Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/58

32 apparent; and I incline to the belief that they belong to the age of the latter, that is, that they are Upper Devonian. Their position as regards the Middle Devonian limestone of Bindi favours this view.

Further again to the eastward numerous traces of ancient conglomerate beds are met with resting on the summits of the mountains, as, for instance, near Carrabungla, at the sources of the Limestone, Tambo, and Reedy Rivers, on the flanks of the Cobboras near Cowombut, and again further to the south and east at Combyingbar, near the boundary-line of this colony. These are similar to those with which the groups at Mount Tambo and the Snowy Bluff commence, and may, with much probability of truth, be regarded as parts of the Great Upper Palæozoic arch, of which the Iguana-Creek beds seem to be the lowest.

According to an opinion expressed to me by Professor M'Coy he regards the marine limestones and shales of the Native-Dog Creek and Cowombut as somewhat older than the Iguana-Creek beds, and this would accord very well with the facts I have observed in the field; for at the former places the last traces of these marine beds are found to be resting on the Snowy-River porphyries in the bottoms of basins, while high up on the intervening mountains (the Cobboras) are the old conglomerates of which I have spoken.

It will have been remarked that almost invariably the Buchan and Bindi Limestones, wherever met with, are seen to lie in basins of the older rocks. The same peculiar feature is to be remarked in the Upper Devonian limestones and calcareous shales of the Native-Dog Creek and of Cowombut, and in the Mount-Tambo and Snowy-Bluff beds. The explanation is afforded by the section at Mount Tambo and by a view of the north side of the Snowy Bluff.

We there see that the preservation of these beds is due to the circumstance that in the folding of the earth's crust, after the laying down of the Devonian strata, certain portions formed what may be termed "pockets" in the lower "palæozoic formation." Subsequent denudation left these "pockets" below the general surface, and erosion by the rivers has worn them out into basins by the removal of the limestones and shales more rapidly than the refractory and silicified rock masses in which they generally rest. In Mount Tambo and the Snowy Bluff we may see the converse of the process which I have indicated as having taken place in regard to the Devonian limestones. In those instances the synclinal "pockets" consisted of siliceous rocks which have proved harder to remove than the older formations in which they were enfolded.

(g) Avon Sandstones.—Of this group of strata I am at present able to say but little. Mr. A. R. C. Selwyn, in the "Notes on the Physical Geography, Geology, and Mineralogy of Victoria," Exhibition Essays, 1866, speaks of them as being "yellow and brownish red coarse-grained sandstones and micaceous freestones with numerous impressions of plants. Very good specimens of Lepidodendron