INDUSTRIAL MINERALS DIAGRAM 20. 



223 



before it is fit for use. Sulphur burns in air with a blue 

 flame, but when it is heated out of contact with air, it volatilises 

 and becomes deposited in the form of a powder which is called 

 flowers of 'sulphur. This is what is used in agriculture. 



Slates and Schists. All laminated rocks are called in a genen 1 

 way schists. To extract slate, large square blocks of suitable size 

 are first detached in the quarry ; it is then carried to the work- 

 shops, where workmen called splitters divide it into sheets of 

 variable thickness, and these are the slates which are used for 

 roofing houses. Slates are also used to make school writing- 

 tablets, billiard-tables, and whetstones. 



Coal. Co.il is one of the most precious productions of the earth. 

 It is generally found at great depths, whence it must be procured 

 by mining, and the aid of powerful machinery. 



The districts where coal is found are called coalfields. Coal 

 belongs, like the schists, to the lowest sedimentary deposits. It 

 is generally arranged in thin layers, near each other, and very 



Coal Mine. 



slanting. When a shaft has been dug deep enough to reach 

 these layers, galleries are pierced into them, by means of which 

 the coal is brought to the foot of the shaft, from whence it is 

 raised by machinery. These galleries are generally very narrow, 

 and just large enough to allow a small waggon to pass. The coal 

 and rock are then separated. 



Coal mines are subject to special dangers which do not exist 

 in other mines to the same extent. Water is often abundant in 

 them, and must be pumped out day and night to allow the work 

 to proceed, for if the pumps should stop for only a minute, it 



