90 HARBOUR CONSTRUCTION. 



its quality, which can be done with more certainty in this way 

 than by merely examining the surface or by taking borings. 

 Where possible, exposed faces, old quarry workings, or buildings 

 in which stone from the district has been used, should be care- 

 fully examined, with the view of ascertaining its " weathering " 

 properties. 



Overburden. An important point for consideration in select- 

 ing a site for a quarry is the amount of " overburden" 1 which 

 would have to be removed before the stone could be obtained. 

 Seeing that the cost of removing this must generally be added 

 to the cost of the stone, it is evident that, if the quantity should 

 be large, it would represent a material increase in the cost of 

 the work, and might point to the expediency of looking further 

 afield for a more suitable site. 



Since the disposal of overburden, when it has been removed, 

 may entail the purchase of additional land, it will be necessary 

 to roughly estimate the quantity. This calculation will be 

 facilitated by the knowledge gained in sinking the trial pits 

 before named. 



It will generally be found desirable to lay out a quarry floor 

 at such a level, and with such inclination (say 1 in 500, or even 

 flatter), that it will drain itself. As, however, the quality of 

 stone often improves as the depth increases, it may be found 

 advisable to sink the floor to a considerable depth, and rely 

 upon pumping to keep the quarry free from water. This latter 

 system is adopted in nearly all the Scotch granite quarries, a 

 very small pump being found sufficient, in most cases, to keep 

 the floor dry. When, however, large quantities of stone are 

 required, such an arrangement would be found to possess serious 

 drawbacks. 



A convenient and sometimes economical plan for disposing 

 Of " overburden " or other useless material, is to form an embank- 

 ment with it upon the quarry floor as the quarry working 

 advances, the cranes and lines for waggons being mounted upon 

 the top of it. The working floor of the quarry may thus be 

 10 to 12 feet, or other convenient depth, below the crane floor. 

 This will be governed by the quantity of rubbish to be disposed 

 of, and may be varied from time to time as circumstances require. 

 In thus disposing of overburden, etc., due provision for drainage, 

 either by natural flow or by pumping, must not be lost sight of. 



1 Earth or other material overlying the rock ; also called " baring," " tirr," etc 



