredging. 



Second, as to purity: A long series of analyses of marl at another 

 prominent Michigan plant gave the following limits of results. These 

 are calculated on a dry basis, the 55 per cent or so of water which the 

 marl carried when reaching the plant being neglected. 



Silica (SiO 2 ) 1 .40 to 8.60 



Alumina (A1 2 O 3 ) . 55 



Iron oxide (Fe 2 O 3 ) .25 



Lime (CaO) 54.60 



Magnesia (MgO) 1 .25 



Carbon dioxide (CO 2 ) 42 . 90 



Organic matter . 05 



1.30 

 1.54 



46.20 



2.78 



36.30 



10.50 



It will be seen from these results that the manager should expect to 

 receive material carrying often very high percentages of water and 

 frequently containing a large amount of organic matter and other im- 

 purities. 



Examining marl deposits. Owing to the nature of the material 

 and the physical conditions under which it occurs, the work of examin- 

 ing and valuing a marl deposit presents certain features of difficulty 

 peculiar to itself. As in any other prospecting work, the two factors 

 which require determination are respectively the extent of the deposit 

 and the composition of the material. 



