THE GEOLOGY OF MITTAGONG. _ 309 
obscure the true character of the country. (b) Very little’ 
has been said by previous observers as to the origin of the 
iron ore deposits of the district. They occur in two types. 
(1) Connected with basalt cappings situated on more or 
less elevated ground. (2) Deposits from ferruginous springs 
on low level ground. 
We did not examine class (1) in detail as they are of no 
economic importance. Their mode of origin has already 
been referred to. 
As regards the second class of deposits, they occur 
usually on the lowest ground in the vicinity; they are 
irregular shaped shallow masses of no very great magnitude 
(running up to 150,000 tons). In most cases the deposition 
can still be seen going on, in one or more small springs 
occupying crater-shaped depressions on the surface. The 
best known of these deposits occurs close to the Fitzroy 
Tron Works and has an area of 54 acres. Three of these 
*‘craters’’ appear on the surface. From one of these there 
is a considerable flow of water, another gives a small 
trickle, the third none at all. The water is used medicinally 
by the residents and contains besides other mineral sub- 
stances ‘0712 parts per 1000 of bi-carbonate of iron. The 
deposition of the iron is brought about in the shallow crater-. 
like depressions by exposure to the air, when carbonic acid* 
is evolved and the iron is deposited as a hydrated peroxide. | 
The composition of a sample of this ore analysed in the — 
Department of Mines is as follows :— 
Hygroscopic moisture (100° C.) ... snr he 
Combined water Ay B ie Ae wag LOOT 
- Ferric oxide ... an ae he och AOS 
Silica ... 2B. tas pas a seek AD 
Ree heer i OS 
mio ae “h Sag . ee ‘76 
AGE tal t535 fa oe epee HO. os 
