Page:A sketch of the physical structure of Australia.djvu/24

12 A. Tasmania.—Granitic and metamorphic rocks are described by Count Strzelecki as existing in large mass in the N.E. corner of Tasmania, and in still larger force over its S. W. portion. They shew themselves also at three other smaller districts in the centre and on the northern coast. The rest of the island seems to be generally formed of great intersecting ridges of greenstone, enclosing plains or valleys composed of the palæozoic formation. Some of the more massive greenstones are evidently anterior to the palæozoic formation, as sections may be seen in which horizontal beds of sandstone abut against cliffs of greenstone, or even repose upon steps and ledges of that rock. In other cases, however, sandstones of the palæozoic formation appear to be capped by immense masses of greenstone, and they are certainly often broken through by basaltic and other igneous rocks. The following details on the geology of Tasmania are extracted from a paper of mine in Journal of Geological Society for 1847.

The Valley of the Derwent River.

Along the S.W. side of the valley of the Derwent runs a bold range of flat-topped hills, of which one of the principal promontories is Mount Wellington, rising immediately behind Hobarton to a height of 4,200 feet above the sea. The upper portion of this range is composed of massive greenstone, often forming rude columns of great size, frequently as much as ten feet in diameter. The lower slope of this range, and much of the country forming the opposite side of the valley, is composed of the palæozoic rocks. These lie generally in a nearly horizontal position, and I believe abut horizontally against the greenstones; but as I never found a clear section near the junction of the two, I cannot positively say