MINERALOGICAL NOTES: No. IV.— ORTHOCLASE m 

 NEW SOUTH WALES 



By C. Andersox, M.A., B.Sc, Mineralogist. 



(Plates xlviii.-lii.). 



Macroscopic crystals of felspar are of common occurrence in the 

 granitic area of Northern New South Wales and have been noted 

 by various observers, but so far no crystallogi-aphic description 

 has appeared. It is proposed in this paper to describe and illus- 

 trate some of the jnore interesting orthoclase crystals comprised 

 in the Australian Museum collection, plagioclase felspars being 

 reserved for a subsequent article. 



The specimens figured were with one exception acquired by the 

 Trustees from Mr. D. A. Porter, of Tamworth, to whom also I am 

 under obligation for particulars of their finding and mode of oc- 

 currence. Mr. E. C. Andrews, of the Geological Survey of New 

 South Wales, who possesses an extensive knowledge of the 

 granites of New England, has been good enough to examine the 

 collection, and has given me valuable information regarding the 

 field I'elations of the rocks in which the felspars are found. 



For identification purposes Becke's method was employed ; by the 

 use of a liquid with a refractive index greater than those of ortho- 

 clase and about equal to the mean index of albite, orthoclase was 

 easily distinguished from plagioclase. This method was supple- 

 mented by observation of the extinction angles on cleavage Hakes. 

 The crystal forms were determined by inspection corroborated by 

 measurement with a contact goniometer. 



CocKBURN Creek. 



(Plate xlviii., fig. 1). 



A single specimen from •' Beadle's Conditional Purchase, "^ 

 Cockburn Creek, near Tamworth, is in the Museum collection. 

 It consists of an aggregate of glassy crystals of a typical 

 adularia habit, accompanied by small brownish crystals of 

 axinite. The felspar presents the simple combination c (001), 

 m (110), X (101), c and x having a tendency to oscillate with one 

 another and give a somewhat rounded termination. 



