218 
Geology of Sydney. 
ally. The layers are thrown into folds, and, as the 
pressure increases, the folds become more acute. 
There is ample proof that the Silurian rocks were 
compressed and folded in a somewhat similar manner. 
The upper folds were planed off by denuding agencies, 
leaving the series of inclined rocks shown in the 
illustration. 
The highly inclined and fine-grained Silurian 
rocks are usually referred to as Silurian “Slates." In 
one sense the term is not correct, as no true slates are 
found in this colony. All our Silurian sedimentary 
Fig. 61. — Illustration showing how horizontal layers of rock are, by 
lateral compression, bent in folds. 
rocks split along the bedding planes. True slates, 
such as the Bangor roofing slate, cleave into thin 
slabs, but in a direction independent of the bedding 
planes. Slaty cleavage is induced in rocks by intense 
pressure. In Fig. 64 we have a series of rocks gently 
folded at one end of the section and highly inclined at 
the other end. A slaty cleavage, denoted by fine lines, 
is developed in the whole series. But it will be noted 
that, whereas the cleavage and the bedding lines 
