Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/45

Rh GBOLOGY OP NORTH GIPPSLAND, VICTORIA. 19

of that of the granites ; but the localities are, generally speaking, between the Tambo and Snowy Rivers, and extending between similar limits north of the Great Dividing Ilange. The granite, on the contrary, is found in all places where denudation has been sufficiently deep. Where I have been able to examine the contact of the quartz-porphyries and the Silurian strata, as at the Omeo Plains, I have found the latter broken up and penetrated by dykes of felstone — which is in places granular (apparently owing to decom- position), in other instances, however, compact, and in such cases, almost without exception, more or less distinctly quartziferous. The forms of Silurian rocks which I have observed to have been invaded b} T the quartz-porphyries are the argillaceous schists and quartzites of the Omeo Plains, and the indurated slates and sand- stones of Bindi ; and we thus see that the irruption of the quartz- porphyries has been subsequent to the invasion and alteration of the Silurian strata by the granites. Near Bairnsdale the Silurian has been broken through by enormous masses of quartz-porphyry and allied rocks, which now are seen as the triad group of hills, Mount Taylor, Mount Lookout, and Mount Alfred ; and here the sedi- mentary rocks have either apparently undergone no change, as at Bulumwaal, or, as at Clifton, have been altered to very nearly the true Hornfels condition. In the river-gravels derived from the neighbourhood of those hills fragments of perfect Hornfels are frequent, showing that the change has been a common one there.

Where I have been able to observe the relations of the quartz- porphyries and the granites, as at the Snowy River near Turnback, and not far from where the latter rocks have invaded the black graptolite slates of Deddick, I have found that the quartz-porphyries have come up through the granite in mountain masses and with a well-defined line of contact. The boundary of the two rocks at Turnback seems to be along a north and south line, and may indicate a great fault ; if so, it is almost the only fault which I have been, so far, able to recognize.

At Turnback we again see the same general relations of the Silurian, the granite, and the quartz-porphyries that I have already pointed out as being indicated at Omeo Plains.

All these divisions — the Silurian, the granites, the quartz-por- phyries — may from one point of view be regarded as forming a group in the geological series. In this aspect they constitute the great " rock-foundation " of North Gippsland on which the younger formations rest.

I find all over the district that this group has been subject to enormous denudation during Palaeozoic time, and that, broadly viewed, the first great stratigraphical break may be placed here. The Sections figs. 1 and 2 (pp. 6 <fc 1 2) will further illustrate my views on this subject. This first " Horizon " may be also regarded as marking the division between the Lower and Upper Palaeozoic times. Below it we have, as at Deddick, Lower Silurian slates with Diplograpsus recfangularis, M'Coy *, as the oldest ; and at Gibbo Biver, and pro-


 * ' Prodromus of the Palaeontology of Victoria,' decade i. p. 11. Frederick

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