Page:A sketch of the physical structure of Australia.djvu/25

13 that they do not pass under them,—that the greenstones of the hill-tops are not a thick capping resting on the palæozoic formation. In ascending Mount Wellington from Hobarton we first pass over a great thickness of white and yellow sandstones nearly horizontal; above these are shales and thin beds of limestone, likewise horizontal; over which again other sandstones are found. These rocks occur to a height of 2,500 feet above the sea, and apparently form a solid mass of that thickness at least. Above this point greenstone alone is to be seen, forming a mass 1,700 feet thick at least. Its total thickness depends of course on the undecided question, of whether it be a capping to the palæozoic rocks, or what I believe is more probable, a solid mass with the sedimentary beds resting against its sides.

Both the sandstones and limestones are quarried at several points. At Mr. Hull's limestone quarries at Tolosa, about four miles from Hobarton, I found dark grey limestone, sometimes compact, sometimes finely laminated, with fragments of shells and corals. The beds of limestone were about two feet thick, and in one place were some beds of soft brown sandstone interstratified with thin beds of limestone. These sandstones were scarcely consolidated, and fell to pieces on being taken from the quarry. They often contained fossil shells, both Spiriferæ and Productæ, quite perfect in appearance, but so much decomposed as not to bear extraction, falling into white powdery fibrous carbonate of lime. I procured from other parts of these quarries the following fossils :—