114 
Geology of Sydney. 
formed in the lagoon, as well as the wind-swept sand- 
hills, were depressed beneath the waters and fresh 
sedimentary deposits laid down above them. 
The absence of coarse grit in much of the shale 
requires an explanation. The troubled waters rushing 
down from the hills brought grit and pebbles without 
a doubt, but these were kept back by the water 
having to find its way to the lagoon by filtering- 
through a thick undergrowth of reeds and fern. This 
is not a mere assumption — you can see reeds and ferns 
thickly imprinted on this block of shaley sandstone, 
and, as is well known, the shales are carbonaceous, and 
could only have derived this carbon from a dense 
vegetation surrounding and partly filling the lagoons. 
We shall see now hoiv this might account for the 
deposition of extremely fine sediments that form our 
shales. A process not at all dissimilar is yet going on 
m the interior of this colony. The Macquarie is 
slowly filling up the flat country beyond Cannonbar 
and the marshes further down. I have noticed, 
no matter how coarse the sediments are, that 
the river carries swiftly past .Dubbo and Warren, yet 
the sediments laid down on the plains far away from 
the river are invariably fine. Back from the river, the 
water flowing gently through the grass is perfect! v 
limpid, having deposited its load of sediment — being 
filtered, in fact — in finding its way through the 
thickly-growing grass. I have seen perfectly limpid 
and crystal-clear water “backing up” through the 
