228 
Geology of Sydney. 
But it must not be thought that because shells make 
up the rock, all limestones were therefore formed in the 
sea. There are also fresh-water limestones, but the 
character of the shells always settles whether we are 
dealing with a fresh-water or a marine limestone. 
A second class — and this includes our cave lime- 
stones — shows, together with fossil shells, the calcare- 
ous framework of corals. Now, all corals are marine, 
and therefore these limestones are of marine origin. 
o 
Enormous areas in tropical seas are now covered with 
growing coral, each polyp separating carbonate of lime 
from the ocean waters. Some belts of Silurian 
limestone, near Molong, must have resembled these 
modern coral-reefs, for they are made up almost 
entirely of masses of the beautiful chain coral known 
to science as Halysites Australis . 1 
A third class of limestones is composed almost 
exclusively of the remains of crinoids. Many vast 
beds of limestone are nothing more than masses of the 
broken stems of these strange and beautiful creatures. 
Encrinites are also known by the name of sea-lilies or 
crinoids, from the Greek word lerinon. meaning* a 
lily. A few of these creatures survive to the present 
time, but they abounded in forest-like masses in 
Silurian days. They were known in mediaeval times 
as St. Cuthbert’s beads, and are referred to in “ Mar- 
mion” — 
“ On a rock by Lindisfarne, 
St. Cuthbert sits, and toils to frame 
The sea-born beads which bear his name.” 
1 Etheridge. “ lteoords oi the Australian Museum.” (See Fig. 70.) 
