August, 1935. The Queensland Naturalist 
79 
Tuff” occurs therefore between the schists on which it is 
resting and the Ipswich beds above, and is found outcrop - 
ing along the margin of the schists. 
Above, or after, the Ipswich strata, there followed a 
great thickness of sandstone, known as “Bundamba,” 
which, in turn, was followed by the “Walloon” coal 
measures of Jurassic age. 
Upon and into these sediments, perhaps partly in late 
mesozoic times, perhaps wholly in the Tertiary period, 
there were extruded and intruded great thicknesses of 
volcanic material, both lava flows and fragmentary ejecta, 
of the dark basaltic as well as the light-coloured trachytic 
or rhyolitic types. The final effort of the vulcanicity in 
this region appears to have taken the form of extensive 
flows of basaltic lava, covering wide areas and forming a 
lava plain or plateau. 
Rain falling on the plateau collected into numerous 
more or less parallel streams, with a general northerly 
trend. In the course of time these streams have carved 
their valleys down through the volcanic material, leaving 
intervening portions of the plateau still at high elevations. 
One stream, Back Creek, has not yet cut its way through 
the basalt, and runs, on Beechmont, at an elevation of 
1700ft. for some miles, while its immediate neighbours on 
either side, the Nerang and Coomera, are in valleys 1200ft. 
below. 
Briefly, the region is characterised by deep valleys 
separating remnants of a formerly continuous plateau, 
consisting of a complex of volcanic rocks resting on 
mesozoic strata, and the still older greywacke, on which 
the mesozoic strata themselves rest. 
The geology so far sounds reasonably clear. Our prob- 
lem, however, occurs in the relationships of the varying 
types of igneous rocks to each other in the field, their 
positions relative to the greywacke, and to the mesozoic 
strata, and the nature of the surface on which they have 
been extruded. The evidence of vulcanism on a grand 
scale in South-east Queensland, has naturally been a sub- 
ject of intense interest to local geologists, and the subject 
of much enquiry as to its age and sequence. With a grad- 
ually increasing knowledge of the field relationships views 
have changed from time to time, but we are not yet in a 
position to be definite. 
Formerly some attributed all or most of the volcanic 
phenomena to the mesozoic period. Latterly, the most ac- 
cepted view regarded all as Tertiary, and divided the vol- 
canic rocks into a lower and upper basaltic phase, with 
an intervening trachytic or rhyolitic phase. This was cor- 
