28 



land 1 cement industry became prominent in this country, the 

 term marl has been used to cover several very different sub- 

 stances. The following three uses of the term will be found 

 particularly common, and must be guarded against when such 

 reports are being examined in search for descriptions of de- 

 posits of oement materials. 



(i.) In early days the term "'marls" and " marly tes" were 

 used to describe deposits of calcareous shales and often cov- 

 ered shales wihich were not particularly calcareous. This use 

 of the term will be found in many of the earlier .geological re- 

 ports issued by New York, Ohio, and other interior States. 



(2.) In New Jersey andi the States southward bordering 

 on the Atlantic and Gulf of Mexico, the term marl is commonly 

 applied to deposits of soft chailky or unconsolidated limestone, 

 often containing considerable clayey and phosphatic matter. 

 These limestones are of marine origin, and not related to the 

 fresh-water marl deposits which are the subject of the present 

 chapter. 



(3.) In the same States as are included in the last para- 

 graph, but particularly in New Jersey and Virginia, large de- 

 posits of the so-called 'green sand marls" occur. This mater- 

 ial is, in no way, related! to the true marls (which are essential- 

 ly lime carbonates), but consists almost entirely of an iron 

 silicate,, with very small percentages of clayey, calcareous, and 

 phosphatic matter. 



Origin or marls. The exact cause of the deposition of marls 

 has been the subject of much investigation and discussion, par- 

 ticularly in the past few years, since they have become of econ- 

 omic importance. The reader who wishes to obtain further de- 

 tails concerning this question will do well to refer to the fol- 

 lowing series of papers. 



(i.) Blatchley, W. S., and Ashley, G. H. The Lakes of 

 Northern Indiana, and their associated marl deposits, in 25th 

 Ann. Kept. Indiana Dept. Geology and Natural Resources, pp. 



31-321. 



(2.) Davis, C. A. A contribution to the natural history of 

 marl. Journal of Geology, Vol. 8, pp. 485-497. 



(3.) Davis, C. A. Second contribution to the natural his- 

 tory of marl. Journal of Geology, Vol. 9, pp. 491-506. 



(4.) Davis, C. A. A contribution to the natural history of 

 marl. Vol. 8, pt. 3, Reports Michigan Geological Survey, pp. 

 65-102. 



