J". A. Thomson — Rocks of Western: Australia. 147 



diamond-shaped, with the lower part of the diamond more obtuse-^ 

 angled than the upper ; from the lower angle of the diamond there 

 descends what is probably a fairly wide straight-sided slit, but its 

 edges seem always more or less broken away ; in the best-presers^ed 

 specimens two long and very slender feeler-like processes project 

 into the aperture from either side of the upper end of the slit ; when 

 all the fragile part has been broken away the aperture is enlarged into 

 a fairly symmetrical diamond. 



This species is very strongly associated with the subzone of Offaster 

 pillula (in the zone of Actinocamax quadratus) and the immediate 

 neighbourhood of that horizon, occurring with great constancy, though 

 always scarce. Otherwise I have only found it in the Uintacrinus 

 band in Hampshire and Eent and in the Marsupites band in Hampshire, 

 and then only most exceptionally. 



EXPLANATION OF PLATE VII. 



Fig. 1. Bhagasostoma Sussexiense. Part of a branch, x 12. . ^ 



,, 2. Ditto. Another part of the same branch, x 21. 

 ,, 3. Ditto. The same specially lighted to show up the outlines of the 



zooecia and avicularia. 

 ,, 4. Ditto. Part of another branch, x 12. 

 ,, 5. Ditto. Ditto. X 12. 



,, 6. Ditto. Cross section through a branch, x 12. 

 ,, 7. B. palpigerum. A fragment of a large zoarium from the Uintacrinus 



band, Margate. Nat. size. 

 ,, 8. Ditto. Part of the same, x 12. 

 ,, 9. Ditto. Ditto. X 21. 



,, 10. Ditto. Part of a branch from the subzone of 0^asierpiZZ«Za, Seaford. 

 X 12. 



{To be continued.) 



II. — The CLASsincATioisr of the Kocks of the Western Australian 



■ GOLDFIELDS. 



By J. Allan Thomson, B.A., B.Sc, F.G.S. 



Contents. 



I. Objects of this Paper. 

 II. Difficulties of Field Geology in Western Australia. 



III.- General Statement of the Problem, and Discussion of Previous Literature. 

 IV. Eesults of the writer's Petrographical Studies. 

 V. Principles of Classification. 

 VI. Suggested Classification. 

 VII. Detailed Petrographical Studies. 



I. Objects of this Paper. 



rnHANKS to the descriptions published by visiting mining engineers 

 J_ and the more detailed work of the Geological Survey of the State, 

 the general geological features of those parts of Western Australia 

 in which mining is carried on are now well known. A surprising 

 degree of uniformity of geological structure and mode of gold 

 occurrence is revealed over an extent of country unparalleled elsewhere 



