MAGNESIAN LIMESTONE. 145 



Crystallisation is a purifying as well as hardening process, 

 and the Carrara marble, which is sent from the Apennines to 

 all parts of the world, is found in large masses of dazzling 

 whiteness, embedded in stone of a very dark colour, con- 

 sisting of various impurities, such as animal and vegetable 

 matter, particles of flint, &c., which have been driven to the 

 surface by the process of crystallisation. Part of this dark 

 stone is usually left on the blocks of marble as a proof of its 

 quality. 



The most highly transparent crystals of carbonate of lime 

 are those known as Iceland spar, which is as clear as glass 

 and quite colourless. 



Some limestones, such as those of Westphalia, contain a 

 large amount of iron which, though probably derived from 

 sea-water, is more than could be separated from it either by 

 animals or vegetables, though both take up small quantities, 

 and it is to iron that red coral owes its colour. The bulk of 

 the limestone-iron was, however, probably separated by a 

 purely chemical process, being deposited by the water in the 

 place of a corresponding quantity of carbonate of lime. 



So, too, with the magnesian limestones, called dolomite, 

 which contain, some of them, more than thirty per cent, of 

 carbonate of magnesia, while fresh coral contains hardly so 

 much as one. It has been suggested that, as the ash of cer- 

 tain fresh-water plants contains a good deal of magnesia, 

 some dolomites may have been formed by them, just as it is 

 thought that some dark- coloured, carbonaceous limestones 

 may have been formed by the decay of different species of 

 Chara, a lime-absorbing plant, which grows in great profusion 

 in some lakes of North Germany. 

 K 



