96 
The Queensland Naturalist. 
Oct., 1920^ 
were hurled forth were collected in part around the various 
centres of activity, and on the road a few chains from the 
quarry accumulations of this volcanic ash in the bedded 
form known as volcanic tuff may be seen. Subsequent 
flows of ‘basalt near the tliiY have somewhat ' disturbed it, 
but its fragmental characters, and also its bedded characters,, 
are clear. 
The 'volcanic ash was expelled from small vents such 
as are shown in the quarry, and probably there were a large- 
number of them distributed over the area in which the tuff 
was formed. 
The jointing in the rock is the result of contraction of 
the volcanic rock on cooling, and if the rock is very homo- 
geneous. then the jointing is likely to be regular. The 
olivine basalt in the quarry is a very homogeneous rock,, 
consequently the jointing is very regular. When the molten 
rock was poured out there were two cooling surfaces — -the^ 
upper surface of the flow and the floor on to which it was 
poured. 
As the molten mass was very homogeneous the surface 
contraction exerted itself equally in. all directions, and when 
the actual fracture took place the contraction acted towards 
points equally spaced throughout the plane of the surface*;, 
the distance aj^art of these points was governed by the 
rigidity of the mass and the rate of cooling: in this quarry 
the points were about two feet apart. F'ractures developed 
along directions at right angles to lines joining these points, 
and if the mass had been thoroughly homogeneous and the 
contraction perfect, the system of cracks or joints would 
have been hexagonal. The centres are not uniformly 
spaced, however, and the resulting columns are not regularly 
hexagonal, but have different numbers of sides, four, five, 
or six ; the columns also are of different lengths. 
Before the system of shrinkage, cracks, or joints had’ 
developed in the cooling basaltic flow, the two vents above- 
referred to were formed, and the system of jointing became 
interfered^ with. . The walls of the vent furnished new 
cooling planes, and the jointing of rock developed parallel 
to the walls, so that close up to the walls of the vent the 
jointing is vertical and the basalt has the appearance of 
being wrapped around the vents in a series of concentric 
layers. As the joints pass away from the vents they take 
on a more and more horizontal position, and the peculiar 
development of concentric saucer-shaped joints between, 
the two vents resulted. 
Iddings, J. P. Igneous Rocks, vol. I., p. 320. 
