BY H. I. JENSEN. 135 



rhyolites, masses of red orthoclase porphyry consisting of decom- 

 posed albite and orthoclase phenocrysts in a red weathered base, 

 probably essentially orthoclase. This base is, in sections 

 examined, found to be aphanitic but holocrystalline; originally 

 some of it has been glassy, but devitrification has set in. This red 

 porphyry is apparently an old trachyte which has reached the 

 surface in a fluid condition. Its association with rhyolite recalls 

 that of the Maroochy district trachytes with rhyolites, and a 

 similar association at Battersby's, near Woodford, in the 

 D'Aguilar Range. 



The Mount Archer rhyolites are, like the orthoclase porphyries, 

 of a reddish colour, sometimes wholly aphanitic, sometimes with 

 phenocrysts of quartz visible in hanrlspecimen. Some are 

 columnar; some are banded and some are not. 



Sp. No. 74 is perfectly uniform in appearance in handspecimen 

 and might be mistaken for a red fine-grained quartzite. Sp. No.75 

 exhibits in handspecimen curious markings all over. Whichever 

 way the specimen is broken these markings are seen. They 

 resemble hieroglyphics. 



Sp. No. 104 (Plate xv., fig.27) presents a different appearance. 

 It is tinely laminated, and the laminae are curving in various 

 ways, giving evidence of flow-structure. 



Sp. No. 105 is composed of alternate bands of cryptocrystalline 

 quartz and volcanic glass (black pitchstone). 



Sp. Nos.75 and 104 were microscopically examined. 



Sp. No.75. Texture : the hieroglyphic structure of the hand- 

 specimen is seen under the microscope to be due to numerous 

 branching and intersecting veins of microcrystalline quartz, which 

 interlace in the fashion of stockworks. A differentiation of the 

 magma into silica and felspathic material has taken place in con- 

 solidation. Crystallisation is subsequent and due to devitrifica- 

 tion. The masses enclosed between the quartz veins consist of 

 an aphanitic, cryptocrystalline to microspherulitic base with a 

 few felspar phenocrysts. 



