Silurian. 
217 
horizontal bedding of the Hawkesbury Sandstone and 
the Permo-Carboniferous, as seen in Figs. 71 and 
23. The angle of inclination which a tilted bed of rock 
makes witli the horizon, is called the angle of dip. On 
page 242 limestone is shown dipping to the west. The 
illustration herewith pictures some books standing 
on edge, just as Silurian rocks arc often seen to stand. 
Tri this instance the rocks or books dip in the opposite 
direction, and the edges showing on the surface are 
called “ outcrops.” The Silurian rocks now standing 
on edge were once spread out horizontally, layer above 
Fig. 60 — Illustration showing the meaning of dip, strike and outcrop. 
layer. Earth movements on a vast scale folded 
and compressed them. Denudation, at some sub- 
sequent period, removed the upper arches of the 
folds, and left highly inclined strata so familiar 
to us. In a small way this has been illustrated 
experimentally by Sir James Hall. Fig. 61 is a 
reproduction of his drawing. A number of coloured 
cloths are laid one above the other, to represent 
different stratified rocks ; a weight is placed above 
them, and then the whole series is compressed later- 
ourepops 
