CALCIUM CARBONATE AND SILICA 31 3 



Chalk, limestone, and coral are composed chiefly of calcium 

 carbonate, and consist almost entirely of the remains of animal 

 organisms. 



Marble, also, is a form of calcium carbonate. It is really 

 limestone, which, by the effect of great pressure and heat in the 

 earth's crust, has become much harder and crystalline. 



Silica. More widely spread throughout the earth's crust even 

 than calcium carbonate is the compound silica, which consists 

 of the oxide of an element termed silicon, and which is repre- 

 sented by the formula Si0 2 . This oxide enters into the forma- 

 tion of by far the greater number of minerals and rocks, being 

 combined with metallic oxides in the form of silicates ; indeed, 

 in the free state or thus combined, silica forms more than half 

 the weight of the earth's crust. In the pure state it occurs 

 both crystalline and amorphous. Two crystalline varieties are 

 known, one, tridymite, is unimportant, while the other, quartz, 

 is a frequently-occurring and highly-interesting mineral. If the 

 quartz is quite clear and transparent, it is known as rock-crystal, 

 and is the Brazilian pebble from which lenses are made for eye- 

 glasses, &c. Sometimes the oxide of a heavy metal is present, 

 colouring the quartz. 



Many sands are made up entirely of grains of quartz which 

 have become more or less rounded by continual rubbing against 

 one another in water. If the sand becomes compacted by the 

 introduction of a cement and by the action of great pressure a 

 sandstone is formed. 



Amorphous Silica. This is found in the form of (1) 

 chalcedony and its varieties, (2) jasper and its varieties, (3) 



Chalcedony is known having all sorts of tints. It is often 

 regarded as a mixture of quartz and opal ; it is familiar as the 

 well-known red stone used in signet-rings and called carnelian. 

 Agate is a variegated chalcedony, composed of different coloured 

 bands. Flint is generally of a black or dark gray colour, and is 

 found in nodules or bands in the chalk formations of Surrey, 

 Kent, &c. 



Jasper is an opaque, impure form of silica, of a red, brown, or 

 yellow colour. 



Opal. This species of naturally-occurring silica always con- 

 tains water in varying amounts. It is supposed to contain some 

 quartz as well as amorphous silica. It is often used in jewellery ; 



