232 Geology of Sydney. 
with elfin dwellers, and so added something of awe to 
the fascination that is ever linked with the mysterious 
and the unknown. 
From a geological standpoint the origin and 
formation of limestone caves becomes intelligible when 
we recall the fact that limestone is the most soluble 
of rocks, and is readily acted on by percolating 
waters. W e note that the sea excavates caverns in sand- 
stones along our coast. This is the result of mechanical 
action. The force of the waves tears masses of 
loosened rock away, and hurls back the fragments, so 
as to grind and pound the cliffs. The action of water 
on limestone is quite different. Surface waters dissolve 
limestones. The effect is chemical and corrosive, 
rather than mechanical and erosive. Rivers erode 
channels in slate rocks, and carry away the eroded 
materials in suspension. The mud and sand that 
discolour the waters of a swollen river are merely the 
ground-up rocks being swept along in suspension. 
Limestone, on the other hand, is dissolved, and the 
rivers of the world carry vast quantities of carbonate 
of lime in invisible loads to the sea. Of course, lime- 
stone rocks are eroded and torn away as well, but the 
hollowing out of caves is due mainly to the dissolving 
of the limestones. Rivers will carve out a valley or a 
gorge in the hardest rocks. The finest examples of 
river action are the famous canons of Colorado, with, 
in some places, almost vertical sides three and four 
thousand feet deep. The valleys on the Blue Moun- 
tains are in their own way not less remarkable, as the 
