346 
Geology of Sydney. 
to a mass of ironstone that has expanded th rough 
the alteration of its constitution by chemical 
action. Similar examples are not at all un- 
common, but need closer study. 
13. Finally, the fossils of the Hawkesbury-Wianamatta 
Series are rare indeed — so rare that every frag- 
ment preserved well enough for identification is of 
great scientific value. In this direction even a 
beginner may render a lasting service to tho 
Republic of Science. 
There are lines of cliff about Manly and other places, 
standing inland, and not now washed by the sea. Does 
this point to an elevation of the land, or to the accumulation 
of sand along their bases ? 
It is probable that the sea recently spread across the 
low-lying land from the Bay to the Ocean Beach, Manly. 
The ground between Bondi and the Harbour was, it is 
possible, also below sea-level in times geologically recent. 
A study of these localities should throw light on the origin 
of the present conditions, and decide whether these are due 
to a change in the level of the land or to the mere accumu- 
lation of blown sand. 
The Hawkesbury River is a “ drowned v river valley, 
into which the sea- water flowed subsequent to a depression 
of the land. How is this proved ? Could the valley be 
cut out by the present river, supposing the land to stand at 
its present level ? 
The valley of the Nepean affords splendid material for 
the study of the adjustment of streams to the structure of 
rocks. Traces of old river systems may also be studied on 
