670 DR. J. A. THOMSON OX THE [Dec. I9I3,. 



of these fundamental divisions, at least on chemical grounds* 

 Nevertheless, the following classification may be suggested : — 



Goldfields genetically associated with 



(1) Bocks of Pacific type, for instance : 



(a) andesites and dacites (New Zealand, Transylvania, etc.) ;. 



(b) gr'anodiorites (East Australia, North America, etc.) ; 

 and probably (c) dolerites (India) ; 



(d) hornblende-schists (India). 



(2) Rocks of Atlantic type, for instance : 



(a) phonolites (Cripple Creek) ; 



(b) foyaite (Portobello, New Zealand). 1 



(3) Bocks of spilitic type, for instance : 



(a) quartz-dolerite and albite-porphyry (Kalgoorlie) : 



(b) albite-granite (Bavensthorpe, Western Australia). 



It is interesting to note that tellurides are found in goldfields 

 belonging to each of the major divisions proposed above, with 

 andesites of Pacific type in Transylvania, with phonolites of 

 Atlantic type at Cripple Creek, and with quartz-dolerites and 

 albite-porphyries of spilitic type at Kalgoorlie. The two most 

 important gold-occurrences not connected with rocks of Pacific- 

 type are tellurite fields. 



(d) The Relationship of the Quartz-Dolerite Series to the 

 Fine-Grained Amphibolites and Greenstones. 



The facts that are clear are that the fine-grained amphibolites. 

 and greenstones are the older series, and have been contact-altered 

 by the peridotites and porphyrites. It has already been suggested 

 that the former rocks are most naturally regarded as an altered 

 series of lavas or tuffs. If this view be accepted, it remains to be 

 considered whether this volcanic series arose from the same magma 

 as the intrusives : that is, whether the relationship between the two 

 series is analogous to that of the lavas and intrusives of so well- 

 known a district as Skye. Had the fine-grained amphibolites, or 

 some of them at least, the composition of spilites, a positive answer 

 could at once be given to this question. But, although no analysis- 

 of the Kalgoorlie examples has yet been made, it is unlikely that 

 they will be found comparable with the spilites, and most probable 

 that they will be found to agree with normal (labradorite) basalts. 

 Even then, it will still be possible to regard them as magmaticalty 

 related to the quartz-dolerite series : for it is clear that the albi- 

 tizing power of the magma was a late development during intrusion 

 and cooling. The question, however, is best left open at present,, 

 until a closer study and comparison of the fine-grained amphibo- 

 lites and their relatives in other goldfields has been made. 



1 See G. II. P. Ulrioh, in F. W. Hutton & G. H. F. "Clinch, 'Report on the- 

 Geology & Goldfields of Otago ' Dunedin, 1875, pp. 165-68; and P. Marshall, 

 ' The Geology of Dunedin ' Q. J. G. S. vol. lxii (1906) pp. 391-93. 



