CHAPTER XXV. 

 FRESH-WATER MARLS. 



MARLS, in the sense in which the term is used in the Portland-cement 

 industry, are fine-grained, friable limestones which have been deposited 

 in the basins of existing or extinct lakes. So far as chemical composi- 

 tion is concerned, marls are practically pure limestones, being usually 

 composed almost entirely of calcium carbonate. Physically, however, 

 they differ greatly from the hard, compact rocks to which the term 

 limestone is more commonly applied, for the marls are granular, loose, 

 non-coherent deposits. These curious physical characters of marls, 

 as compared with ordinary limestones, are due to the peculiar condi- 

 tions under which the former have been deposited. Samples of marl 

 from different localities will on comparison be found to exhibit consider- 

 able variations, and these arise in large part from differences in local 

 conditions during deposition. 



As explained on a later page, differences of opinion exist as to the 

 exact cause of the formation of marl deposits. The points in con- 

 troversy are of no particular practical importance, and may be dis- 

 regarded in the present brief statement of facts. It may safely be 

 said that marls are deposited in lakes by spring or stream waters carrying 

 lime carbonate in solution. The actual deposition of the marl is in 

 part due to purely physical and chemical causes, and in part to the 

 direct or indirect action of animal or vegetable life. The result in 

 any case is that a calcareous deposit forms along the sides and over 

 the bottom of the lake, this deposit consisting of lime carbonate, mostly 

 in a finely granular form, interspersed with shells and shell fragments. 



Various uses of the term "marl". A warning to the reader con- 

 cerning other uses of the term "marl" may profitably be introduced 

 here. The meaning above given is that in which the term marl is com- 

 monly used in the cement industry at the present day. But in geological 

 and agricultural reports, particularly in those issued before the Portland- 

 cement industry became prominent in this country, the term marl has 

 been used to cover several very different substances. The following 



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