LITHOLOGY. 
55 
that a microscope is required to distinguish tlieir structure — the 
former would be the first to decompose in consequence ; for ex- 
ample, among the numerous sandstone buildings in this country, 
those which have been constructed with a fine grained stone are 
usually in the best state of preservation ; and it may generally 
be considered as a sort of rule, that the same description of work 
can be performed in less time if the stone be fine-grained than 
if it be coarse. 
Particular cases may come under an architect's consideration^ 
wherein the specific gravity of stone is of serious importance, 
such as landing steps occasionally under water, where there may 
be a rapid current, or in other situations, subject to the influence 
of powerful waves, especially of sea-water, such circumstances 
will require that a heavy stone should be used, because all bo- 
dies immerged are reduced in weight precisely equal to that of 
the bulk of water which they displace. The lightest stone I 
have ever found is from Gatton, in Surrey, weighing lOSlbs. to 
the cubic foot ; now, if a similar stone were used in water, it 
would be reduced sixty-three pounds (the weight of a cubic foot 
of water) ; therefore it would be like building on land with a 
material weighing only forty pounds to the foot. The heaviest 
building stones that have yet come under my notice are the 
dark grey varieties of sandstone from the vicinity of Swansea, Gla^ 
morganshire ; Abercarne, Monmouthshire; the Forest of Dean, 
Gloucestershire, and the neighbourhood of Dundee ; some of which 
are even heavier than granite and quite as durable, weighing 
upwards of one hundred and seventy pounds per cubic foot. 
Hence it will be perceived that such stones are peculiarly adapted 
for docks, harbours, breakwaters to bridges, &c. In situations 
constantly or alternately under sea-water, sandstone is preferable 
to limestone, because it is not so liable to be perforated by Saxi- 
cava rugosa, Pholas, or other boring mollusca, which frequently 
honeycomb calcareous stones within their reach, to the depth of 
several inches. 
