64 T. W. E. DAVID. 
the gentle western slope of the fold. The summit of the fold is 
reached at the point where the old line of railway above the Zig- 
zag intersects the ancient river gravels. From here to the top of 
the Zigzag the strata resume their easterly dip, which increases 
rapidly until at the point where the septum is reached it amounts 
to 30°, and near the base of the septum to 50°. The exact shape 
_of the fold has been determined by studying the bending of the 
well marked bed of clay shale interstratified with the Hawkes- 
bury Sandstone about twenty feet below its surface. There is 
no evidence that shearing has taken place either in the fold or 
in the septum. The septum is about fifteen chains long, and the 
strata composing it dip at an average angle of 38°, the dip of the — 
current bedding being as high as 70°, measured from the present 
horizon, but never making an angle of more than about 26° with 
the true bedding planes. (See diagram 2, Plate 2.) 
The section above referred to, shews how the ancient river 
channel has partaken in the folding. Near the foot of the escarp- 
ment the septum becomes united to the trough; and here again, 
up to the present I have not been able to find any evidence of 
shearing. The fact that brittle rocks such as the Hawkesbury 
Sandstones have been bent so sharply and to such an extent | 
without any considerable fracturing, suggests that the bending 
movement was probably extremely slow. As regards the amount 
of displacement which has resulted from the folding, it is found 
that if the normal easterly slope of the sandstone platform be 
produced over the bottom of the trough near Emu Plains, the 
surface of the Hawkesbury Sandstone is about two hundred and 
fifty feet below its normal level. ‘The question here suggests itself 
—has the movement produced an upheaval of the eastern escarp- 
ment of the Blue Mountains, or a depression of the coastal strip, 
or both? If positive elevation has resulted, evidence of such should 
be afforded by a lessening of the easterly slope of the Blue Moun- 
tains as it approaches the top of the fold, so as to make the surface 
to the west of the fold slightly concave. Such a concavity exists, 
but only to a very limited extent; on the other hand there is 4 
