274 HARBOUR CONSTRUCTION. 



levelling of small stone being placed directly upon it, the surface 

 may first be covered, and thus sealed, by a layer of soft concrete 

 in small bags ; or the rubble, after having been roughly levelled, 

 may be covered with canvas, and a bed of mass concrete 

 deposited, as described on pp. 266 et seq. 



When a mound has been formed by depositing stone of all 

 sizes in due proportions, it is practically impermeable, and in 

 such cases a levelling of small broken stone and chippings is 

 probably the readiest means of forming a true bed ; but when 

 only selected stone, to the exclusion of small, has been used, 

 about one-third of the mound consists of voids, and there is 

 necessarily a rush of water through it as each wave rises against 

 the face of the superstructure. 



In a mound thus formed, a levelling bed of small stone, unless 

 secured as above described, or in some other efficient way, would 

 be almost certain to work down into the voids of the mound, to 

 the manifest injury of the work. Other precautions, necessary 

 in using small stone for foundation work, were noticed when we 

 were considering rock foundations (pp. 268 et seq.). 



