A wonderful diabase sill, some 200ft. 
high. capped with the same thickness of 
sedimentary rock, stretches from the 
mainland at Passage Point right down 
the eastern coast line of North Bruny, 
doubtless underlying much of that island. 
There the diabase appears to disappear, 
or not to rise so high for the 14 miles 
or so of Adventure Bay, and it reappears 
again at Fluted Cape. In the gap 
formed between the last-named cape an<; 
Cape Frederick (Henry the waves have 
exit out the land surface of soft sand¬ 
stones and formed the wide sweep of Ad¬ 
venture l>n\. 
'I he “fluted" oi’ “organ nine" structure 
of many of the prominent diabase head¬ 
lands forms a striking scenic feature. 
From a distance the diabase appears to 
have formed in columns, as is frequent 
with the basalts of the lava flows, but 
looking closer it is soon seen that the 
“columns” are usually irregular in shape, 
size, and angles. Most are roughly quad¬ 
rilateral with the inner side merged in 
the general mass of the icliff. Often the 
edges are rounded off, and other geomet¬ 
rical figures are formed. But probably 
this columnar formation is not structural 
in origin, as with the basaltic columns of 
iBurnie. "The Giants Causeway,” Ire¬ 
land. and elsewhere, but are merely due 
to erosion. These cliffs exposed to the 
sci are continually dripping with sea 
mists ami vapours, and are exposed to 
the full effects of the hot sun. and the 
penetrating frosts, Thes,- great agents 
of erosion crack the rock surfaces along 
lines of weaknrss, which, in this diabase^ 
appears to he in vertical lines through 
the mass at intervals of from two to six- 
feet lines of w< akness caused, no doubt 
by tonsional stresses when the mass was 
cooling. 
in the solid diabase there is no trace- 
of icohimnal structure, but as the agents 
of erosion eat into a cliff face the cracks 
are widened, and deepened until a series 
of furrows are cut into tile cliff. These 
give the columnar appearance from a dis¬ 
tance. Near the top of the cliff, nr on 
the outer edge of prominences, these ero¬ 
sion cracks meet, at right angles to the 
cliff face, and so complete the column. 
In many cases on most diabase headlands 
residuals of diabase stand up often to 
50ft. as columns. These have been iso¬ 
lated by the en'argement of cracks until 
the surrounding rock has vanished. These 
columnar erosion residual- arc to be seen 
best on the end of Cape dtaoul. There 
are also several good spreimens along the 
top of the cliff face from Penguin island 
to Cape Connella. 
A very noticeable feature of the topo¬ 
graphy of the district is the way in 
which the diabase prominences of Fluted 
Cape and Cape Connella. after rising 
10(H) feet sheer from the sea floor, slope 
very rapidly down nearly to sea level 
half a mile inland, forming in section 
a pyramid with its highest point on 
the top of tile cliffs. This coastal bat¬ 
tlement of rock is not a feature tliat. 
may be expected, blit it is found else¬ 
where around Southern Tasmania, al¬ 
though nowhere else is it so pronounced. 
A more usual development would be a 
gentle slope from the centre of the island 
to the sea. with perhaps a line of cliff's 
at the sea border down to which the 
land sloped. 
The explanation of the reason for 
the sea -edge being the highest land in 
the vicinity may be found in the de¬ 
velopment of the topography of, the 
island. Almost certainly the diabase 
lifted great thicknesses of sedimentary 
rock, which after its intrusion lay 
above it, but have since vanished. 
Streams worked down this uplifted plat¬ 
eau in accordance with drainage levels 
caused by the uplift. They ate rapidly 
through the soft sandstones until they 
struck the harder diabase below, lint 
then they had formed valleys from which 
they could not escape, and therefore had 
to keep on cutting through the harder 
rock. In time the softer overlying 
sandstones were entirely removed by 
tributary rills and rain wash, hut the 
risers hud to keen on flowing in the 
original channels formed when they were 
eating through the sandstones. They 
are thus what are called “consequent'’ 
streams. 
With the removal of the sandstones 
the intrusions of diabase stand out as 
residuals. These the rivers have litile 
affected beyond generally rounding their 
contour. While the sandstones still 
covered the island the three creeks that 
flow northward into the south of Ad¬ 
venture lav captured most of the drain¬ 
age of the plateau. They were pro- 
27 
