QUARRYING. 101 



cloven with "plugs and feathers." Such holes are drilled in 

 rows, and upon their accuracy, not only of position, but also of 

 direction, the success or failure of any " cut " will greatly depend. 



Explosives. Dynamite, guncotton, and other quick explosives, 

 will be found to crack granite and similar hard rock for a con- 

 siderable distance from the charge. When rock is only required 

 as rubble, these explosives may often be used with advantage ; 

 but when blocks of stone are required for ashlar-work, their 

 general use is inadmissible. Insipient cracks thus formed are 

 often very difficult to detect, and in some cases are only dis- 

 covered when the stone parts under the mason's hammer, possibly 

 after a considerable amount of labour has been expended upon it. 



In driving headings for mines, small charges of guncotton or 

 of nitro-glycerine compounds will be found of great service, they 

 being, as a rule, far superior to powder for such work. 



When rock is liable to be too much broken up by ordinary 

 blasting, seam-firing may be resorted to. 



Seam-firing. In the first instance, holes are drilled along the 

 line of the seam or joint which it is intended shall form the 

 limit of the mass to be dislodged. The charges are then so pro- 

 portioned that they shall only have power enough to push the 

 mass forward to the extent of a few inches. When this has been 

 done, and a sufficient time at least four hours allowed to 

 elapse to ensure safety, powder is deposited in the seam, and 

 tamped with such suitable material as may be at hand. Tamp- 

 ing seams is by no means an easy or perfect operation, because, 

 owing to their narrowness and irregularity, it is often impossible 

 to ram the tamping properly, and we have, therefore, to be 

 satisfied with the best we can do. In seam-firing, extreme local 

 pressure and shock are avoided, and the energy of the exploding 

 charge is much more evenly distributed over the back of the 

 mass, which is thus heaved out with but little noise or disruption. 

 This system is largely adopted in quarrying granite for monu- 

 mental purposes, and I have found it very useful where it has 

 been difficult, from the output of ordinary blasts, to find sufficient 

 good stone for large ashlar-work. 



Some kinds of rock may be readily quarried by drilling lines 

 of holes, and bursting off pieces, of the dimensions required, by 

 means of " plugs " and " feathers " or wedges. This system cannot 

 well be adopted excepting where the rock is stratified. It 

 is, however, extensively resorted to in many sandstone and 



