112 HARBOUR CONSTRUCTION. 



Felspar crystals vary much in colour, but they may be readily 

 distinguished from quartz by their opacity, and by the fact of 

 their being soft enough to admit of being easily scratched by the 

 point of a knife. 



The fracture of good granite should show a fresh-looking, 

 bright, compact surface. A yellowish shade in grey granites, a 

 brown appearance in pink granites, or a dull fracture in either, 

 are pretty sure indications of decay. 



Chalk is not suitable for sea-works, unless, possibly, some of 

 the harder kinds, which may sometimes be employed with 

 advantage in positions where they will not be subjected to 

 erosion. Most chalks, however, are liable to become soft under 

 the influence of sea-water, and the amount of wear which chalk 

 cliffs within reach of the sea usually display should be taken 

 as a warning against placing much faith in such a material. 



Many of the limestones, oolites, and sandstones are very 

 suitable for breakwater construction, preference being given to 

 the hardest and heaviest of them. They are, however, all but 

 especially the two first named subject to the attacks of the 

 Pholas dactylus, which is a boring mollusc. Stones attacked by 

 these shellfish are often completely honeycombed by the holes 

 which they bore. The limestone used in the construction of 

 Plymouth breakwater has suffered greatly from them, neces- 

 sitating renewal in some places. 



Another small mollusc (Saxicava) is a stone-borer, but is less 

 formidable than the Pholas. 



Quartzite, or metamorphic sandstone, is a very hard and 

 durable rock, well adapted for sea- works. The Holy head break- 

 water is almost entirely constructed of it. It is generally 

 obtainable in large masses, but it is so hard as to render working 

 it almost impossible. It, like other stones, however, requires to 

 be selected. 



In a quarry of this stone which I worked for some years, 

 seams occasionally occurred which were permeated by fine white 

 clay; and although the stone generally was exceedingly hard, 

 that obtained from these seams was friable and unfit for use in 

 exposed situations. 



It is well, as far as possible, to ascertain the weathering 

 qualities of stones by noting the condition of exposed faces in 

 old quarry workings, buildings, etc. As a rule, those stones 

 which absorb least water are the best. 



