Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/49

Rh GEOLOGY OF NORTH GIPPSLAND, VICTORIA. 23

to the Native-Dog Creek, to the Toonginbooka River, or to the Little River.

Many of the agglomerates are extremely interesting. They are formed entirely of angular or slightly rounded fragments, varieties of felstone, or of quartz-porphyries, and with, in some instances, fragments of granite ; these fragments are of all sizes from almost dust up to several feet in diameter. When the base is light in hue, or dark red, the contrast to the variously coloured and textured fragments is striking. Generally, however, the weathered surfaces are dull, and only show the fragments standing out in relief.

The whole of these rock masses have evidently been subject to great changes ; they are almost universally quartziferous, the quartz being more or less perfectly crystallized in double pyramids ; and I suspect that a great part of the apparent felstones is merely altered silicified ash.

The general series of this formation, as seen in the deep gorge of the Little River, may be about 2000 feet ; the stream has cut a con- stant succession of falls in its rock-bound chasm. The lowest rocks visible are allied to the quartz-porphyries with occasional agglome- rates ; in the upper parts there are agglomerates, ash, and felstones, all much consolidated and siliceous, and penetrated by irregular branching veins and dykes of hard white felstone. In this gorge I found some of the beds of coarse ash beautifully distinguished from each other, not only by marked planes of deposit, but by the different texture of the beds themselves. The whole of tho series appears to be of subaerial origin.

All these considerations have led me to believe that in the Wom- bargo Mountain we may recognize the site of a Palaeozoic volcano, the central mountain being the denuded core round which some small portions of the vast masses of ejectamenta still remain grouped.

The Cobboras, St. Pancras Peak, Mt. Statham, may be indicated as presenting quite similar appearances to those of Wombargo, and taken together may possibly represent a somewhat north and south line of volcanic orifices extending southward through the Buchan country. The isolated mountains which I have mentioned as stand- ing in the Marine Tertiary area convey to my mind a strong sugges- tion of similar origin and similar age. Such are Mount Taylor and Mount Nowa Nowa, in the neighbourhood of each of which the Tertiary gravels are largely composed of felstones and other igneous rocks belonging to the " Snowy-River Porphyries."

The relative proportions, as given in the sketch section, of the central quartz-porphyry and the surrounding felstones, are no doubt quite conjectural, and the former I believe to be far in excess of the truth. I merely wished to indicate as nearly as possible the general genetic relations, as I believe them to be, of these most interesting rock masses.

(e) Middle Devonian. — I have already pointed out that the an- cient volcanic materials of the Snowy-River country rest upon the Silurian, and are overlain by Middle-Devonian marine beds. On