340 



CEMENTS, LIMES, AND PLASTERS. 



Physical characters of marl. Marl as found in existing lakes may 

 contain as high as 50 per cent of water, while even the dry marl-beds 

 occurring in swamps or marshes will carry 15 to 25 per cent of moisture. 

 This moisture, together with the fine granular character of the marl 

 itself, gives it a sticky, putty-like character. In .color pure marl is 

 white, but it usually contains so much organic matter as to give even 

 the better samples a grayish or yellowish tint, while the more impure 

 marls may be very dark gray in color. 



Marl usually contains very little sand or grit, though some of its 

 shells and lime carbonate particles may give it a gritty feeling when 

 examined. Such shells, etc., can, however, be usually crushed between 

 the fingers, which will serve to distinguish them in the field from sand 

 grains. Though as sticky as clay, marl is markedly lighter in weight, 

 owing to the high percentage of moisture which it contains. 



The natural fineness of marl is a matter which is of direct interest 

 to the Portland-cement manufacturer, because of its effect on the 

 cost of grinding the raw material. Marls differ quite widely in this regard, 

 some being fine-grained throughout, while others contain considerable 

 percentages of coarse material, including shells, etc. 



The sieving tests tabulated below * were carried out by Prof. Davis 

 on samples of marl from three Michigan localities, and serve to show 

 the differences in fineness above noted. 



TABLE 156. 



FINENESS OF CRUDE MARL. 



(DAVIS.) 



1. Cedar Lake. 



2. Littlefield Lake. 



3. Michigan P. C. Co., Coldwater. 



The weight of the marl is also a matter of economic interest. A 

 wet marl, as dredged from a lake bottom carrying from 50 to 60 per cent of 

 water, may average about 2000 Ibs. per cubic yard, so that a cubic yard 

 of such material would contain only about 800 to 1000 Ibs. dry marl. 

 A dry marl taken from a well-drained marsh or swamp may run as 



* Vol. 8, pt. 3, Reports Michigan Geol. Survey, pp. 74-77. 



