Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/40

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A. W. H0WITT ON THE PHYSICAL GEOGRAPHY AND

to over twenty feet in width. A preliminary microscopic examina- tion of a number of these in thin sections has shown me that they are varieties of compact felstones, diorites, and basalts ; but many have evidently undergone great mineral changes, partly due to metamorphism, which has, I believe, also generally affected the already metamorphosed schists, and partly due to alterations which have affected the dykes alone. I am, at present, unable to assign a classificatory position to these.

Some of these intrusive dykes or masses are no doubt Palaeozoic, but others are undoubtedly connected with the outflows of the Ter- tiary dolerites, which will be mentioned later*.

Epidote is of frequent occurrence, both in the intrusive dykes and in the schists themselves, and forms a marked feature in the Omeo rocks.

The subjoined sketches (fig. 3) will show the relations of such intrusive dykes to the schists and granites of Omeo.

Fig. 3. — Aphanitic Dyke at Sandy Creek, Tambo River.

A. Section. Dip N. 10° W. at 80°.

B. Ground-plan. Strike N. 80° W.

The rock through -which the dyke cuts is a metarnorphic granite.

These crystalline schists extend across the valley of Livingstone Creek towards the Omeo Plains, where they appear to pass into the argillaceous schists. These latter still distinctly show the former condition of the beds, being alternations of altered slates and sand- stones. The former have become spotted or nodular argillaceous schists; the latter quartzites. The connexion of these with the crystalline schists is seen in a gully near the Livingstone Swamp, from which I have taken the subjoined sketch (fig. 4).

most valuable paper "On the Microscopic Structure and Composition of Bri- tish Carboniferous Dolerites," read before the Geological Society of London, June 24, 1874 (p. 529 of the Quarterly Journal of the Society, vol. xrx. 1874), and have used the term " dolerite" as including all those Tertiary vol- canic rocks of which basalt is the compact form. I have, however, still used the restricted term melaphyre in one instance.
 * I have in this paper followed the suggestion made by Mr. Allport in his