The Stones in the Streets. 
341 
that is accountable for the tendency of some of 
the sandstone to disintegrate and fret, particu- 
larly under projecting ledges and mouldings. 
Hardly anything has been done experimentally 
to settle the point. Mr. Wilkinson accounted for 
it as follows : — 
“ Sydney is built on the Hawkesbury Sandstone, and 
many fine edifices of the city can testify as to the value 
of the stone for building purposes. Some of it, however, 
contains an objectionable quantity of salts. Where this is 
the case the stone weathers, not on the surfaces exposed to 
rain, but on the sheltered or under-surfaces of the cornices 
of buildings. The cause may be easily explained. As the 
moisture in' the stone evaporates on coming to the surface, 
the. salt held in solution crystallizes therefrom, and, by the 
well-known power which the minute crystals exert when 
forming, they force asunder the grains of sand, and thus by 
degrees slowly disintegrate the rock ; whereas the upper 
surface of the stone is exposed to the rains, which con- 
tinually wash away the salt as it appears, and so prevent 
its crystallization.” 
There is hardly a limit to the number of localities 
where good stone may be found. The very 
best stone, however, hitherto quarried, comes 
from the quarries of Mr. Robert Saunders, 
Pyrmont. 
2. The blue metal used for road-making is an andesitic 
basalt, from a volcanic flow of Permo-Carboniferous 
age at Kiama. The same stone is also dressed 
into square blocks for paving. Basalts from 
Pennant Hills and Prospect are also used. 
