138 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



round a kernel of sanidine is seen. The phenoeryst is- 

 2 mm. long, and the dyke is no doubt associated with the 

 "granophyre" which Dr. Woolnough-'^ has described from 

 the northern base of Mount Alford. 



The most characteristic pitchstone, however, is the 

 one which occurs over the whole of the southern area, as 

 inclusions in the acid pyroclastic material. It is remarkably 

 uniform in nature and occurs in boulders of all sizes. It 

 is found in great abundance at Mount Lindsay and forms 

 the major portion of the agglomerate which makes up the 

 top dome of Mount Lindsay. The pitchstone itself occurs 

 in tremendous blocks many tons in weight, and when fresh 

 has a beautiful glassy black appearance. It has a conchoidal 

 fracture, and the white felspar phenocrysts which occur 

 abundantly through it show up well. In thin sections it is 

 seen to be made up of a light-brown glass and it has 

 phenocrysts of sanidine, anorthoclase, ciuartz and occasion- 

 ally augite. These phenocrysts are usually corroded and 

 embayed and often the glass is seen entering along the 

 cleavage cracks. {See Plate XII., fig. 1.) The glassy 

 groundmass shows perfect fluxion structure and it is 

 traversed by perlitic cracks. In several places through the 

 sections small angular fragments of felspar may be seen, 

 and these have apparently resulted from the disruption 

 of the phenocrysts while the groundmass was still in a 

 mobile state. Around these fragments the fluxion structure 

 is particularly pronounced, and their sharp angles are in 

 great contrast to the rounded corners of the larger pheno- 

 crysts which have remained intact. The specific gravity 

 is 2-39. 



Cracks occur abundantly in the phenocrysts and are 

 of interest as they are seen to lia'"e the same characters 

 as those described in a perlitic pitchstone from the Tweed 

 River by Smeeth.^' 



Th^ cracks are much moie abundant in the quartz 

 than in the felspar, and in some cases they pass continu- 

 ously from the matrix into the quartz crystals and also 

 into the felspar cystals. One does not, however, see the 



'^ Pioc. Roy. Soc, N.S.W., xlv., p. 150. 

 "Proc. Roy. Soc, N.S.W,, for 3894 (xxvii.). 



