A Treatise On The Principles And Practice Of Harbour Engineering

86 HARBOUR ENGINEERING. rocks such as quartzite. Many of the harder sedimentary rocks, though suitable in other respects, are unfortunately subject to the depredations of two troublesome molluscs, the Pholas dactylus and the Saxieava, both of which attack limestone and sandstone. Limestone blocks at Plymouth breakwater have had to be replaced by granite blocks on account of the ravages of the Pholas, which has already been mentioned in connection with its attacks on timber structures. Boring its holes in close proximity to one another, it honeycombs masonry work until it brings about its destruction. The weights and specific gravities of stone suitable for maritime purposes are somewhat as follows. It will be understood, of course, that there is often a considerable range of weight in material of the same class, according to locality, owing to variations in composition and texture. Weight and Strength of Stone. Weiglit in Ibs. per cub. ft. Crushing Load in Ibs. per sq. in. Specific Gravity. ; Granites. 160 - 190 8,000 - 14,000 2’5 to 2’97 Basalts and Traps . 170-199 8,000 - 16,000 2'65 to 2-97 Limestones 130-170 3,000- 9,000 2’03 to 2’65 Sandstones 150-170 2,000- 8,000 2-34 to 2’65 Granite has been used in the construction of two notable breakwaters in this country —those of Plymouth and Portland. The stone used at Plymouth came from the quarries of Colcerrow and Roughtor in Cornwall and Pewtor in Devonshire. Penryn in Leicestershire, in addition to Cornish quarries, supplied stone to Portland, where a large quantity of the local limestone was also used. Holyhead breakwater was built of Anglesea stone, which, nominally a granite, is really a quartzite. Alderney breakwater consista maiuly of the native Mannez stone, a sandstone grit of such extraordinary hardness as to exceed that of the neighbouring Guernsey granite. Obtainment.—Next to the selection of a stone cornes the question of the facilities for its obtainment and the cost of conveyance. Certain breakwaters have been so fortunate as to be located in the immediate neighbourhood of a suitable quarry. In other cases stone has had to be transported from some distance. Generally speaking, upon a rocky coast stone is likely to be fairly pleutiful and cheaply procurable; on a sandy shore, where stone is not so accessible, other forms of construction, such as fascine work, may commend themselves to preference on economical grounds. Quarrying’.— The art of quarrying is one which is often applied to spécial purposes: some quarries being mainly worked for building blocks and others almost entirely for setts and road metalling. Obviously, neither of these departments claim any attention here. Stone which is required for breakwater purposes is of an intermediate character—not so small as for macadam, nor so regular as for architectural work. The rubble which is