BY W. H. BYRAN, B.SC 161 



ion is shown by a series of disconnected dykes, which, break- 

 ing across the strike of the schists extend from the cross 

 roads at Indooroopilly school to the carriage drive in Mt. 

 Cootha Reserve, the hne of intrusion in this case being 

 almost parallel to the axis of intrusion of the granites. 



An interesting feature in connection with these rhyo- 

 lites is the fact that some of them have been found to be 

 slightly auriferous. Mr. Rands^ refers to an intrusion — 

 probably that in the road cutting near Indooroopilly school — 

 in which ' very minute specks of goid were visible." An 

 assay of an average sample, however, '" gave but a mere 

 trace of the precious metal." 



vii. The Porphyries. 



This is a field name given to a series of rocks, which 

 differ in many respects from the rhyolitic dykes. They 

 form a distinctive type, and seem to be the result of a rather 

 later series of intrusions than that which produced the 

 more acid rocks. The rock is readily recognised in the 

 field since it always weathers into black, rounded boulders^ 

 somewhat rough and pitted on the outside. It is thus 

 easily distinguished from the sharp angular fragments 

 resulting from the weathering of the other mere acid type 

 of intrusion. The typical rock is porphyritic with numerous 

 phenocrysts of idiomorphic felspar — brown from alteration 

 — showing zoning in the hand specimen, and a few smaller 

 phenocrysts of quartz set in a brown cr grey felsitic ground- 

 mass. 



The rock easily undergoes decomposition, making it 

 extremely difficult to collect fresh specimens. Under the 

 microscope, the quartz phenocrysts are seen to have rounded 

 outlines, to be deeply corroded and to be surrounded by 

 marked reaction rims. The felspar is very altered being 

 replaced by an aggregate of muscovite and similar secondary 

 minerals. In spite of this alteration the idiomorphic 

 outlines of the original crystals are still definitely preserved. 

 Both the felspar and quartz phenocrysts are very s milar 



1. Op. cit. p. 1. 



