Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/444

334 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 9, nected, though all traces of this have since disappeared by denudation. Yet the assumption that the first three veins especially are intrusive appears to be singularly at variance with the mode of their mineral connexion with the reef. The feature generally observable between intrusive dykes and the surrounding rock (a division-line between the quartz and granite) is completely absent. The granite appears here, in fact, not at all unlike a zone of impregnation, inasmuch as the quartz, some distance above the veins, shows first scattered crystals and particles of felspar; these increase in quantity; plates and small nests of black mica appear, and, whilst their number also augments, the mixture becomes more and more fine-grained, and the passage to typical granite is insensibly completed, the reverse process of change of this rock to reef-quartz commencing again a few feet below. The definite thickness given to each granite- vein in the previous description has only reference to the zone of typical granite; if those portions of the reef above and below which bear already a marked granitic character were to be included, the width of each vein would perhaps be double that given. Even the gold and the other minerals associated with it in the reef take part in the above passage, for they have been found several inches deep in the granite; in the centres of the veins, however, none could be detected. Mr. Salter, the Manager of the Alliance Company, presented to the Melbourne Public Museum several fine specimens, showing the gradual change of quartz to granite, examples of vein-granite with specks of gold, pyrites, &c., and also one specimen from the lowest (fourth) vein, consisting of quartz, granite, and metamorphic rock. Between the two latter, however, the passage appears not to be so gradual, on account of the black colour of the metamorphic rock, caused by its very micaceous character along the line of contact, though in some places the two rocks appear like different conditions (textures) of the same magma.

As regards their intrusive character, there is also an inexplicable discrepancy in the behaviour of the upper three and the lowest granite-vein, viz. that if this last vein be considered to have faulted the reef (a supposition all the circumstances certainly point to), no reason can be assigned why the other veins should not have done the same, seeing that they intersect the reef at angles equally favourable for the production of this phenomenon. A shifting of the reef might perhaps have been caused by the second vein; but as, from the mode of intersection (figs. 5 & 5a), it would have taken place in the direction of the strike, i. e. the surface-portion of the reef moved southward, no displacement might be apparent; with the other two veins, however, it ought to be as plain as with the fourth granite-vein.

The minerals hitherto observed as associated with the gold in the Nuggetty Reef are the following: —

Iron-pyrites. Occurring generally in patches, strings, and, though rarely, in druses of small cubical crystals in hollows.

Arsenical pyrites. Like the former; seldom as perfect crystals imbedded in solid quartz.