FOR GREAT BRITAIN AND IRELAND. 91 



Beton. Of late years Mr. Walker has introduced from 

 France the use of beton as a substitute for backing. This 

 artificial concrete is sometimes used in enormous blocks. 

 I have seen Cherbourg blocks of 50 tons prepared in boxes, 

 whose sides and tops are removed after the concrete has 

 set, in order to be again similarly employed. The pro- 

 portions used at Cherbourg by M. Rebeille were two of 

 sand or fine gravel to one of Portland cement ; and the 

 blocks thus prepared began to set in about two hours, were 

 as hard as Roman cement in 24 hours, and in the course 

 of a few months became nearly as hard as stone. 



The first layer would be arranged across the pier, so as 

 the better to fit the irregularities of the bottom, and above 

 that they might be arranged lengthways of the pier, so as 

 to form its outer and inner walls, the space between being 

 filled with common rubble or beton. 



DESTRUCTION OF STONE. 



The Pholas bores through wood, blue lias and marble 

 limestone, hard and soft shales, sandstone, clay, and is much 

 found in peat deposits. 



The saxicavcs make the limestone blocks holed like a 

 honeycomb ; pieces break off, and the creatures commence 

 work again on a new surface. 



Mr. Stevenson quotes Professors Forbes and Hanley 

 regarding this (* Hist. Brit. Mollusca,' vol. i. p. 104). 



OF IRON. ITS DURABILITY. 



Promenade-piers are more often built of iron piles than 

 other materials, but not being calculated to afford shelter 

 the writer will not go into this material to any great extent. 



