Vol. XXVIU 



lull 



] Excui'sioii to Sydenham , Bulla, and Diggers Rest. 5 3 



tourmaline, and andalusite. On passing west down Deep 

 Creek the hornlels beds gave place to beds of a very tough 

 conglomerate, very much like the Kerrie conglomerate near 

 Riddell. The pebbles were dented in such a way as to show 

 the immense pressures to which these conglomerates had been 

 subjected in consequence of protracted earth movements. 

 From lithological comparisons with similar conglomerates, 

 these are probably of Uj^per Ordovician age, although they 

 may be basal Silurian. On leaving this spot a traverse was 

 made north for a mile and a half over the granite, whose highest 

 portion stood about no feet above the basalt plain around. 

 This granite outcrop must once have been at a much higher 

 level, as evidenced by the huge tors or monoliths left out- 

 standing on the topmost portions. On arrival at the Bulla 

 school-house. Master Moore, son of a local resident, had pro- 

 vided milk and hot water for the party. These were very 

 acceptable, and enabled us to enjoy a cup of tea with the 

 lunches we had brought. After lunch we inspected sections of 

 great masses of kaolinized granite near Bulla. This shows 

 itself to be a much decomposed granite, in which the biotite 

 has completely chloritized, or even become quite bleached, 

 while the quartz remains loosely embedded in a mass of kaolin 

 from the complete hydration and carbonation of the felspars. 

 This kaolinization may be due to the gradual attack of surface 

 waters bearing carbon dioxide, in which case an analysis of 

 tlie material should reveal the presence of carbonates. On the 

 other hand, the decomposition of the granite in this area may 

 be due to the operation of vapours containing fluorine, boron, 

 and, perhaps, to a less degree, chlorine, which attack the 

 granite during the latest stages of its intrusion. This process 

 of alteration by hot vapours is known as pneumatolysis, the 

 results being well shown in the carclazyte or kaolin deposits of 

 Cornwall. A recent paper by Mr. J. H. Collins (Q.J.G.S., 

 vol. Ixv., No. 258, p. 155) records an interesting example of 

 pneumatolytic action on a granite. With regard to the 

 Bulla area, the kaolinized granite is distinctly local, and seems 

 to have some connection with the fact that it occurs in that 

 portion of the granite which is, or has been, covered by the 

 basalt flows, while the higher portions of the granitic mass, 

 which have presumably never been covered by the basalt, 

 have escaped kaolinization. The process may have begun by 

 carbonated water soaking in along that portion of the granite 

 just overlapped by the basalt, and may have extended down- 

 wards, and in under the basalt, from the junction. The apparent 

 absence of fluorite and tourmaline from the kaolinized granite 

 does not favour the pneumatolytic theory, as one would expect 

 to get these minerals formed where hot fluorine, boron, and 



