1'24 VOLCANIC AREA. OF EAST MORETOX, ETC., DISTRICTS, Q., 



(4) Order of consolidation. 



1. Sphene 



2. Magnetite 



3. Apatite 



4. Hornblende 



5. Plagioclase 



phenocrysts 



6. Orthoclase 



phenocrysts 



/ Orthoclase, quartz 

 7. <. and plagioclase of 

 ' the base 



The constituents of the base crystallised almost simultaneously, 

 although the plagioclase present commenced and finished crystal- 

 lisation a little before the other minerals, forming definite 

 microliths. Orthoclase (sanidine) and albite crystallised together 

 with the quartz in cryptographic patches. 



(5) Nomenclature and affinities. — The texture of this porphyry 

 is that of a hypabyssal rock, the felspar being of two generations, 

 and a ground mass being present. In microscopic structure it 

 shows close resemblance to certain granophyres described by 

 Iddings from Obsidian Cliff, Yellowstone, National Park;* and 

 to some granoph} 7 res and rhyolites described by Hague from the 

 Eureka district, Nevada,! in which micropegmatitic phenocrysts, 

 consisting of intergrowths of quartz and sanidine, were noted. 

 Hague also notes certain phenocrysts with irregular boundaries 

 which merge into the groundmass by increasing abundance of 

 inclusions. Both these features are abundant in the typical 

 Pt. Arkwright porphyry. It may therefore be termed Grano- 

 phyric Porphyrite. The magmatic name as seen from the norm 

 is Andose (p. 171). (For analysis, p. 169.). 



*U.S. Geol. Surv. Seventh Ann. Rep. 1885-1886. 

 f U.S. Geol. Surv. Monograph XX. "Geology of the Eureka "District." 



