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Joseph Hooker states that "there are about 180 genera, out of 600 in South-western Australia, that are either not found at all in South-eastern, or that are represented there by a very few species only, and these 180 genera include nearly 1,100 species." _Geological Explanation of the Differences of these Two Floras._--These facts again clearly point to the conclusion that South-western Australia is the remnant of the more extensive and more isolated portion of the continent in which the peculiar Australian flora was principally developed. The existence there of a very large area of granite--800 miles in length by nearly 500 in maximum width with detached masses 200 miles to the north and 500 miles to the east--indicates such an extension; for these {495} granitic masses were certainly once buried under piles of stratified rock, since denuded, and then formed the nucleus of the old Western Australian continent. If we take the 1000-fathom line around the southern part of Australia to represent the probable extension of this old land we shall see that it would give a wide additional area south of the Great Australian Bight, and form a continent which, even if the greater part of tropical Australia were submerged, would be sufficient for the development of a peculiar and abundant flora. We must also remember that an elevation of 6000 feet, added to the vast amount which has been taken away by denudation, would change the whole country, including what are now the deserts of the interior, into a mountainous and well-watered region. But while this rich and peculiar flora was in process of formation, the eastern portion of the continent must either have been widely separated from the western or had perhaps not yet risen from the ocean. The whole of this part of the country consists of Palaeozoic and Secondary formations with granite and metamorphic rocks, the Secondary deposits being largely developed on both sides of the central range, extending the whole length of the continent from Tasmania to Cape York, and constituting the greater part of the plateau of the Blue Mountains and other lofty ranges. During some portion of the Secondary and Tertiary periods therefore, this side of Australia must have been almost wholly submerged beneath the ocean; and if we suppose that during this time the western part of the continent was at nearly its maximum extent and elevation, we shall have a sufficient explanation of the great differ
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