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mams of such organisms as the foraminifera, corals, and mol- 

 lusks. Most of the thick and extensive limestone deposits of 

 the United States were probably deep-sea deposits formed in 

 this way. Many of these limestones still show the fossils of 

 which they were formed, but in others all trace of organic ori- 

 gin has been destroyed by the fine grinding to which the shells 

 and corals were subjected before their deposition at the sea- 

 bottom. It is probable also that part of the calcium carbonate 

 of these limestones was a purely chemical deposit from solution, 

 cementing the shell fragments together. 



A far less extensive class of limestones though important in 

 the present connection' owe their origin to the indirect action of 

 organisms. The "marls," so important today as Portland ce- 

 ment materials, fall in this class. As the class is of limited ex- 

 tent, however, its method of origin may be dismissed here, but 

 will be described later in the section on Marls. 



Deposition from solution by purely chemical means has uns- 

 doubtedly given rise to numerous limestone deposits. When 

 this deposition took place in caverns or in the open air, it gave 

 rise to onyx deposits and to the "travertine marls" of certain 

 Ohio and other localities ; when it took place in isolated portions 

 of the sea through the evaporation of the sea water it gave rise 

 to the limestone beds which so frequently accompany deposits 

 of salt and gypsum. 



Varieties of limestone. A number of terms are in general 

 use for the different varieties of limestone, based upon differ- 

 ences of origin, texture, composition, etc. The more important 

 of these terms will be briefly defined. 



The marbles are limestones which, through the action of heat 

 and pressure, have become more or less distinctively crystalline. 

 The term mart, as at present used in cement manufacture, is ap- 

 plied to a loosely cemented mass of lime carbonate formexl in 

 lake basins as described on a later page. Calcareous tufa and 

 travertine are more or less compact limestones deposited by 

 spring or stream waters along their courses. Oolitic limestones, 

 so calK-d because of their their resemblance to a mass of fish- 

 roe, are made up of small rounded grains of lime carbonate. 

 Chalk is a fine-grained limestone composed of finely comminuted 

 shells, particularly those of the foraminifera. The presence of 

 much silica gives rise to a siliceous or cherty limestone. If the 



