BY W. N. BENSON. 161 



of the magnetite syenite-porphyry of Kiruna, Lappland, though 

 originally acid, has been still further albitised(22). Nothing 

 analogous to the features claimed by these authors as evidence of 

 albitisation, has been noticed in the keratophyres of the Nundle 

 district. 



The development of magnetite in the keratophyres presents 

 many features of interest. ho far as can be learnt, the only 

 rocks, at all analogous to these, are the Pre-Cambrian magnetite- 

 syenite porphyries of Lappland, and a few isolated and less 

 investigated occurrences in the Urals and elsewhere. The 

 analogy is very clear, if we compare our rocks with the descrip- 

 tions and illustrations in the papers of Sundiusl23), Geijer(24,25), 

 and Lundbohm{26). Sundius, while employing Geijer's term, 

 magnetite-S3'enite-porphyry, suggests that keratophyre would be 

 a more suitable designation. It will be of interest, therefore, to 

 summarise the views that have been put forward as to the origin 

 of the Scandinavian rocks. The magnetite-syenite-porphyries 

 are in intimate association with great deposits of iron-ore, and 

 the explanation depends on the view adopted as to the origin of 

 the iron-ores. Backstrom considered the iron-ores were of hydro- 

 pneumatolytic origin, belonging to the last phase of volcanic 

 activity(27). The volatile iron-salts rose through the igneous 

 rocks, and, coming into contact with the sea- water above, were 

 precipitated as magnetite. This hypothesis was supported Ijy 

 De Launay(28). On the other hand, Hogbom considered the ore 

 was the result of a differentiation from a syenitic magma(29), and 

 Stutzer supported this view, adding to it the statement that 

 pneumatolysis has played an important minor role in the forma- 

 tion of the ore(30). Geijer has studied the question in great 

 detail. His monograph on the Kiruna field(31)is, unfortunately, 

 not accessible in Sydney, but he has published an abstract of it 

 in Economic Geology(24), and, more recently, a general review of 

 the mode of occurrence of the iron-ores of Lappland(25). He 

 supports the view of the magmatic origin of thfe iron-ores, as also 

 of the magnetite-syenite-porphyries, believing that the latter 

 differentiated out from the normal syenite-porphyry of the dis- 

 trict, and had a lower temperature-range of crystallisation. He 



