THE GEOLOGY OF MITTAGONG. 319 
the track crosses a large outcrop of rock, which is gray on 
the weathered surface, but when broken into discloses a 
dark blue colour like that of the basalts. This rock breaks 
readily into flakes with a conchoidal fracture, the non-basic 
character of the rock is shown by the fact that the flakes are 
translucent on thin edges. This outcrop was of large 
extent, but the boundaries were either hidden by talus 
slopes, or formed cliffs which rendered mapping almost 
impossible. This dark trachyte contains a large proportion 
of zegerine (soda augite) in its composition. 
A similar association of trachytes of like composition 
occurs in the Glass House Mountains, Queensland. Indeed 
Several sections made by Mr. H. I. Jensen from rocks 
brought ky him from that locality show minute micro- 
scopical resemblance. The blue hornblende mentioned 
above also occurs in the trachytes of the Warrumbungle 
Mountains near Mudgee. This similarity in structure in 
the trachytes extending along the western slope of the 
present Dividing Range is of much interest and will doubt- 
less repay investigation. (See Section IV. Nos. 2 and 3). 
1.—ec. Intermediate Tuffs and Breccias.—The tufis are 
in all cases much decomposed and in many instances merge 
into breccias. 
(1) Tuff cone, half mile west of the Gib. 
(2) Tuff bed, one and a half miles on Berrima road. 
(3) Tuff, west slope of Jellore. 
(4) Tuff, three miles east-north-east of the Gib. 
(1) This volcanic cone rises sharply from lower levels 
round Bowral, just to the west of the southern end of the 
tunnel. The top is fairly flat and oval in shape, being 
approximately 100 yards long by 30 yards wide. The cone 
is composite in character, the eastern slope consisting of 
decomposed syenitic breccia and vesicular lavas in which 
the steam holes have been filled with calcite. Towards 
