File:The Quarterly journal of the Geological Society of London (12826331995).jpg

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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .N-OV. 4,
line separating that colony from Victoria, is crateriform, and is in the
midst of a low, flat, and sandy district, with low ranges of tertiary
limestone. I am not aware whether it has erupted much lava.
It is impossible at present to define the exact limits of the areas
occupied by the most recent volcanic rocks, or perhaps always to
determine the age of the various basalts or lavas ; but we may draw
some useful general conclusions, nevertheless, from the information
already accumulated.
Over nearly the whole extent of Victoria there are masses of in-
trusive basalt, in some places columnar, in others in rude angular
blocks, breaking through both the granite and the palaeozoic strata,
which clearly, from the mode of their occurrence, are unconnected
with the recently-extinct volcanos. Such masses are often found in
close proximity to the newer basalt, as at Melbourne ; and of these
Mr. Selwyn's descriptions are most excellent. He draws a proper
distinction between the older, close-grained basalt, as it generally
occurs, much decomposed, and of a nodular structure, and the re-
cent rock, which is commonly very vesicular and amygdaloidal * .
From the similarity in lithological character, as he observes, it is,
however, sometimes impossible to separate the one from the other.
I have seen the newer basalt, in close proximity to a crater, both
vesicular and very dense, and the latter undistinguishable from some
specimens of undoubtedly older rock. Neither from their mineral
constituents nor specific gravity is it possible to say, from a hand-
specimen, whether the basalts are new or old. The following
section (fig. 4) shows, in a synoptical form, the manner in which these
formations occur.
Fig. 4. — Diagram showing the relations of the Basalts in Victoria.
a. Porous lavas (recent), b. Younger basalts, c. Older basalts, d. Quartz-
ose drift (Pleistocene ?). e. Tertiary fossiliferous strata (Miocene ?).
/. Palaeozoic rocks, g. Granite.
The older basalt, c, in all probability, was erupted by submarine
volcanos ; and we have no reason to suppose that it did not flow
in considerable streams, and fill up inequalities in the ancient sea-
bed. Extensive denudation (of which during that period we have
abundant proofs) would account for the removal of the cappings or

• See Geological Surveyor's Report (p. 7). 1854.