Narrabeen Shales. 
187 
ing up to 72 per cent, of iron, is a constituent of basalts, 
tuffs, and allied igneous rocks. We are familiar with 
the yellow and red tints of iron rust, or oxide of iron. 
The magnetite of the original rock is now altered into 
many varieties of iron oxide, and acts as the colouring 
constituent of the shales. Iron, in its various forms, 
has been called Nature’s colour-box, and the rich, warm 
hues of the chocolate shales, the soft yellows of the 
sandstone, and the rich reds of the ironstone bands are 
excellent examples of the variety and life given to 
rocks by iron oxide — Nature’s universal dye. 
Summary of the Trias or Trias-jura. — 
Between the Paheozoic and Mesozoic rocks there was 
a great break, during which the “ differentiation,” as 
it is called, of the vital functions of animal life 
became more marked. There was also a progress in 
“ Cephalization,” or head development. Other 
characters appeared, which are so obvious that even 
a slight acquaintance with geology is sufficient to 
impress one with the vast advance towards higher 
types made in the “ great interval ” between Palaeo- 
zoic and Mesozoic times. 
At the opening of the Mesozoic period the country 
around Sydney was undergoing depression. The 
basin now occupied by the Hawkesbury-Wianamatta 
series was either a great brackish or fresh- water lake or 
a land-locked sea, into which sediments were carried by 
rivers that have long since disappeared. The tract of 
land that supplied the sediments has also disappeared. 
Plants, many quite new, were abundant, and reptiles 
