250 
Geology of Sydney. 
noxious gas, and distributing a thousand streamlets of acid 
water over the surrounding country, and which, as it drains 
off, not only wears the rocks it passes over, but dissolves 
them in minute quantities, especially such as contain much 
lime, and then, laden with its various compounds, flows off 
to the distant sea, where reef corals, lying in fringing 
banks round the coast, are slowly absorbing the lime from 
the water around them, and building the fragile coatings 
that protect them during life. Slowly as the land sinks, 
the coral bank increases in height, for reef corals can only 
live near the surface of the water ; and soon a considerable 
thickness has been obtained ; while below the upper zone of 
live corals lies a vast charnel-house of dead coral skeletons ; 
then comes a change : suitable temperature, or some other 
essential condition, fails, killing out all the corals, and 
through long ages other deposits accumulate over them, 
gradually crushing and consolidating the coral bank into 
a firm rock. At last a convulsion of the earth’s crust 
brings it up from the buried depth in which it lies, leaving 
it tilted on its edge, but still, perhaps, beloAv the surface of 
the ground ; rain, frost, and siioav slowly remove what 
covers it, until it lies exposed again to the sunlight, but so 
changed that only for the silent but irresistible testimony 
of the fossil forms of Avliich it is composed, it were hard to 
believe that this narrow band of hard grey rock was once 
the huge but fragile coral bank glistening in the bright 
waters with a thousand hues. And now the process 
is repeated ; the decaying vegetation of the surround- 
ing forest produces the carbonic acid, the rains spread 
it over the ground, which is now the most favourable 
for being dissolved, and the consequence is that the 
acid water saturates itself with the limestone rock, and, 
