312 
Geology of Sydney. 
great grinding-mills, where large pieces of stone 
go in at one end, and only fine sand and mud come 
out at the other. 
“A stream moving along at the rate of about half- 
a-mile (880 yards) an hour, which is a slow rate, can 
carry along ordinary sandy soil suspended in a cloud- 
like fashion in the water ; when moving at the rate of 
two- thirds of a mile (about 1,173 yards) an hour, it 
can roll fine gravel along its bed ; but when the rate 
increases to a yard in a second, or a little more than 
two miles an hour, it can sweep along angular stones 
as large as an egg. But streams often flow much 
faster than this, and so do rivers when swollen by 
heavy rain. 
“A rapid torrent often flows at the rate of eighteen 
or twenty miles an hour, and then we may hear the 
stones rattling against each other as they are irresis- 
tibly rolled onward ; and during very heavy floods, 
huge masses of rock as large as a house have been 
known to be moved. 
“ These are the two principal ways in which 
streams and rivers act as transporting agents ; they 
carry the finer materials in a suspended state (though 
partly drifting it along their beds) ; and they push 
the coarser materials, such as gravel, bodily along. 
But there is one other way in which they carry on 
the important work of transportation, which, being 
unseen, might easily escape our notice. Every spring 
is busily employed in bringing up to the surface 
mineral substances which the water has dissolved out 
