LIMESTONES. 89 



organic origin 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 very important in 

 the present connection, owe their origin to the indirect action of or- 

 ganisms. The " marls", so important to-day as Portland-cement 

 materials, fall in this class. As the class is of limited extent, however, 

 its method of origin may be dismissed here, but will be described later in 

 the chapter on marls, pages 334-347. 



Deposition from solution by purely chemical means has undoubtedly 

 given rise to numerous important limestone deposits. When this depo- 

 sition 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 locali- 

 ties; when it took plac.e 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 differences 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 pres- 

 sure, have become more or less distinctly crystalline. The term marl as 

 at present used in cement manufacture is applied to a loosely cemented 

 mass of lime carbonate formed in lake basins as described in more detail 

 in Chapter XXV. Calcareous tufa and travertine are more or less com- 

 pact limestones deposited by spring or stream waters along their courses. 

 Oolitic limestones, so called because of 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, par- 

 ticularly those of the foraminifera. The presence of much silica gives 

 rise to a siliceous or cherty limestone. If the silica present is in com- 

 bination with alumina, the resulting limestone will be clayey or argil- 

 laceous. 



Chemical composition of limestone. A theoretically pure limestone 

 is merely a massive form of the mineral calcite. Such an ideal lime- 

 stone would therefore consist entirely of calcium carbonate or carbonate 

 of lime, with the formula CaC0 3 [CaO + C0 2 L corresponding to the com- 

 position calcium oxide (CaO) 56 per cent, carbon dioxide or carbonic 

 acid (C02) 44 per cent. 



