THE GEOLOGY OF MITTAGONG. 307 
I. INTRODUCTION. 
The huge eruptive mass of ‘“‘The Gib’” has probably 
attracted attention from the time of the earliest settlers, 
and was termed trachyte by the earlier geologists. 
Previous Observers: 
Rev.W. B.Clarke in his ““Sedimentary Formations,’’ mentions 
occurrence of fossils in the shales. 
©. S. Wilkinson, described the coal measures of Mittagong 
very fully in “Annual Reports Mines Dept. N.S.W., 
(1879, p. 215; 1882, pp. 141-2; 1890, pp. 206 —11.) 
J. B. Jaquet, Chapter iIv., in his Memoirs on “‘ Iron Ore 
Deposits of N.S.W.,”’ treats of the Chalybeate Spring 
Deposits of Mittagong. 
R. Etheridge, Junr., in ‘“‘ Invertebrate Fauna of the Trias,”’ 
mentions Triassic fossils in the local railway cuttings. 
H. F. Pittman, in ** Mineral Resources of N.S.W.,’’ mentions 
Iron ore deposits, p. 199 and p. 205; Diamonds p. 395; 
Mineral Water p. 448; Building stone pp. 444-5. 
Mittagong is situated at the north-east extremity of a 
triangular valley whose apex divides the Mittagong Range 
at the angle nearest Bowral which is known as the Gap. 
This valley forms part of the watershed of the Nattai 
River, which runs northward through deep gorges in the 
Hawkesbury Sandstone. The Gib extends along the south- 
east boundary forming a large area of intermediate char- 
acter. Standing on the Gib and looking north, we see first 
the Mittagong valley consisting of sandstones and shales 
which are altered and intruded by numerous dykes of 
trachytic character. Further northward, beyond a steep 
escarpment are the anthracite mines where the Newcastle 
measures outcrop, surrounding what is evidently the eroded 
boss of a syenite laccolite. Beyond are the steep gorges 
+The name “Gib” is the popular abbreviated form of the name 
Gibraltar Rock” given to this mass on the Government maps. 
