DIABASE OF TASMANIA. 255 
examinations by Mr. W. H. Twelvetrees of specimens of the 
Cataract Hill rock, Launceston, a fairly typical representa- 
tive of the Tasmanian diabase (or dolerite). Its essential 
constituents were found to be plagioclase-felspar, augite, and 
a little iron oxide, the augite crystals showing ophitic struc- 
ture on the felspars, and the result of the examination proved 
the whole character of the rock to be that of one which had 
crystallised below the earth’s surface. A subsequent paper 
by Messrs. Twelvetrees and Petterd(1°) gives in admirable 
detail the results of further microscopical examinations, 
which fully confirm this conclusion, and coincide with that 
derived from other evidence already cited. 
How, then, is the presence of this uncovered holo-crystal- 
line igneous rock on the summits of so many mountains to 
be explained? What was its original covering, and how 
has that covering been removed! Chief among the diffi- 
culties that seem to have been encountered in pursuing such 
inquiries are—the apparent vastness of the area occupied 
by the mountain-cappings, the absence of visible traces of 
their former covering, and the great vertical height to which 
those cappings are supposed to rise above their actual con- 
tact with the adjacent sedimentary rocks. The last of these 
suppositions is a natural one, resulting from the almost in- 
variable concealment of the line of contact by accumulations 
of talus, &c., at the foot of the cliff-faces exposed; the two 
former are due to our very scanty knowledge of the immense 
mountain area which has never been geologically surveyed. 
Messrs. Twelvetrees and Petterd suggest (op. cit., p. 54) that 
“the faces and cliffs which we now see are subterranean 
sections lifted for our inspection by some earth movement ”’ ; 
but, if this is to be taken as meaning that they may have 
been lifted after consolidation, the theory could hardly be 
reconciled with the condition of the adjacent sediments 
where they are exposed to view. Mr. R. M. Johnston has 
discussed (op. cit., p. 21) the possibility of a lava-flow welling 
up to the surface by gigantic fissure eruptions, and attaming 
in places a thickness of 2000 feet or more before cooling, 
and suggests that the lower portion of such a mass would, 
through slow cooling, exhibit the structure characteristic of 
igneous sills. Such a condition is certainly conceivable if 
sufficient traces could be found of the upper portions of the 
original mass to show a gradual transition from sub-aerial 
lava to holo-crystalline diabase; but no such evidence is yet 
forthcoming. 
(°) Transactions Royal Society, Tasmania, 1898-9, p. 47. 
