184 
Geology oe Sydney. 
day. Plants grew thickly around the water’s edge. 
Their remains form thick carbonaceous materials, am: 
the fact that they are so perfectly preserved shows they 
were not drifted far, but grew close by where we now 
find them. We must picture the area now occupied 
by the Narrabeen Shales as a land-locked lake of con- 
siderable extent. Rivers from the surrounding high- 
lands swept into this lake. A dense vegetation fringed 
the waters, and the hill sides were covered by the 
straight-stemmed and graceful Phyllotheca. The 
sediments brought down by the rivers and deposited 
layer above layer we now recognise as the Nar- 
rabeen Shales. But it is obvious that a great tract of 
hill country must have been worn away to furnish 
material for the accumulation of so vast a mass as we 
see in these shales. What, it will be asked, was the 
nature of the country that was used up, so to speak, 
in the building of the Narrabeen Shales ? The hills 
were formed of Permo-Carboniferous strata, with 
lavas and volcanic ash deposits — the result of the 
volcanic outbursts towards the close of the preceding 
Permo-Carboniferous period. Part of the Narrabeen 
Shales are, therefore, said to be built up of re- 
distributed volcanic material. Much, if not all, of 
the volcanic materials, however, in these beds was, in 
Professor David’s opinion, derived from “contempo- 
raneous volcanic eruptions, which produced layers of 
greenish-grey ash or tuff.” 
The peculiar red colour of the chocolate shales can 
be thus accounted for. Magnetite, a mineral contain- 
