230 
Geology of Sydney. 
Many extinct crinoids had stems from fifty to 
seventy feet long, and Buckland calculated that some 
of them had 150,000 little bones and 300,000 contractile 
bundles of fibres to move them. The crinoids culmi- 
nated in Mesozoic times, and then practically died out 
without giving origin to anything higher. 
Turning now to Australian limestones, we can 
note the types referred to, but it often happens that 
crinoids, corals, and shells are all preserved on the one 
slab. But, as the crinoids, as well as the corals, are 
exclusively marine animals, the marine origin of the 
limestone is evident. 
Caves in General. 
Caves are found in almost all kinds of rock. 
They are more common in limestone and basaltic 
rocks than others. In the basalts, caves are formed 
either by violent igneous action, or the flowing away 
of molten masses, leaving an arch of consolidated or 
hardened material standing around. Caves are also 
formed in volcanic rocks, through the removal of 
softer portions of the mass by denudation or erosion. 
The well-known Blow-hole at Kiama is an example 
of this last — a dyke of softer material being carved 
into by the sea, while the main mass of basalt offers 
more resistance to the action of the waves. 
It may be well to explain here, that the “ caves ” 
in the sandstones on the Blue Mountains should be 
more properly termed rock-shelters. They consist, for 
the most part, of an overhanging ledge under which 
