Silurian. 
219 
almost coincide towards the right hand of the section, 
at the other end the cleavage lines are at right angles 
to the bedding planes. It will thus be seen how the 
cleavage characteristic of true slate is often quite 
independent of the planes of stratification or bedding ; 
but cleavage planes may also coincide with the bedding 
plane. On the Blue Mountains no true cleaved slates 
are known, the slaty character of the Silurian rocks 
being due to the original planes of stratification. 
When the bedding-planes are close, and the bedding 
well marked, they are known as flagstones. 
Origin of the Silurian Slates. 
The evidence at present available points to the 
fact of these rocks having been laid down in the waters 
of a sea. All the forms of Silurian life known as 
fossils are Marine. We have shells, corals, trilobites, 
and crinoids, in fine-grained sediments, telling of an 
open sea far away from shore deposits. The Silurian 
is notable for its beds of limestone, made up almost 
entirely of the remains of corals, crinoids, and shells. 
Limestone. 
Limestone is a general term for all rocks the basis 
of which is carbonate of lime or calcium carbonate. 
As this chapter may be read by the beginner, or by 
visitors to our Caves who may have no special 
knowledge of geology, it is desirable to explain 
clearly some every-day terms. 
Calcium is ... ... a metal and one of the elements. 
