254 PROCEEDINGS OF SECTION C. 
of Permo-Carboniferous sandstone, mudstone, &c., so 
hardened that they yield slowly to the eroding agencies; 
while, farther away from the outcrops of the diabase, the 
same rocks have been carved into deep or wide bays by the 
encroachments of the sea. By far the most interesting of 
the accessible sections, as showing evidence of intrusion, 
elevation, and contact-metamorphism, is that at Frederick 
Henry Bay, to which reference has already been made. On 
the North and East Coasts the conditions are much less 
favourable for definite conclusions, but at Port Sorell there 
are patches of altered sandstone to be seen on exposures of 
greenstone sheets. At Okehampton Bay, on the East Coast, 
a band, which is ordinary compact grey shale away from the 
centre of disturbance, has been altered by similar agency 
into a black chert-like mineral, with conchoidal fracture. 
The overlying sandstone has been somewhat hardened, and is 
traversed by an irregular vertical jointing, an indication of 
shrinkage during a process of cooling. In a small bay, 
south of Triabunna, a similar intrusive sheet has converted 
the coal-measure sandstone into a kind of tesselated pave 
ment, showing the tops of five-sided prisms a few inches in 
diameter. Other similar mstances observed without any 
systematic exploration might be mentioned, but these may 
be accepted as affording sufficient proof of the intrusion of 
the diabase into all the sedimentary rocks, except those of 
Tertiary age. The latter, in relation to the greenstone, 
appear to have been deposited after it and the associated 
sediments had been subjected to extensive and long con 
tinued erosion and denudation. 
II. The conditions under which the diabase presents itself 
elsewhere throughout the eastern half of Tasmania, especi- 
ally where it is seen occupying the summits of all the moun- 
tains and principal hills, open up questions much more diffi- 
cult of solution than that of its relative age. The essential 
distinction between this rock and the basaltic lavas often 
in close connection with it, and the consequential difference 
between their modes of occurrence, have not always been 
sufficiently recognised. Some writers, even up to the present 
time, have regarded these huge and widely distributed cap- 
pings as the remnants of vast lava flows from a series of 
volcanoes now no longer in evidence. But the structure of 
the diabase, at the first glance, suggests that it was not 
ejected or cooled under sub-aerial conditions. This view 
was confirmed in 1896(9) by the report of microscopical 
(?) Transactions Royal Society, Tasmania, 1896, p. 89. 
