Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/43

Rh GEOLOGY OP NORTH GIPPS1AND, VICTORIA. 17

in rapidity of change and in the nature of the change itself. In examining any of the isolated granite areas, the Silurian strata are found in the neighbourhood to maintain their general direction of strike and dip until in near proximity to the granite, where they exhibit signs of pressure and of alteration. The beds are thrust in all directions, and extensively broken up by irregular joints. Granite veins are found to traverse these contact-margins, showing in different localities the same general features, and especially the distinct contact of the sedimentary rock and the granite vein, as clearly as elsewhere the contact of the granite with the mass of intrusive dykes is shown. The stratified rocks themselves are greatly altered, either resembling fine-textured gneissose or mica- ceous schists, or forming dense crystalline rocks in which the planes of deposit can barely be distinguished by wavy lines of various shades of colour. The extreme form of this series is Horn- fels. The changes resulting in the former series of alterations seem to be heralded by a micatization of the rocks ; in the latter by the appearance of chiastolite-like markings, which can often be still distinctly recognized in an enlarged form in the Hornfels.

A microscopic examination of thin sections of the Hornfels series has led me to believe that, in some cases, the markings I have referred to are probably due rather to the somewhat different aggregation of materials than to the introduction of fresh elements into the rock masses.

Sometimes both forms of alteration are seen in the same area, and occasionally in near proximity, as at Dargo Flat, where the mica- ceous alterations are seen, especially on the northern margin (Dargo River), and the indurated rock series on the south side (Orr's Creek).

These alterations of adjacent rocks extend for uncertain distances from the visible granite surfaces ; and I think that this may be accounted for on the belief that the granite continues nearer to the present surface in some directions than in others — in other words, that it is due to the granite surface being as highly uneven under- ground as we see it to be where denudation has laid it bare.

In some parts of North Gippsland the rock masses which have apparently been invaded by granites do not present so much the alterations I have described as a general silicification of the strata, by which great portions have assumed the character of quartzites, as, for instance, Delegete Hill and the Bowen Mountain. But it seems to me significant of some connexion between the changes seen and the appearances of invasion by the granites, that the intensity of those alterations in the sedimentary strata varies in an inverse proportion to the distance of the former. But in these tracts there are also places Avhere tho rock masses present just such alteration as I have before mentioned ; for instance, Marriott's Mountain, where the Silurian slates and sandstones have, near the granite, assumed a finely gneissose appearance.

The annexed section (fig. 5), sketched from a cutting in Orr's Creek, Dargo Flat, will illustrate the contact appearances generally seen.

Q.J. G.S. No. 137. c