344 
Geology of Sydney. 
offered as to their occurrence. Small scales of graphite are 
also commonly noticeable in the sandstones, and no satisfac- 
tory explanation has hitherto been given regarding the 
origin of this graphite. The residue left after washing 
a quantity of crushed sandstone is often observed to con- 
tain very minute green stones, not unlike sapphires. The 
true character of these has not been decided. Barite is 
also far from uncommon in the shales, and forms crystals in 
joints in the sandstones. Some explanation is needed as to 
the origin of this barite. 
AtNarrabeen, and also near Rookwood, the sandstones 
and shales become markedly calcareous. No one has fur- 
nished any proof as to whence this calcium carbonate was 
derived. « 
It may be useful to indicate a few directions in which 
even a student might do some acceptable original work : — 
1. No map has been yet compiled showing the basaltic 
dykes around Sydney. 
2. More information is needed concerning angular 
blocks of shale occurring in the Hawkesbury- 
Wlanamatta Series, and which seem to Mr. Wil- 
kinson to be indicative of Glacial Action in some 
form. 
3. Additional knowledge as to the extent and general 
character of the basalts on the Blue Mountains. 
4. The chemical composition and microscopic struc- 
tures of basalt are subjects hardly touched. 
5. In the Hawkesbury Sandstones we find areas of 
rather coarse and very fine materials, the occur- 
rence of which in juxtaposition needs elucidation. 
6. There is room for much research in determining the 
areas that were denuded to furnish material for 
