648 DR. J. A. THOMSON ON THE [Dec. I913, 



outlines of the felspars are retained, and the alteration-products 

 of the presumed pyroxenes still form, distinct pseudomorphs, the 

 structural analogy with quartz-dolerites is complete. It follows 

 that the alteration, as compared with the original rock, consists in 

 albitization of the felspars, and in conversion of the ilmeirite into 

 leucoxene and of the pyroxene mainly into chlorite and carbonates. 

 The question arises whether this alteration has beeu effected directly 

 on the fresh quartz-dolerite, or after it had been first uralitized. 

 This will be discussed in the concluding part of this paper. 



The greenstones of ' The Mile ' seldom contain so much chlorite 

 as the above-described rocks ; and, by a breaking-down of the sharp- 

 ness of the pseudomorphs it is not always easy, and sometimes 

 impossible, to distinguish the distinctive elements and structure of 

 the quartz-dolerites. The felspars are seldom entirely destroyed, 

 but have lost their rectilinear outlines, and what remains is much 

 obscured by inclusions of chlorite, carbonates, and sericite. The 

 felspars of the micropegmatite have often completely disappeared. 

 Consequently, it is no longer 'possible to isolate by exclusion 

 certain aggregates of chlorite and carbonate as pseudomorphs of 

 ophitic pyroxenes ; and the rocks are, to all appearance, structure- 

 less aggregates of chlorite, carbonates, felspar, quartz, sericite, 

 leucoxene, and apatite. Nevertheless, there are certain relict- 

 structures of wonderful persistence, even under extreme alteration 

 and after considerable shearing. Leucoxene is never completely 

 absent, even in rocks in which much secondary magnetite or pyrite 

 is present, and it preserves the large deeply-embayed forms which 

 characterize ophitic ilmenite. Again, groups of quartz-grains 

 extinguishing together, but widely separated by a structureless 

 •aggregate of chlorite and carbonates, are found in the most altered 

 rocks. Mr. Card, 1 who supposed that these represented fragments 

 of still larger quartz-crystals undergoing replacement, aptly 

 compared them to islets of an archipelago (fig. 2, p. 649). The 

 common crj'stallographic orientation of each ' archipelago,' and 

 the abundance of apatite in the ' islets ' and the intervening 

 ' sea,' combined with the presence of micropegmatite in the less 

 altered rocks, seem conclusive evidence that the quartz once 

 formed part of a micrographic intergrowth. Not only has replace- 

 ment caused the felspars to disappear, but it appears also 

 to have obliterated the finer particles of quartz, and to have 

 encroached on the larger. While it ma}' be admitted that 

 such arcliipeliigos of quartz might arise in the manner suggested 

 by Mr. Card from the quartz of a granite, their conjunction with 

 leucoxene leaves no possible parent but an intrusive basic rock 

 containing quartz. 



As a secondary structure in these greenstones may be noted the 

 occasional development of the carbonates into large euhedral 

 rhombohedra — cutting indifferently across the junctions of the 

 former minerals, and enclosing quartz, leucoxene, and muscovite. 

 Often the composition of the carbonates varies from place to place 

 1 Eec. Geol. Surv. N. S. W. vol. vi, pt. 1 (1898) p. 23. 



