gcls_WFP_234

THE BRISBANE TUFF.

The Brisbane Tuff, which is more usually called "Porphyry" in the trade, is a stone which has been extensively used over a long period.

It is an invaluable material to Brisbane owing to its utilisation for road-making, kerbing and building purposes. It occurs along a belt stretching for several miles through Brisbane in a north and south direction and has an average width of perhaps a half-mile.

The stone is a variable one as regards colour and compactness. It needs very careful selection, which is easily understood when one realises its mode of origin. It is formed from a volcanic ash of a rhyolitic nature and has been subsequently consolidated and hardened very largely by the passage through it of solutions containing silica. The influence of the silicification has not been equal in all parts of the mass so that the stone varies a great deal in the same quarry. All shades of colour from white to pink, green, yellow, brown and purple are obtainable; these colours are due to the influence of iron and manganese oxides.

An examination under the microscope shows the stone to be made up of occasional small crystals of quartz, orthoclase, and plagioclase, set in a devitrified felspathic ground mass which often shows the peculiar curved outlines of the originally glassy particles.

Several important structures have been built of this material in the rough-dressed condition with sandstone used for the facings, and as the tuff has been well selected the use of the stone has been attended with much success.

The tuff is multi-coloured with a predominant pink to purple tint so that the lighter coloured sandstone gives the necessary relief.

[Handwritten signature]
Rotarian H.C. Richards
Professor of Geology
University of Queensland