1892. 



THE SEQUENCE OF PLUTONIC ROCKS. 297 



constituents, especially iron-oxide and lime, from the included rock. 

 The lenticles of rhomben-porphyry show very interesting structural 

 features in their external portions. In the interior they are massive, 

 and the large porphyritic felspars give the characteristic rhombic 

 sections; near the margins they are foHated, and the porphyritic 

 felspars have been deformed into long narrow lenticular folia. The 

 structural modification is exactly similar to that which would be 

 produced by dynamo-metamorphism of a regional character. Here 

 it is associated with contact-metamorphism and is a purely local 

 phenomenon. 



We must now return to the consideration of the mutual relations 

 of the rocks of plutonic masses. The general question of the 

 differentiation of plutonic magmas has been discussed in a recent 

 paper by another Scandinavian geologist, Professor Vogt (4). The 

 immediate object of this paper is to explain the origin of certain ore- 

 deposits, and especially those of the Ekersund district in Norway, 

 and of Taberg in Sweden. The one in the former case is ilmenite, 

 and in the latter titano-magnetite. In both cases the ores are 

 intimately associated with basic eruptive rocks, in which the same 

 minerals occur as accessory constituents ; but the actual relations of 

 the ore-deposits to the surrounding rocks are not the same in the two 

 cases. At Taberg the ore occurs in the centre of a mass of olivine- 

 hyperite into which it passes by insensible gradations. The ore 

 itself contains a small quantity of olivine and felspar. As the rock 

 is traced outwards, felspar increases in quantity, and pyroxene makes 

 its appearance. Thus it passes into a hyperite rich in magnetite and 

 olivine, and finally into normal olivine-hyperite. Such relations 

 clearly imply that the ore-deposit has been formed by a local concen- 

 tration of ferriferous molecules in a plutonic magma, or else by the 

 local accumulation of magnetite grains or crystals after separation. 

 The author inclines to the former hypothesis. 



The Ekersund ilmenite deposits form dyke-like masses in a 

 rock mainly composed of labradorite. The other rocks in the same 

 eruptive district are hypersthene- and biotite-norites, and enstatite 

 granite. The relative ages of the different plutonic masses have not 

 yet been determined, but the ore-deposits appear to be distinctly 

 later than the surrounding labradorite-rock, for they occasionally 

 contain included fragments. Microscopic examination of the mutual 

 relations of the individual constituents of the ore-deposit does not 

 support the view that it has been formed by the mechanical accumu- 

 lation of solid crystals or grains of ilmenite, and the author concludes 

 that the concentration was produced in the magma without the 

 actual separation of minerals. In other words, he supposes that a 

 magma having the composition of the ore was produced by diffusion 

 processes such as those referred to at the commencement of this 

 article. As illustrating the nature of these diffusion processes, he 

 refers to a remarkable dyke occurring near Huk in the Christiania 



