068 IJR. J. A. THOMSON ON THE [DeC. I9I3, 



felspars were first saussuritized. ■ On the whole, without being quite 

 positive on the point, I favour the view that the albitization pre- 

 ceded the uralitization, and, farther, that (as in the case described 

 by JBailey & Grabham) the more acid part of the quart z-dolerite 

 niass was affected by the albitization, while the more basic part 

 was not, and was subsequently converted into amphibolite. Never- 

 theless, if it should be subsequently shown that the uralitization 

 preceded the albitization, it would not invalidate the general con- 

 clusions of this paper, for it is certain that the quartz-dolerites 

 were partly sheared before the intrusion of the porphyries. 



Although it is difficult to give actual proof of it, there is every 

 reason to believe that the lodes as such were formed after the 

 intrusion of the porphyries. The latter never cut the lodes, but 

 sometimes form their walls. Now, if the albitization were due to 

 emanations from the cooling quartz-dolerite magma, it preceded the 

 intrusion of the porphyries, which did not take place until the 

 quartz-dolerite had been sheared, and, in consequence, the formation 

 of the lodes must be due to a later set of solutions. But the rocks 

 most intimately associated with the lodes, and on which the vein- 

 solutions have left their mark : namely, the bleached greenstones, 

 do not differ essentially from the greenstones. Albitization and the 

 concomitant processes that gave rise to the greenstones constituted 

 the incipient vein-alteration, extreme carbon ation and sericitization 

 with minor silification the final vein-alteration. Consequently, if 

 the lodes are later than the porphyries, the albitization and accom- 

 panying changes must have followed, or at least must have continued 

 until after the intrusion of the porphyries. Confirmative evidence of 

 this view is afforded by two observations : first, that the porphyrites 

 have been albitized, and that some of the porphyries appear to be 

 albitized porphyrites; and, secondly, that the greatest development 

 of the bleached greenstones lies in those parts of the quartz-dolerite 

 mass that surround the porphyry intrusions. 



The solutions producing the alterations at Ivalgoorlie must have 

 been very similar in nature to those that produced the albitization 

 in Britain. Mr. Dewey & Dr. Flett have described the latter as 

 follows :, — 



' The composition of the pneuinatolytic emanations cannot be exactly defined, 

 but it is certain that they consisted of water and soda and silica in solution ; 

 probably also carbonic acid was abundant, and many other substances may have 

 been present.' (Geol. Mag. dec. 5, vol. viii, 1911, p. 246.) 



The early solutions in Ivalgoorlie which caused the albitization 

 must have been of this nature, with soda and silica relatively 

 abundant; while the later solutions must have contained more 

 carbonic acid aud, perhaps, also potash. The solutions probably 

 altered gradually in nature as the underlying magma cooled, and in 

 the late stage at which the lodes were formed ' many other sub- 

 stances' must certainly have been present ; but their discussion does 

 not come within the scope of this paper, and must await a fuller 



