Oct., 1920 
The Queensland Naturalist. 
95 
ill the quarry is one which is to be highly commended, and 
wliich in the future wilLearn the grateful thanks of students 
of geology, and those interested in natural phenomena. 
The quarry, which is used for obtaining material for 
road making, pitching, and kerbing purposes, is situated 
on the edge of the range, near Toowoomba, and the stone 
worked is a fine-grained olivine basalt — a volcanic rock 
which has been poured out on to the surface from a volcano. 
Owing to the quarr}' having been worked extensively 
for some time, the excavations are large and the clifif faces 
show structures of various kinds. The feature which first 
arrests attention is the manner in which the stone in the 
quarry breaks, as shown by the cracks in the faces of the 
quarry. The regular cracks are due to a well-developed 
system of joints which are, generally speaking, either vertical 
or horizontal in their disposition. When the vertical joints 
are the more pronounced, columnar structure results, and 
this type occurs in most of the quarry. The regularity of 
the jointing is interfered with in several parts of the quarry, 
but at two places in particular there are very marked 
interferences with the jointing. 
These places mark the points where two volcanic vents 
have been forced through the volcanic rock, and the jointing 
of the rock alongside the vents is approximately parallel 
to the wall of the vent — that is to say, vertical. 
The horizontal jointing in the rock between the two 
vents ])asses into vertical jointing alongside the vents, so 
tliat a concentric series of saucer-shaped joints is developed 
between the two vents. One of the vents has been exposed 
through being worked into, and it shows evidence of having 
been plugged up with boulders and smaller fragments of 
volcanic rock which have fallen back into the vent after 
having been blown out ; this material is known as volcanic 
agglomerate, and it is mucli more altered and decomposed 
to-day than the surrounding rock. The other vent has not 
yet been exposed, but from the disposition of the jointing 
ft is clear that the wall has been very closely approached. 
1 he olivine basalt rock in the ciuarry was poured out 
on the surface in what is known as Upper Cainozoic times — 
a period quite young geologically — and in all probability the 
material was poured out of a crack or fissure, there being 
several fiows. The openings appear to have become choked 
later on, and, owing to the openings being restricted, the 
latter periods of the activity were much more explosive in 
Ihcir character. The volcanic ash and finer fragments which 
