12 MINERAL COMPOSITION OF STRATIFIED ROCKS. [Ch. H 



Calcareous rocks. — This division comprehends those rocks which, like 

 chalk, are composed chiefly of lime and carbonic acid. Shells and corals 

 are also formed of the same elements, with the 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. White chalk is sometimes 

 pure carbonate of lime; and this rock, although usually in a soft and 

 earthy state, is occasionally 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 ap- 

 plied to a rock in which the grains are partly calcareous and partly sili- 

 ceous, or to quartzose sandstones, having a cement of carbonate of lime. 



The variety of limestone called " oolite" is composed of numerous 

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

 usually a small fragment of sand as a nucleus, around which concentric 

 layers of calcareous matter have accumulated. 



Any limestone w^hicli 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 is in many cases 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 ascertained by 

 applying to the surface a small drop of diluted sulphuric, nitric, or mu- 

 riatic acids, or strong vinegar ; for the lime, having a greater chemical 

 affinity for any one of these acids than for the carbonic, unites imm.e- 

 diately with them to form new compounds, thereby becoming a sulphate, 

 nitrate, or muriate of lime. The carbonic acid, when thus hberated 

 from its union with the lime, escapes in a gaseous form, and froths up 

 or efi'ervesces as it makes its way in small bubbles through the drop ot 

 liquid. This effervescence is brisk or feeble in proportion as the lime- 

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

 foreign matter mixed with the carbonate of lime. Without the aid ot 

 this test, the most experienced eye cannot always detect the presence ot 

 carbonate of lime in rocks. 



The above-mentioned three classes of rocks, the siliceous, argillaceous, 

 and calcareous, pass continually into each other, and rarely occur in a 

 perfectly separate and pure form. Thus it is an exception to the general 

 rule to meet with a limestone as pure as ordinary white chalk, or with 

 clay as aluminous as that used in Cornwall for porcelain, or with 

 sand so entirely composed of siliceous grains as the white sand of Alum 

 Bay in the Isle of Wight, or sandstone so pure as the grit of Fontaine- 



