ANORTJJOCLASE BASALT FROM MAPLETON, QUEENSLAND. 161 



Anorthoclase Basalt from Mapleton, 

 Blackall Range, South-Eastern 

 Queensland. 



J3y II. C. Richards, D.Se., Professor of Geology and 

 Mineralogy. University of Queensland. 



(Plate IV.) 

 ■{Sead before the Eoyal Society of Queensland, ilst August, 1922.) 



Some time ago, while investigating the volcanic rocks 

 of the Blackall Range some 60 or 70 miles north of Bris- 

 bane, in the Mapleton area, the author was attracted by a 

 basaltic flow which contained numerous lozenge-shaped 

 phenocrysts of felspar. Subsequent microscopic examina- 

 tion bore out the prediction that they were crystals of 

 anorthoclase felspar. 



Mr. G. J. Saunders, B.E., il.Sc, who was completing 

 his honours course in Geology at the University in 1918, 

 ifindly undertook the complete chemical analysis of the 

 rock; and an inspection of this shows the presence of 7-02 

 per cent, of alkalies — an excess of nearly 3 per cent, over 

 the combined alkalies in the average analysis of basalt 

 .as given by Daly.^ 



Although many rliyolitic and trachytic rocks with very 

 ■decided alkaline characters have been described from 

 Southern and Central Queensland b.y Dr. H. I. Jensen and 

 the author, up to the present there has not been any 

 record of a sub-basic or basic alkaline rock containing 

 anorthoclase. 



This paper, therefore, constitutes the first record in 

 Queensland of a basic alkaline lava containing 

 iinorthoclase. 



FIELD OCCURRENCE. 



About 2 miles to the south-west of the township of 

 Mapleton, in the proximity of the Mapleton Falls, where 

 the road leaves the top of the range to descend into the 



• R. A. Daly, ' ' Origin of Igneous Eocks. ' ' 



