MATERIALS. 



in 



Name of stone. 



LIMESTONE 



Mountain ... 

 Plymouth ... 

 Lias 

 Magnesiau ... 



OOLITE 



Portland stone Best bed 

 Top bed 



Roach bed 



Bath stone .. 



Weight of 



a cubic foot 



in pounds. 



... 165 



... 156 



... 150 



... 150 



... 180 

 ... 158 

 ... 165 



... 180 



... 175 



166 



Basalt and kindred rocks, in consequence of their high 

 specific gravity and great hardness, are very suitable for use in 

 forming the rubble-mound bases of breakwaters, and in other 

 situations where dressing is not required. However, some 

 descriptions of basalt, which is only a variety of trap-rock, 

 disintegrate rapidly when exposed to the weather, so care is 

 required in making a selection. 



The several descriptions of granite and syenite are by far 

 the most useful, and best adapted for general harbour work. 

 These have a fairly high specific gravity, and are sufficiently 

 hard to offer substantial resistance to erosion and rough usage. 

 They can, at the same time, be readily cut and dressed, and, for 

 the most part, they weather well, and are obtainable in large 

 masses. 



Speaking generally, granites are distinguished from syenites 

 by the presence of mica, which in the latter is replaced by 

 hornblend. It would, however, be difficult, if not impossible, to 

 draw hard and fast lines between granites, syenites, and por- 

 phyries, as they merge imperceptibly into each other, not only 

 as regards their mineral constituents, but also in colour and in 

 the arrangement of their crystals. Thus we have porphyritic 

 granites, syenitic or hornblendic granites, and so forth. 



Granites decay chiefly by the decomposition of the felspar 

 which they contain, and also by the mica absorbing moisture, 

 which renders it liable to disintegration by frost. 



The yellow or brown appearance which syenites or granites 

 exhibit during decomposition is due chiefly to the oxidation of 

 the iron which they contain. 



