72 
Geology of Sydney. 
AQUEOUS ROCKS. — Rocks of Aqueous origin 
are divided into classes accordingly as they were 
formed by 
(а) the mechanical power of moving water ; 
(б) the intervention of some chemical agency j or 
(c) by the help of plants or animals. 
Rocks formed by the mechanical force of moving 
water are simply consolidated deposits of mud, sand, 
gravels, or shingle. These deposits were brought down 
by a river to an estuary or sea, or were produced by the 
action of the sea in wearing away a coastline. The 
denuded material is swept away into leas troubled waters, 
where it is laid down to form rock. In the course of 
time pressure alone, or pressure aided by chemical 
action, will consolidate the sand to sandstone, and the 
loose gravel to conglomerate, while the finer materials 
will settle down and form a shale or a slaty claystone. 
Aqueous rocks that owe their origin to some form 
of chemical action are exemplified in many of our 
limestones, particularly in the “formations” seen in 
limestone caves. Bands of ironstone that are seen in 
stratified rocks are also due to chemical reactions. 
Water holding carbonic gas in solution (most natural 
waters hold this gas dissolved) is able to dissolve 
limestone. This carbonate of lime or limestone will 
be held in solution only so long as the requisite 
carbonic acid gas is present. Directly the carbonic acid 
gas, from any cause, passes off, the carbonate of lime is 
dropped down as a fine white precipitate. Some 
