Shells and corals are also formed of the sanW5ii^eiits 1 .with ifie 

 addition of animal matter. To obtain pure lime, it is necessary 

 to calcine these calcareous substances, that is to say, to expose 

 them to heat of sufficient intensity to drive off the carbonic acid, 

 and other volatile matter, without vitrifying or melting the lime 

 itself. White chalk is often pure carbonate of lime ; and this 

 rock, although usually in a soft and earthy state, is sometimes 

 sufficiently solid to be used for building, and even passes into a 

 compact stone, or a stone of which the separate parts are so 

 minute as not to be distinguishable from each other by the naked 

 eye. 



Many limestones are made up entirely of minute fragments of 

 shells and coral, or of calcareous sand cemented together. These 

 last might be called " calcareous sandstones ;" but that term is 

 more properly applied to a rock in which the grains are partly 

 calcareous and partly siliceous, or to quartzose sandstones, having 

 a cement of carbonate of lime. 



The variety of limestone called " oolite" is composed of numer- 

 ous small egg-like grains, resembling the roe of a fish, each of 

 which has usually a sma*ll fragment of sand as nucleus, around 

 which concentric layers of calcareous matter have accumulated. 



Any limestone which is sufficiently hard to take a fine polish 

 is called marble. Many of these are fossiliferous ; but statuary 

 marble, which is also called saccharine limestone, as having a 

 texture resembling that of loaf-sugar, is devoid of fossils, and a 

 member of the metamorphic series. 



Siliceous limestone is an intimate mixture of carbonate of 

 lime and flint, and is harder in proportion as the flinty matter 

 predominates. 



The presence of carbonate of lime in a rock may be ascer- 

 tained by applying to the surface a small drop of diluted sul- 

 phuric, nitric, or muriatic acids ; for the lime, having a stronger 

 chemical affinity for any one of these acids than for the carbonic, 

 unites itself immediately with them to form new .compounds, 

 thereby becoming a sulphate, nitrate, or muriate of lime. The 

 carbonic acid, when thus liberated from its union with the lime, 

 escapes in a gaseous form, and froths up or effervesces as it 

 makes its way in small bubbles through the drop of liquid. This 

 effervescence is brisk or feeble in proportion as the limestone is 

 pure or impure, or, in other words, according to the quantity of 

 foreign matter mixed with the carbonate of lime. Without the 

 aid of this test, the most experienced eye cannot always detect 

 the presence of lime in rocks. 



The above-mentioned three classes of rocks, the arenaceous, 

 argillaceous, and calcareous, pass continually into each other, 



