Page:Quarterly Journal of the Geological Society of London, vol. 35.djvu/56

30 30 A. W. HOWI'Xl ON THE PHYSICAL GEOGRAPHY AND

beds are more or less coarse conglomerates, sandstones, sandy or slaty shales, all of a reddish or yellowish colour, and narrowly re- sembling similar beds in the Iguana-Creek series. We have also here a great thickness of felstones, in which occur not only the wavy un- dulating lines of various colour, but also the angular fragments of different colour and texture such as are seen so frequently in the ash and fine agglomerates of the Wombargo Mountain and other localities of the Snowy-River porphyries.

Near the summit the Snowy Bluff is encircled by a high precipice. The rugged face is worn into cavernous hollows as the component rock varies in hardness, and is seamed in all directions by a multi- tude of joints. It is only to be ascended in a few places, where rain-gullies have cut through from the upper grassy slopes of the mountain ; but elsewhere is the inaccessible haunt of the Eock Wal- laby.

This series, some thousand or more feet in apparent thickness, consists of various beds of a basic igneous rock, either porphyrinic with plagioclase prisms, as in the lowest bed visible, or dense in texture, or vesicular and amygdaloidal, as in the upper parts. But everywhere the joints are lined or filled by quartz, chalcedony, or yellowish-green epidote ; and the cavities are either geodes of quartz and epidote, or filled by those minerals severally or by chalcedony. An examination of this precipice and of its various component por- tions, as well as of the underlying and overlying strata, has led me to conclude that these basic igneous rocks are interbedded, and not intrusive.

A preliminary examination of some thin sections of these rocks which I have prepared for the microscope at once disclosed to me the familiar appearance of close-grained dolerite or basalt, in which either the plagioclase or the magnetite were predominant, and in which quartz, chalcedony, and epidote have been very largely intro- duced as secondary minerals. The plagioclase has in many cases been altered from the centre outwards by (apparently) chloritic minerals, and the magnetite has become still further oxidized, so as to show the translucent blood-red colour of haematite.

In accordance with the views which appear now to meet with general acceptance*, these rocks, as being probably dolerites or basalts of pretertiary age, would be classed under the restricted term of Melaphyre, or perhaps they may with still greater propriety be called Upper Devonian Dolerites.

In a paper which I am preparing for the next Progress Report of the Geological Survey of Victoria I hope to be able to describe more fully the microscopic as well as the macroscopic peculiarities of these most interesting rock masses.

In proceeding to the eastward of the Mitchell River no traces of

' Die Porphyrgesteine Oesterreichs,' p. 136 ; Allport, " Microscopic Structure and Composition of British Carboniferous Dolerites," Quarterly Journal of the Geo- logical Society of London, 1874, vol. xxx. pp. 529, 580. Other references might be given, but these may suffice.
 * Zirkel, ' Untersuchungen, &c, der Basaltgesteine,' p. 198 : Tschermak,