178 
Geology of Sydney. 
The Narrabeen Shales. 
The Narrabeen Shales, which underlie the Hawkes- 
bury Sandstones, are so named from the fact that these 
beds are well exposed on the coast about Narrabeen, to 
the north of Manly. These Narrabeen Shales form the 
lowest beds of the Hawkesbury-Wianamatta series. 
They are also at the basement of the sediments of 
Mesozoic age. Between their lowest layers and the 
coal-bearing rocks below there is a geological break, 
representing a vast period. Around Sydney, the Nar- 
rabeen Shales are seemingly conformable to the under- 
lying Palseozoic rocks. The fossils, however, show that 
a vast change had come over the face of the earth 
between the deposition of the Permo-Carboniferous 
and the overlying Narrabeen Shales. In fact, in most 
parts of the world, between Primary and Secondary 
rocks, there is a great change in both marine and 
terrestrial life, as stated on page 144. Here, close to 
Sydney, we have rocks in contact, but which are 
separated in age by a great interval difficult to mea- 
sure in years. Daring this “ interval ” — not, perhaps, 
synchronous all the world over — nearly all the plants 
that characterised the coal period have disappeared, 
and the oldest known mammal appears. With the close 
of the Palseozoic, marked here by the junction of the 
Narrabeen Shales and the Permo-Carboniferous, all the 
corals with septa in groups of four ( Rugosa) dis- 
appear for ever. Graptolites and Trilobites have long 
since become extinct, and the greater number of the 
lamp-shells named Bracliiopoda ceased to live, although 
