io6 



HARBOUR CONSTRUCTION. 



astonishing how soon pieces of the most refractory rocks, such 

 as quartz or granite, become rounded and worn down when 

 subjected to such action. 



The rapidity with which concrete blocks, even of the hardest 

 quality, are worn away by the motion of shingle in front of or 

 amongst them is well known to most engineers ; and blocks of 

 granite, although much more durable than concrete, are similarly 

 affected. In illustration of this, I give the following sketch, 

 which represents a large red granite boulder one of several 

 which until recently protruded from a shingle beach on the 

 south side of Peterhead Bay. 



Seaward 

 Side 





FIG. 19. 



It must not be imagined that erosion by sand or shingle is 

 due merely to their being rolled about by the waves, because it 

 is a fact that they are taken up by them and dashed against 

 opposing objects with enormous force. I have seen the face of a 

 sea-wall, composed of hard 6 to 1 Portland cement concrete, 

 starred by the impact of pebbles driven against it during a 

 storm to such an extent that it looked almost as if it had been 

 subjected to rifle-fire. 



The projections which were left upon the large and very 

 hard quartzite stones forming the sea face of the superstructure 

 of Holyhead breakwater, have already been much reduced 

 and rounded by the grinding of rubble and shingle against 

 them. 



