ECONOMIC ASPECT OF SALT UEGION. 199 



Vault-shaped quarries. — Where gunpowder is used, the quarries are 

 usually worked in the shape of a vault, sloping downwards at an angle of 

 about 60°. The vaulted shape of the cutting in the salt-rock and over- 

 lying bank of debris is to give some security against falling in, and the 

 sloping position is chosen to give stability to the debris and waste salt 

 which had been filled into the empty space, created by the preceding excava- 

 tion. The quarry men cannot afford to remove a large quantity of the over- 

 lying debris at a time, and thus lay bare a large surface of the rock-salt 

 upon which to work, and having to work successively, they just take as 

 much debris away as will enable them to go on with the salt excavation 

 for the time being. When they have excavated deep enough to make 

 the removal of the salt difficult, they start afresh from above, commen- 



A. 



cing by throwing the overlying debris down into the hollow which they 

 have left. By the time this hollow is filled, a certain surface area of 

 rock-salt is again laid bare, having the shape of a ring segment from 4 

 to 20 feet broad, extending round the margin of the former vault. The 

 area being ready, the excavation of the rock-salt is again taken up. 

 A cutting is first made along the new inner boundary by means of the 

 pick-axe, and then blasting is commenced. A blast takes off about 20 

 or 25 maunds of salt, and as the blasting proceeds, the cutting is carried 

 ahead along the inner wall so as to form a fresh vault gradually as the 

 old vault disappears ; the face of the debris which had been filled in is 

 laid bare, but remains standing, owing to its sloping surface, down which 

 the road leading to the bottom of the quarry is carried. Had the exca- 

 vation been worked down vertically and the debris been thrown into the 

 previously excavated space when the salt was removed by the new work- 

 ing, earth-falls would occur from the face of the debris. 



To this type of quarry, the general features of all, more or less, 

 approach. Where the c heading' is more than average, we even find real 

 mines on a small scale, though never without daylight, whilst with 

 exceptionally little debris the quarries arc more open with a larger work- 

 ing surface. 



( 303 ) 



