Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/350

 upper portion of the same formation, and occasionally throughout the other beds.

At Gympie, one of the richest quartz-reefing districts in the colony, the auriferous area is confined to veins traversing a crystalline diorite, or within a certain limit of its boundary, marked by the presence of fossiliferous diabase tufas.

Of the crystalline rock at Gympie Mr. Aplin, who reported on the district, gives the following description : —

" In irregular-shaped masses or broad dykes this rock occurs at intervals over a large portion of the area of this gold-field, but is most prominently developed within a zone half a mile or more in width, having a longitudinal direction of about N. 60° W., and embracing the space from the ' Lady Mary ' hill to that on which the Gympie township commences.

" It is in the decomposed upper portions of this rock, which weathers brown and argillaceous, though retaining its compactness, that the quartz veins traversing it are found to be so highly productive.

" In its ordinary condition it is excessively hard, and is the most formidable obstacle the miners have to contend with, some from the very surface, and others at varying but comparatively shallow depths."

Of the lode-stones outside a certain limit from this crystalline rock he says : — " The quartz-veins associated with the slates or other sedimentary rocks not connected with, or affected by, the greenstone have hitherto been found not to contain gold in paying quantity."

Mr. Hacket, who devoted a much longer time to the survey of the Gympie gold-field, confirms Mr. Aplin's opinion on the latter point, as he says in his report : —

" Skirting the greenstone-belt on both sides is a series of siliceous slates and quartzites, highly metamorphosed, and in many cases jasperized, banded, and cleaved.

" In this latter formation no gold has yet been found, although many reefs have been explored."

Mr. Hacket, however, seems to have thought that the crystalline rocks were interstratified with the slates &c.

My own impression, formed from two short visits to this locality, was that the crystalline rock was in all cases intrusive, whilst the so-called " fossiliferous greenstone " was a tufaceous deposit, partly contemporaneous with, partly the result of the denudation of the crystalline rock which represented the core or cores of Devonian trap-vents.

The analysis of the two varieties of rock above mentioned, viz. the so-called Gympie "greenstone" and the "fossiliferous greenstone," gave the following results : —