Revieu-s — J. H. L. Vogfs Formation of Iron Ores. 83 



As illustrating, in a general way, the effects of such diffusion, a 

 dyke, 10 metres wide, occurring near Huk, in the Christiania 

 district, is described. The centre of the dyke is a mica-syenite- 

 porphyry, consisting of large crystals of orthoclase lying in a light 

 red, fine-grained ground-mass. About one or two metres from the 

 junction the ground-mass takes on a grey colour due to increase in 

 the magnetite. As the junction is approached, the felspars decrease 

 in size, plagioclase takes the place of orthoclase, and the ground- 

 mass becomes darker and more compact. The actual margin is 

 formed of a black rock, rich in iron, and containing a few por- 

 phyritic crystals of felspar (plagioclase) and mica. Pyrite increases 

 as the junction is approached. The microscope shows that magnetite 

 and biotite are very abundant in the marginal rock, which may be 

 described as a kersantite. Chemical analysis establishes the fact 

 that phosphoric acid is also more abundant at the margin than in the 

 centre. The amount found corresponds to '51 per cent, of apatite 

 in the centre and 144 per cent, at the margin. There is nothing in 

 the appearance of the dyke to suggest that there have been two 

 intrusions. The transition from one type of rock to the other is 

 gradual, and can only be accounted for by supposing that the magma 

 became differentiated, before consolidation, by a concentration at the 

 margin of the chemical compounds of the first formed constituents. 



This case is so interesting that we quote below (1) the composition 

 of the original magma as reckoned from actual analyses ; (2) the 

 composition of the centre; and (3) the composition of the margin: 



47 

 20 



..• ( r 



5-5 



7-5 



4 



1 



3 



Having illustrated in this way the general, principles of magmatic 

 concentration, the author describes in detail the ilmenite-deposits 

 of the Ekersund-Soggendal district. They are found in a region 

 composed of plutonic rocks which may be grouped under the following 

 terms : — labradorite-rock (a norite extremely poor in ores and 

 ferro-magnesian constituents), hypersthene-norite, biotite-norite and 

 enstatite-granite. Dykes (exclusive of the ore-deposits) traverse 

 the district. They may be classified, in the order of their formation, 

 as follows : — 



I. Ilmenifce-liyperstlieiie-labradorite dykes of pegmatitic character. 



II. Norite dykes. 



III. Diabase dykes, generally containing olivine. 



Titanite is absent from the labradorite-rock and the true norites, 

 but is found in the enstatite-granite. All the above rocks evidently 

 belong to the same general period. They are the products of the 

 consolidation of one magma-basin. 



The ilmenite-deposit of Storgangen may be regarded as forming 

 a dyke 3 kilometres long, and 20 to 70 metres wide. It consists of 



