25 



with the potash granites of south-eastern Ireland. The second is 

 the rock recorded from Moolyella, Pilbarra Goldfield, Western 

 Australia/i'^ a tin-bearing albite pegmatite which* cuts across 

 a mass of granite. Mr. Gibb Maitland apparently assigns this 

 albite- rock to the pneumatolytic stage of the crystallization of 

 the granite. 



The third instance is recorded by Emmons and Calkins (20) 

 as occurring in connection with a grano-diorite at Cable, in 

 the Philipsburg Quadrangle, Montana. "The essential con- 

 stituents are nearly pure albite- and biotite; the accessories 

 are zircon, rutile, and apatite; . . . chlorite is present 

 in small amount." "A small exposure of pegmatite occurs; 

 It is essentially an aggregate of snow-white albite 

 crystals . . . small polygonal interstices between which 

 are filled with green chlorite. Quartz and potash felspar are 

 absent. The accessories are zircon and rutile. . . " 



This occurrence seems analogous to that of Rosetta Head. 

 R. L. Jack (21) records albitite in the Hundreds of Roberts 

 and Miltalie, Co. Jervois, Eyre Peninsula, the analysis of 

 the former being given above, but there is nothing in his 

 description of the occurrences to tell definitely whether they 

 are associated or connected with the more potassic granites. 



(5) RED (potash) APLITE. 



The main outcrop of this rock occurs near Middleton 

 Beach, and its field characters have already been described. 

 A small dyke, to be presently described, is very similar both 

 in appearance and in mineralogical characters, and may 

 quite probably be genetically connected. 



Microscopically the habit of the red aplite is fairly 

 typically aplitic, the constituents being mostly allotriomorphic 

 to subidiomorphic, and there has been a certain amount of 

 strain, resulting in undulose extinction and in slight peri- 

 pheral granulation. The minerals present are quartz and 

 felspar, with a few flakes of muscovite and a little limonite 

 after magnetite; ferro-magnesian constituents are absent. 



Tlie felspar comprises microcline and albite, both heavily 

 kaolinized and stained with iron oxide, which gives the rock 

 its red appearance. Microcline predominates, but owing to 

 alteration the relative proportions of the two felspars are 

 impossible of measurement. 



Microcline occasionally shows a slight tendency to pris- 

 matic habit, and traces of microperthitic structure are seen. 



(19) Geol. Surv. of W. Aiistr., Bull. No. 15, pp. 12 and 2o. 



(20) U.S. Geol. Surv., Prof. Paper 78, p. 97. 



(21) Jack: Op. cit. 



