We find that the Upper Silurian Limestones of the Gibbo River, and also those of the Limestone River, which I regard as probably of the same age, are part of the great general group of Lower Palæozoic formations which I have described by the term "the Lower Palæozoic foundation" of North Gippsland. We may therefore not unreasonably infer that those great changes in the earth's crust continued to the close of the Silurian period. The surface of all these Silurian strata shows the long ages of denudation which must have elapsed before the succeeding Devonian groups, as we see them, were laid down.
This long period seems to have been one of volcanic activity. The "Snowy-River porphyries" show the extension, and the Wombargo Mountain reveals to us the structure, of these ancient volcanos. Isolated hills of quartz-porphyries and allied rocks which have penetrated the Silurian and the granite suggest strongly the idea of cases of similar volcanos from which the ejected materials have been denuded, but which are still to be recognized in the large percentage of felstone and porphyry pebbles in the accumulations of Tertiary marine gravels surrounding them.
That these Palæozoic volcanos were subaerial, and probably terrestrial, may be inferred from the fact that the great piles of felstones, ash, and agglomerates now remaining contain no beds which I can recognize as aqueous; and this inference would accord with the fact that, so far as our present palæontological evidence extends, no Lower Devonian marine strata exist; for the Buchan and Bindi Middle Devonian limestones rest on the Snowy-River porphyries, and these latter upon the Lower Palæozoic rock masses.
But if the Lower Devonian period was thus one of terrestrial conditions, it follows that the succeeding period was one of subsidence of the land. Denudation must have been very great over the Snowy-River porphyries, over the Silurian and the metamorphic schists, the granites and quartz-porphyries, before the marine limestones of Buchan, Bindi, Gelantipy, and elsewhere were deposited.
To the westward, somewhat similar marine conditions accompanied the formation of the Tabberabbera shales and limestones.
During this Middle Devonian period I am unable as yet to recognize any traces of volcanic action.
Succeeding this period, we have equally clear evidence of great and long-continued movements of the earth's crust. The Bindi limestones were denuded; the Tabberabbera shales and sandstones were tilted at a high angle, folded, and apparently denuded even more than the Bindi limestones. We no longer have strata containing a well-marked marine fauna; but the beds of Iguana Creek and of Mount Tambo only yield us somewhat scanty traces of a terrestrial flora; while the shales and limestones of Cowombat and the Native-Dog Creek appear to me to indicate a shallower sea or a nearer proximity to land than the Buchan and Bindi beds.
The Upper Devonian conglomerates, sandstones, and shales suggest the proximity of land; and volcanic activity is again apparent in the felstones and felstone ash, somewhat resembling the older accumulations at Wombayo and the Snowy River. But these volcanic
