98 Ernest W. Skeafs : 



in a shallow quarry near the summit of the hill. The rock is 

 grey, fairly even grained, and felspar, quartz, blaick biotite and 

 a little hornblende are visible in the hand specimen. Its specific 

 gravity is 2.69. Under the microscope (Section oOi) it is no- 

 ticed that both plagioclase and orthoclase are present, that some 

 of the biotite has been altered to chlorite, abundant needles of 

 apatite are included in the generally ragged crystals of biotite, 

 and a little rutile ii5 ^^robably present. The symmetrical extinc- 

 tion angles of the plagioclase lamellae ra-nge from about 11 deg. 

 to 17 deg. The crystals are frequently zoned, the margins being 

 invariably more acid, and are sometimes untAvinned. The cen- 

 tral parts of the crysta)ls correspond to andesine of composition 

 AbjAn.,, the mafl'gins to oligoclase of composition Ab^Auj. The 

 average composition of the plagioclase as a whole is probably 

 near AbgAug. The plagioclase is generally someiwhat kaolinised, 

 and is usuallj'' idiomorphic. The orthoclase, containing some 

 minute irregular intergTowths with albite is, however, fresh and 

 moulded on the plagioclase. The structure of the rock, as a 

 whole, i* hypidiomorphic, and the average grain-size is 1 mm. 



Petrographically, it should be classed with a number if other 

 Victorian granitic rocks as a grano-diorite, on account of the 

 large amount of quartz present, the considerable quantity of an 

 alkali felspar, and the relatively acid character of the plagioclase.- 

 present in this group of rocks. Professor Gregory,-"- following 

 American usage, has suggested the application of this term in 

 preference to Quartz-mica-di(jrite, to which group Dr. Howitt ha.> 

 referred some of them. 



No chemical analysis of this rock is available, but an attempt 

 has been made to determine, quantitatively, its mineral volume 

 composition. From this the bulk mineral composition is found 

 by multiplying the percentage volume of each mineral by its 

 specific gravity. Finally, by accepting analyses of minerals 

 having similar optical properties, an attempt has been made to 

 determine approximately the chemical composition of the rock. 



The method followed in determining the volume percentage of 

 each mineral in the rock is due to Rosiwal.- He has used a 



1 The Geology of Mount Macrcli.n, VictoriiL, I'mi'. Roy. Soc. Victona, 1-1 (l!)0-i), i>. VM. 



2 Verhanrtl d.k.k. Ocol Kfifli.s:uis),. ISils, pii. 14.S, ^•t^^■i\. Tlu' t^uiuititativi.' flii.ssitiiMtioii 

 of Igneous Itooks, l!)0:i, p. '201. J. I'. l(ldiiij;s, Joiinial of (ieolo.M , vol. \ii. (l!)04>, p. 2.')2. 



