52 MINERALOGY. 



limestone in which there are some silica and clay, and 

 sometimes magnesia. Marl, which is of so great use in 

 agriculture, is a mixture of carbonate of lime with clay. 

 When carbonate of lime is deposited from the waters of 

 a mineral spring it is called calcareous tufa. Crystalli- 

 zation is prevented in this case from the constant mo- 

 tion of the water. Chalk, though abundant in England, 

 France, and many other countries, is not found at all in 

 this country, though the other common forms of carbon- 

 ate of lime are present in large quantities. 



101. Calcareous Spar. This name is usually given to 

 the crystallized varieties of carbonate of lime. The crys- 

 tals vary exceedingly from the variety of their secondary 

 forms. Some fanciful names are given to some of them 

 from their peculiar shapes, as dog-tooth spar and nail- 

 head spar. They are various in color, commonly white 

 or light gray, or reddish or yellowish, and sometimes 

 wine-yellow, red, rose, or violet. The crystals of Ice- 

 land spar, so called because they were first brought from 

 Iceland, are transparent, and they exhibit double refrac- 

 tion that is, objects seen through the crystal appear 

 double. Satin spar has its name from the satin lus- 

 tre which its beautifully fibrous arrangement presents. 

 There is a form of carbonate of lime, called aragonite, 

 that crystallizes after a different plan from that of calca- 

 reous spar, and it is much harder. 



102. Marble. This is a granular limestone, the grains 

 being imperfect crystals from their encroaching on each 

 other during their formation. The finest varieties are 

 called statuary marble, the best coming from Carrara, in 

 Italy, the island of Paros, and some other localities in the 

 same quarter. The coarse kinds are the common mar- 

 bles, which are either white or clouded with various col- 

 oring substances. In some marbles there are shells or 

 corals, these being composed of the same mineral with 

 the marble itself. Indeed, it is supposed by some that 

 all marble and other forms of carbonate of lime are made 



