BY H. C. RICHARDS. 137 



phenocrysts consist of quartz, sanidine and an acid plagio- 

 clase somewhat sparingly developed ; these are set in a 

 very fine cousertal groundmass. Micrographic intergrowths 

 of quartz and orthoclase are very common. This rock is 

 very similar to the rhyolite of Glennie's Pulpit at Mount 

 Alford, which is a few miles to the south. Name : Quartz 

 Porphyry. 



Glassy Varieties. 

 Perlite. — This is of rather limited occurrence and is 

 found at the Glass Cutting on the Springbrook Plateau, 

 between portions 117 and 121, parish of Numinbah. It 

 is found at the base of the rhyolite flow, and is many feet 

 thick. It can be seen grading off into the rhyolite and it 

 exhibits beautiful fluxion structure and perlitic cracks. 

 (See Plate XII., flg. 2.) Perlites are abundantly developed 

 and the rock in many places is seen to be composed entirely 

 of them. In colour it is a bluish grey when fresh, but 

 it becomes brown on alteration. The rock has a specific 

 gravity of 2-33. 



Under the microscope, the holohyaline nature of the 

 rock is very evident and the perlitic cracks are abundant. 

 Trichites, cumulites and margarites are present in great 

 abundance, and the cumulites are seen arranged in bands. 

 It is noticeable that the trichites and margarites are 

 arranged along parallel lines, and these intersect the bands 

 of cumulites at an angle of 10°. 



Fifchstones. — These occur as dykes and as inclusions 

 in the acid pyroclastic material in the southern portion of 

 the area. The dyke occurrences are near Cunningham's 

 Gap. Near the Moogerah School at the foot of Mount 

 Alford, there is a green pitchstone dyke through the 

 Walloon measures. There are very occasional phenocrysts 

 of sanidine up to 1-5 mm. long, but apart from this the 

 rock is a glass in which are occasional lath-shaped felspars 

 •05 mm. in length. Trichites are common and spherulites 

 are very abundant and they tend to have an axiolitic 

 arrangement, parallel to the fluxion bands. {See Plate 

 XII., fig. 3.) A number of micro-slides of this rock were 

 prepared for class purposes, and in one section, a pheno- 

 cryst of micrographic intergrowth of quartz and orthoclase 



c 



