INDUSTRIAL MINERALS DIAGRAM 20. 227 



sand. It is generally blackish or grey, but sometimes reddish. 

 It breaks like glass, and is sometimes semi-transparent. When 

 struck with iron, sparks appear, which are particles of iron 

 chipped off and ignited by the concussion. 



Millstone Grit is another kind of silex which is generally of a 

 white or reddish colour, and is hollowed with a number of large 

 and small cavities. In spite of the presence of all these holes, 

 millstone grit is an exceedingly durable stone, with which founda- 

 tions and buildings which are required to possess unusual solidity 

 are constructed. Millstone-grit is so called because it is also used 

 to make millstones. For this purpose stones must always be 

 chosen which are both hard and full of holes, so as to crush and 

 bruise the corn better. Millstone-grit answers these conditions 

 very well, but it is very rare to find quarries in which millstones 

 can be cut in one block, so they are made of several pieces fitted 

 together, and joined by cement. The millstone is then strongly 

 bound with iron, and allowed to dry for a very long time before 

 being used. These millstones are as solid as if they were made 

 out of a single piece. 



Rock-crystal, Agate, Glass. Rock-crystal and Agate are also 

 formed of the same substance as sand, sandstone, and millstone. 

 This substance is silex, flint, or quartz. When it is perfectly 

 pure, it is also perfectly transparent, and forms rock-crystal. In 

 agates, the quartz is slightly coloured, or traversed by veins of 

 different colours. Rock-crystal and agates are extremely hard* 

 and are frequently used to make ornamental articles. 



Glass is made of sand and soda, which is melted together at 

 an intense heat. Fine glass is often called crystal. It is more 

 sonorous, and is cut easier than glass. It is obtained by adding 

 a definite quantity of litharge to melted glass. 



Clays. Clays are earths of exceedingly fine grain, which form, 

 when mixed with water, an adhesive paste, which can be worked 

 in different ways. They are composed of more or less impure 

 alumina. Clays are sometimes bluish, yellowish, or red. Though 

 they can be mixed with water, water cannot easily penetrate them, 



