92 HARBOUR CONSTRUCTION. 



them periodically as the working face recedes. This arrange- 

 ment, however, soon gives trouble. The curves become too 

 sharp, and the lines are distorted by repeated sluing, so that 

 from time to time they require to be taken up and relaid ; the 

 quarrying operations, meanwhile, being seriously interrupted. 



Working Face. Whenever possible, a good working face 

 should be obtained, and in opening out a quarry this should 

 be borne in mind and arranged for. 



When the face is very high, it is sometimes worked in " lifts " 

 or " benches " of from 40 to 50 feet in height, especially where 

 small-hole firing is resorted to; but when mines are used, the 

 whole height of face is generally dealt with in one blast. 



" Strike " and " Dip " of Strata. The direction of the " strike " 

 and " dip " of the strata must be carefully considered in laying 

 out a quarry. In granite quarries, and others in igneous rocks, 

 the run of beds and joints is very uncertain, although in some 

 instances they display a regularity approaching to that of 

 stratified rocks. 



When the " dip " and " strike " of rock can be taken ad- 

 vantage of in quarrying, it will be found to greatly facilitate 

 work. 



Blasting. Quarrying is commonly conducted by firing com- 

 paratively small charges of blasting-powder, or other explosive, 

 placed in holes drilled to such a depth as circumstances may 

 require. 



The weight of charge must depend greatly upon the length 

 of the line of least resistance, the arrangement of beds and 

 joints of the stone, and the extent to which these have been 

 taken advantage of in deciding upon the position of the charge. 

 A common practice, in determining the amount of powder to be 

 used, is to cube the line of least resistance in feet, and divide 

 by a coefficient which varies according to the nature of the 

 rock to be blasted, and which can only be determined, with 

 accuracy, by actual trial and experience. 



For hard, heavy, compact rock, such as granite, quartz, etc., 

 the coefficient will be about 16 or 18, whereas for softer and 

 lighter descriptions of rock it may vary from 20 to 30. 



In some blasts at Holyhead, where the rock is of hard 

 quartzite, the coefficient adopted was from 10 to 12; whereas 

 at the celebrated and successful blast made at Round Down 

 Cliff, Dover, many years ago, on behalf of the South-Eastern 



