THE NONMETALLIC MINERALS. 265 



sition of ciilcareous sediments on sea bottoms; manjM^eds, however, as 

 the oolitic- limestones, show unmistakable evidences of true ehcMuical 

 precipitation. They are in all cases eminently stratilied rocks, though 

 the evidences of stratification may not be evident in the small speci 

 men exhibited in museum colUn'tions. Vari(>i,al names other than 

 those mentioned above are given and which are dependent upon struc- 

 tural features or other peculiarities. A shaly limestone is one partak- 

 ing- of the nature of shale. Chalk is a fine pulverulent limestone 

 composed of shells in a finely comminuted condition and very many 

 minute foraminifera. (See p. 262.) The name chalky limestone is fre- 

 quently given to an earthy limestone resembling chalk. Marl is an 

 impure earthy form, often containing many shells, hence called shell 

 marl. An oolitic limestone is one made up of small rounded pellets 

 like the roe of a fish. The name marble is given to any calcareous or 

 even serpentinous rock possessing sufficient beauty to be utilized for 

 ornamental purposes. 



11^^^^^ — Aside from their uses as building materials, lithographic 

 purposes, etc., as described elsewhere, limestones are utilized for a 

 considerable variety of purposes, the more important being that of 

 the manufacture of mortars and cements. Their adaptability to this 

 purpose is due to the fact that when heated to a temperature of 1,000° 

 F. they gradually lose the carbonic acid, becoming converted into 

 anhydrous calcium oxide (CaO). or quicklime, as it is popularly called; 

 and further, that this quicklime when brought in contact with water 

 and atmospheric air greedily combines with, first, the water, forming 

 hydrous calcium oxide (CaOHjO), and on drying once more with the 

 carbonic acid of the air, forming a more or less hydrated calcium car- 

 bonate. In the process of combining with water the burnt lime (CaO) 

 gives off a large amount of heat, swells to nearly twice its former 

 bulk, and falls away to a loose, white powder. This when mixed with 

 siliceous sand forms the common mortar of the bricklayers, or, if with 

 sand and hair, the plaster for the interior walls of houses. (Specimens 

 Nos. 63U4, 63145, U.S.N.M., fromVermont; No. 53195,U.S.N.M., from 

 Maine, and No. 53168, from Pennsylvania, show the character of the 

 rocks commonly used for these purposes.) Quicklime formed from 

 fairly pure calcium carbonate sets or hardens after but a few days' 

 exposure, the induration, it is stated, being due in part to crystallization. 

 The less pure forms of limestone, notably those which contain upwards 

 of 10 per cent of aluminous silicates (clavey matter), furnish, when 

 burned, a quicklime which slakes nmch more slowly — so slowly, in fact, 

 that it is not infrequently necessary to crush to powder after burning. 

 These same quicklimes when slaked are further differentiated from 

 those already described by their property of setting (as the process of 

 induration is called) under water. Hence they are known as hydraulic 

 limes, and the rocks from which they are made as hydraulic limestones. 



