INDUSTRIAL MINERALS DIAGRAM 19 



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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 

 flowen of sulphur. This is what is used in agriculture. 



Slates and schists. All laminated rocks are called in a general 

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

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

 workshops, 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. Coal i-s 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 coal fields. 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 wagon 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 

 might happen that the galleries would be inundated, and work 

 become impossible. Another danger is fire-damp. The coal in 



