312 T. G. TAYLOR AND D. MAWSON. 
Now suppose that the iron ore was derived from the 
decomposition of the dyke which intersects it, suppose that. 
the decomposition reached as far as the coal seam, and the 
dyke to have an average width of 10 feet, then the deposit 
would represent the decomposition of the dyke for a length 
of over 3,000 feet. Objections to this assumption—that 
the iron has been leached out of such a length of the dyke 
and deposited at the one spot are 
1. The dyke is only seen to outcrop at the surface for a 
length of about 2,000 yards. 
2. That part of its iron contents went to build up another 
ore body situated further along the dyke. 
3. Iron derived from the decomposition of such a length 
would not under ordinary circumstances reach the 
surface at the same point. 
Now the point where the iron ore deposit occurs is about 
the lowest part of the valley between the Gib and the dome 
shaped hill over the anthracite mines, this can be readily 
seen on examining the accompanying section.* 
The sedimentary formation both next the Gib and in the 
anthracite valley is slightly tilted upwards by the syenite, 
“®Gib"” Rock. Intrusive Syenite — 
4 1. 
Fitzroy cao Works. 
has x Iron Ore deposit &, 
Chalybeate Spring 4 
Gt seeuene rn Line. 
nraots 
= 
BS 
SS 
2 
es 
Fig. 3—Section across the Mittagong Valley, showing the probable que 
of the chalybeate deposits. 
+ This section is drawn approximately to nce the data has been aw 
largely from Mr. Jaquet’s map. 
