Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/62

36 They are not only scarped at the sides, but occasionally completely isolated by the sources of streams feeding the adjoining rivers. There is, for instance, evidence pointing to the conclusion that the Dargo and Cobungra High Plains were once continuous across the present valleys of those rivers. This is shown by the Tabletop Mountain, which stands in the valley of the Upper Dargo River.

Underneath these flows is found in places auriferous quartz-gravel, which is now worked by miners. The deposits have not yet been sufficiently opened up to admit of any opinion being formed as to their precise nature or value; but there is little doubt that they will prove to be the ancient beds of those streams which now flow in the valleys a thousand feet or more below.

At Cobungra layers of black clay have been met with in the auriferous quartz-gravel, and contain lignite and leaf-impressions; but these have not yet been determined.

I have nowhere observed the traces of volcanic orifices. It is possible that craters may have been situated in those tracts now eroded into valleys; but, on the other hand, it is somewhat improbable that all volcanic orifices connected with the various lava-flows spread over such a wide district should have been obliterated, or that, if such existed in the areas now seen as valleys, some traces of them should not have remained. During ten years that I have been constantly looking for such evidence I have never met with it. Mount Battery on the Cobungra River certainly appears at first sight to resemble a volcanic cone; but on inspection it will be found that it is merely a projecting spur from the doleritic sheet of the Cobungra River. In places a well-marked columnar structure is seen, vertical to the horizontal flow; and I have no doubt that it is merely a much thicker portion of the sheet which has filled the former valley, and has been subsequently isolated by the river.

Throughout the whole of the district in which these volcanic rocks occur, and which extends only about thirty miles south of the central chain, immense numbers of intrusive dykes and masses of basic igneous rocks are met with. An inspection of thin slices from a great number of these, from all parts of this district, has shown me that a large proportion exhibit the familiar appearance of doleritic and basaltic rocks, and, moreover, of exactly the same general mineral character as the Tertiary dolerites and basalts of the district. These are characterized by a predominance of plagioclase and magnetite.

It seems therefore not impossible that these great doleritic and basaltic flows have been emitted from fissures rather than from true volcanic orifices.

It seems worthy of remark that we should find these traces of Tertiary volcanic activity following the direction of the Great Dividing Range. So far as I am aware, they extend in the same way far into New South Wales.

I have not touched upon the gold-workings of the district: the subject is hardly within the scope or the limits which I had proposed