128 VOLCANIC AREA OF EAST MORETON, ETC., DISTRICTS, Q., 



by an examination of their extinction angles. Zoning due to 

 alternate layers of two felspars deposited in crystallographic con- 

 tinuity also occurs; this is visible without crossed nicols. The 

 plagioclase varies from acid labradorite (extinction angle in 

 symmetrical sections 24°-27°) to albite (extn. angle up to 21°). 

 The latter is found chiefly intergrown with orthoclase. (b) The 

 orthoclase exhibits Carlsbad twinning and is also zoned; the 

 zoning appears to be due to a layer of albite being deposited 

 round the orthoclase, or to alternate layers of these minerals. 

 The core of many albite and orthoclase crystals has a dusty 

 appearance due to an abundance of minute inclusions of glass 

 and hornblende, or sometimes to incipient decomposition. (c) 

 The hornblende is brownish or olive-green, and contains inclu- 

 sions of magnetite and apatite. Most of the crystals are twinned. 

 Twinning plane, parallel to a and c. 



Double refraction : - 022. Pleochroism : r deep greenish- 

 brown or olive; h greenish; a light green. 



Absorption : c > b > a 



Optic sign positive. Plane of optic axes parallel to b. Extinc- 

 tion on edge b/c, 18°; on other lines bisecting cleavages from 10° 

 to 21°, mostly c. 10°. 



(d) The quartz is interstitial and last to crystallise, (e) The 

 biotite is very corroded. Chlorite occurs as an alteration product 

 after biotite and hornblende. 



(4) The order of consolidation is normal. There were, how- 

 ever, several stops in the last stages of crystallisation, giving 

 rise to the zoning of the felspars described above. The plagio- 

 clase and orthoclase commenced to crystallise almost at the same 

 time, but the latter finished last. 



(5) Nomenclature. — In composition and general appearance 

 this rock is a quartz diorite, but on account of its many close 

 affinities with the Pt. Arkwright, Eudlo Ck., and Eumundi por- 

 phyrites I have described it here. The chemical analyses (anal, 

 p. 169) further prove its close relationship to the Pt. Arkwright 

 rock. Tts magmatic name is Andose. 



