27 



Composition. In composition these chalks, or "rotten lime- 

 stones," vary from a rather pure calcium carbonate, low in 

 both magnesia and clayey materials, to an impure clayey lime- 

 stone, requiring little additional clay to make it fit for use in 

 [V.riland cement manufacture'. Analyses quoted from various 

 authors of a number of these chalky limestones are given in 

 Table IV, and will serve to show their range of composition. 



Analyses of Chalky Limestones. 



Fresh-Water Marls. 



Marls, in the sense in whidh the term is used in the Portland 

 cement industry, are incoherent limestones which have been 

 deposited in the basins of existing or extinct lakes. So far as 

 chemical composition is concerned, marls are practically pure 

 limestones, being composed entirely of calcium carbonate. 

 Physically, however, they differ greatly from the compact 

 rocks which are commonly described as limestones, for the 

 marls are granular, incoherent deposits. This curious physical 

 character of marls is due to the conditions under wlhich they 

 have been deposited, and varies somewhat according to the par- 

 ticular .conditions which governed their deposition in different 

 localities. 



A warning to the reader concerning other uses of the term 

 "marl" may profitably be introduced here. The meaning above 

 given is that in which the term marl is commonly used in the 

 cement industry at the present day. But in geological and agri- 

 cultural reports, particularly in those issued before the Port- 



