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is frequently spoken of as east and west, which seems indeed to be the general bearing of the hilly land from Collier Bay to the Gulf of Carpentaria. All the large spaces between and around these ranges of old rocks are occupied by great plains, formed of tertiary rocks in a horizontal position, which, as we have seen, spread unbroken and unchanged sometimes for hundreds of miles.
The total absence of any rocks of an age intermediate between the palæozoic and the tertiary, so far as is at present known or appears probable, is another general result, worthy of remark, both as producing simplicity of structure, and for other reasons.
We are naturally led from these two results to two speculations, one as to the physical geography of the great tract still unexplored in the centre of Australia, another as to the past geological history of the country.
Let us first consider what is probably the character of the unexplored interior of Australia. When we look at the great tertiary plain into which the Murray and other rivers of New South Wales fall on their road to the sea, at the immense tertiary plateau round the Great Australian Bight, that spreads horizontally for an unknown distance into the interior, and which appears to sweep up to the very borders of the hill country of Western Australia (see ante p. 63, and 67, the account of the sand plains), at the great flats, probably tertiary, that extend from the N.W. coast
