FO UNDA TIONS. 265 



an over-end setting-machine, in carrying on work of this kind, 

 will thus be readily seen ; nevertheless, when work is of a fairly 

 straightforward description, the over-end system possesses many 

 advantages, and admits of work being rapidly executed. The 

 relative advantages and disadvantages of the several methods of 

 constructing breakwaters were, however, discussed in the last 

 chapter, and need not be further gone into here. 



Prior to building upon rock, all sand, gravel, or mud which 

 may have collected in pockets, gullies, or elsewhere on the line 

 of the work, should be removed, together with all weed and 

 other sea-growth, the rock surface being thus laid bare before 

 the foundations are put in. 



A small " grab " will often be found useful and expeditious 

 for such work; but it sometimes happens that the site is too 

 confined, or the rock surface too irregular, to admit of one being 

 worked to advantage. When this is the case, the work must be 

 performed by divers, and it becomes very costly. 



When rock is soft, or of a texture which admits of its being 

 easily worked, it is sometimes dressed and carefully levelled by 

 divers for the reception of the foundation course, the blocks of 

 which are laid directly upon the bed so prepared. When this 

 system is adopted, the first course is usually treated as a 

 compensating one, the blocks for it being made of special 

 thicknesses, increasing by, it may be, 6 inches or a foot at a 

 time, and the rock being dressed down to suit the nearest size. 



In other cases the rock is dressed to the level which is found 

 to entail the least amount of work, and the blocks are cut to 

 suit it. 



Where a foundation consists of hard rock, such as those of 

 igneous origin, or of the metamorphic class, this kind of treat- 

 ment is impracticable, and some levelling medium must be 

 introduced between the irregular rock surface and the first 

 course of blocks. 



There can be little doubt that when mass- work can be put 

 in, it makes better and closer work than bags, because, however 

 well the latter may be packed, they cannot make homogeneous 

 work, neither can a sufficiently true bed be formed by them to 

 found blocks upon without their surfaces being trimmed off or 

 levelled in some other way. 



Until recently, however, bags of concrete were commonly 

 used for founding blocks upon, the formation of accurately 



