THE GEOLOGY OF MITTAGONG. 325 
tsFa 
ee cla 
Quarkles ! Impo fe G raphite 
Fig. 7.—Section A B (see fig. 6). 
due in many cases to their filling old river valleys. Hnsuing 
denudation has caused these ancient depressions to stand 
up as ridges. The very varied texture of the basalts 
obtained from localities near Mittagong is well shown by 
the sections. (See Section IV., Nos. 8 and 9.) 
2.—d. Basic Tuffs and Breccias.— 
1. Shaft over Gib Tunnel, (north end). 
2. Tuff on Currockbilly. 
do. Tuff near Frasers Hill. 
1. This example of basic breccia occurs in the spoil heap 
excavated from the shaft used in building the Gib Tunnel. 
A large quantity of Wianamatta shale has also been tipped 
here. The breccia is composed of fragments of shale, 
micaceous sandstone and older basalt enclosed in a dark 
matrix of basalt. It also contains large lumps of what 
appear to be orthoclase. These fragments have evidently 
been torn from the sides of the fissure through which the 
basalt welled. Close alongside and a little nearer the road 
is a basalt knob, which probably marks the centre of the 
boss. The former is a fine specimen of basic breccia not 
being at all decomposed. We were informed that this basic 
neck gave much trouble in excavating the tunnel. 
