THE GEOLOGY OF MITTAGONG. By kil 
This type of deposit is common from near Picton as far 
south as Goulburn (vide J. B. Jaquet’s report). One point 
to be noticed is that these deposits only occur in the 
vicinity of Post-Triassic volcanic action. The faulting 
and cracking of the sedimentary formations due to this 
action would allow the escape of the chalybeate waters to 
the surface, and consequent deposition of the ore. As to 
the source of the iron in the spring water, there are two 
possible hypotheses—that the iron was dissolved (a) out of 
the sedimentary rocks, (b) out of the eruptive rocks. 
(a) Dealing first with the possibility of its being derived 
from the sedimentary formations, we should expect to find 
this type of deposit scattered over the Permo-carboniferous 
coal basin, and that instead of occurring more abundantly 
near the periphery, as is the case, such ore deposits would 
be more extensive towards Sydney the centre of the basin. 
The iron certainly could not be obtained from the Hawkes- 
bury Sandstones, because of their small iron contents; so 
that if obtained from the sedimentary formations it would 
in all probability be derived from the coal measures. Objec- 
tions to this would be that whilst the coal measures occur 
over so large an area in the east of New South Wales the 
iron ore deposits are limited; also that water taken out of 
the coal seams does not contain nearly so large a percentage 
of iron. 
(b) The eruptive Intermediate rocks of the district con- 
tain slightly more than 8* of ferric oxide. Mr. Jaquet 
_ estimates the amount of iron ore as 150,000 tons containing 
70°58? ferric oxide. The decomposed syenite appears as a 
white trap, and would only contain a trace of iron, so that 
we may say that 8% of the ferric oxide in the rock is leached 
out during decomposition. Then we find to produce the 
iron ore deposit 1,323,300 tons of syenite would be required. 
Taking the specific gravity as 2°6 this is equivalent to 
18,185,230 cubic feet, 
