August, 1935. The Queensland Naturalist 
81 
water-worn quartz gravel and silicified wood. The gravel 
and wood show that the junction is not a faulted one, and 
are definitely against any suggestion that the rhyolite was 
intruded along a fault line. They suggest rather that the 
rhyolite flowed onto or against a steeply inclined surface, 
or one which has since been tilted. 
The position of the rhyolite along the boundary of 
the schists, and the presence of fossil wood beneath it, is 
strangely similar to the occurrence of the Brisbane Tuff, 
which has been found as far south as Logan Village, in a 
bore, only 12 miles from Tamborine Mountain. 
An examination of the wood, however, has enabled 
Mr. W. D. Francis to identify it as being of a compara- 
tively recent type of dicotyledonous tree, whereas the dico- 
tyledonous plants are unknown from Triassic rocks, and 
the wood under the Brisbane tuff is coniferous. 
The correlating of the Numinbah rhyolite with the 
Brisbane tuff is also negatived by the association with the 
rhyolitic rocks of Springbrook, and the Beechmont 
National Park plateaus, for this is plainly above some of 
the sandstone and some basalt. Yet the rhyolite belt oc- 
cupies the river bed at Numinbah and the Coomera gorge 
at levels below the sandstone- 
At present, this forms a geological dilemma only to 
be solved by further field work. 
A particularly interesting feature was disclosed on 
our visit to the “Natural Bridge/’ about two miles from 
the border gate, and perhaps, half a mile from the road 
on the eastern side, near the foot of Springbrook. 
Here a stream, Cave Creek, falls through a hole in 
the roof of a cave, some 75ft. across and 150ft. long, con- 
verting. that part of the roof between the fall and the en- 
trance into the so-called natural bridge. 
Set in luxuriant rain forest, with the light streaming 
clown and illuminating the falling water, and the pool 
into which it falls, against the dark background of the cave 
beyond, the scene is one of rare beauty. 
The rock is basalt, and the cave what is known as a 
lava tunnel of a type such as are well known, for instance, 
in the western district of Victoria, and in North Queens- 
land. The origin of such caves is due to the fact that in a 
flow of molten lava the surface and advancing edge forms 
crusts owing to cooling. The advancing edge being the 
coolest part, becomes more viscous than the lava behind 
it, and forms a steep margin to the flow. At times the pres- 
sure behind causes the, steeper edge to give way and per- 
mit the more fuid lava to drain away, leaving, where con- 
ditions are suitable, an empty tunnel in the more con- 
