FRESH-WATER MARLS. 335 



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 deposits of cement materials. 



1. In early days the terms " marls" and " marly tes" were used to 

 describe deposits of calcareous shales and often these terms were 

 extended to cover shales which were not particularly calcareous. This 

 use of the term will be found in many of the earlier geological reports 

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



2. In New Jersey and the States southward bordering on the Atlantic 

 and Gulf of Mexico the term marl is commonly applied to deposits 

 of soft chalky or unconsolidated limestone often containing consider- 

 able 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, but particularly in New Jersey and Virginia, large 

 deposits of the so-called "green-sand marls" occur. This material is in 

 no way related to the true marls (which are essentially lime carbonates), 

 but consists largely of the iron silicate called glauconite, or green sand, 

 with very small percentages of clayey, calcareous, and phosphatic matter. 



The three early uses of the term "marl" above noted all agree in 

 that they apply to deposits of marine origin, while the marls of the cement 

 manufacturer are purely fresh-water deposits. 



Occurrence of marl deposits. Fresh-water marls occur in more or 

 less lenticular or basin-shaped deposits of relatively small size. Their 

 form and local character are both due to the fact that the marls were 

 formed by deposition in lake basins. In many cases these lakes still 

 contain water, and in some instances marl deposition is now in progress. 

 In other cases, however, the lake has entirely disappeared, and the marl- 

 bed now occurs in a swamp or marsh covered with peat or muck. 



The disappearance of a lake in this fashion must be regarded as part 

 of a very natural and almost invariable cycle of events. The existence 

 of a lake at any point along a drainage system is to be considered a 

 somewhat unnatural and temporary condition, and one which will be 

 removed by natural causes as soon as possible. In the glaciated 

 portion of the United States many lakes were formed at the close of 

 the Glacial period. These lakes were due in some cases to the fact that 

 deposits of sand and clay laid down by the glaciers had filled old 

 valleys and dammed the streams occupying such valleys. In other cases 

 the lake basins were formed by the irregular distribution of these glacial 

 deposits, leaving hollows and depressions which subsequently became 

 Billed with water. In either case a lake or pond was formed, and imme- 



