DR. J. A. THOMSON ON THE [Dec. I9I3, 



(/) The Hornblende-Rocks (Pyroxenite-Amphibolites). 



These rocks appear only in a few dumps, both on the east and 

 on the west side of the main dyke of quartz-dolerite, and are 

 intimately associated with, talc-schists and magnesite-rocks. The 

 disposition of these dumps suggests that the rocks either form long, 

 narrow, independent dykes in the peridotites, or that (with the 

 latter) they form a banded complex. 



In hand-specimens and thin sections alike they are found to 

 consist almost entirely of large interlocking crystals of pale fibrous 

 hornblende, without any trace of parallel structure, except in the 

 immediate vicinity of local shear-planes. In places, the presence 

 of brown hornblende of more massive character betokens a small 

 amount of original hornblende. The brown hornblende shades off 

 to green on its edges ; generally it has outgrowths of massive 

 tremolite, such as commonly occur on the original hornblende of 

 uralitic and pilitic rocks. Iron-ores are rare, and are altered on 

 the exterior into leucoxene or granular sphene. Still more rare 

 are felspar (saussuritized), apatite, and biotite ; while a small 

 amount of interstitial micropegmatite has been observed in one 

 specimen. As in all the amphibolites, surface- weathering has 

 produced a variable amount of chlorite and epidote from the horn- 

 blendes. 



The only rational explanation of the mode of origin of the 

 dominant pale fibrous hornblende is that it is uralitic : therefore, 

 the original rock must have been a pyroxenite containing a small 

 amount of brown hornblende. 



Pyroxonite-amphibolites, with or without original brown horn- 

 blende, have so far been found in only a few Western Australian 

 goldfields, but are probably of wider distribution. G.S.M. 3963, 

 from Lennonville, is, as Mr. Gibson 1 has already pointed out, a 

 partly uralitized pyroxenite, and consists only of augite, pale 

 fibrous hornblende, and granular sphene. G.S.M. 8322 (from 

 Mount Desmond, Phillips River Goldfield), which Mr. Simpson & 

 Mr. Glauert have compared with the Kalgoorlie type, 2 is a similar 

 rock in which uralitization is complete. A more complete analogy 

 with the Kalgoorlie type is shown by a rock from Lawlers (G.S.M. 

 7105), containing a small amount of brown hornblende in addition 

 to the dominant uralite. A rock from Raven sthorpe (G.S.M. 8140), 

 described by Mr. Simpson & Mr. Glauert 3 as an altered diabase, 

 shows a much greater amount of brown hornblende, and in this 

 respect constitutes a passage to the lustre-mottled amphibolites. 

 The brown hornblende is almost opaque from the presence of 

 inclusions of iron-ores, arranged for the most part in rows oblique 



i Bull. Geol. Surv. W. Austral. No. 8 (1903) p. 13. 

 ~ Ibid. No. 35 (1909) pp. 26, 28. 

 3 Ibid. pp. 27, 43. 



