THE GEOLOGY OF MITTAGONG. AT, 
later volcanic activity than that causing this tilting. Thus 
an interesting sequence can be traced in these rocks, the 
more acid forming the earliest intrusions, the basic rocks 
following afterwards. 
(b) Under the preceding subdivision the earlier igneous 
rocks are shewn to be Post Triassic. The basalt flows 
which represent the latest members of the series were 
described as overlying pebble drift beds, the age of which 
although not very exactly determined, has been shown to 
he later than early Tertiary, by leaf remains found in a 
similar bed at Wingello. These basalts are homotaxial 
with the pliocene basaltic flows extensively developed at 
Gulgong, and other parts of New South Wales. 
The syenites and trachytes of this district bear an 
intimate relationship to similar rocks of the Glass House 
Mountains in Queensland, the Warrumbungle Mountains in 
New South Wales, and the soda series of Mount Macedon 
in Victoria, and as there is good evidence’ to show that 
these allied rocks are Cretaceous or Cretaceo-Hocene. It 
is probable that the Mittagong intermediate rocks which 
are known to be Post Triassic are of the same age, viz., 
late Cretaceous or early Eocene. 
VII. SUMMARY. 
A regular sequence of igneous rocks beginning in OCre- 
taceous times and extending till late Tertiary has taken 
place in the Mittagong district. The intrusion and effusion 
of the molten magmas was accompanied by faulting and 
fissuring of the sedimentary beds, leading to the production 
of bosses, lavas, sills and dykes. The trachytes of this 
district, like those already mentioned homotaxial with them, 
are remarkable for their high percentage of alkalies, and 
1 T. W. E. David, B.A.. F.R.s., “‘ Note on the occurrence of diatomaceous 
earth at the Warrumbungle Mountains New South Wales.” Proc. Linn. 
Soc. N.S.W., June 1896. 
