18 



silica present is in combination with alumina, the resulting lime- 

 stone will be clayey or argillaceous. 



Chemical composition of limestone A theoretically pure lime- 

 stone is merely a massive form of the mineral calcite. Such an 

 ideal limestone would therefore consist entirely of calcium car- 

 bonate or carbonate of lime, with the formula CaCO3 (CaO-f- 

 CO2), corresponding" to the: composition calcium oxide (CaO) 

 56 per cent. ; carbon dioxide or carbonic acid (CC)2) 44 per cent. 



As might be expected, the limestones we have to deal with in 

 practice depart more or less widely from this theoretical com- 

 position. These departures from ideal purity may taike place 

 along either of two lines, 



a. The presence of magnesia in place of part of the lime ; 



b. The presence of silica, iron, alumina^ alkalies, or other im- 

 purities. 



It seems advisable to discriminate between these two cases, 

 even though a, given sample of limestone may fall under both 

 heads, and they will therefore be discussed .separately. 



a. The presence of magnesia- in place of part of the lime. 

 The theoretically pure limestones are, as above noted, composed 

 entirely of calcium carbonate and correspond to the chemical 

 formula CaCO3. Setting aside for the moment the question of 

 the presence or absence of such impurities as iron, alumina, si- 

 lica, etc., it may be said that lime is rarely the only base in a 

 limestone. During or after the formation of the limestone a cer- 

 tain percentage of magnesia is usually introduced in place of 

 part of the lime, htus giving a more or less magnesian limestone. 

 In the magnesian. limestones part of this calcium carbonate is 

 replaced by magnesium carbonate (Mg CO;}), the general 

 formula for a magnesian limestone being therefore 



x Ca COs+y Mg 03. 



In this formula x may vary from 100% to zero, while y will 

 vary inversely from zero to 100%. In the particular case of this 

 replacement where the two carbonates are united in equal 

 molecular proportions, the resultant rock is called dolomite. It 

 has the formula, CaCO3, MgCO3 corresponding to the com- 

 position calcium carbonate 54.35 per cent. ; magnesium carbonate 

 45.65 per cent. In the case where the calcium carbonate has 

 been entirely replaced by magnesium carbonate, the resulting 



