t «5 ] 

 XX XIX. Proceedings of Learned Societies, 



GEOLOGICAL SOCIETY. 



[Continued from vol. xxxvi. p. 594.] 



November 22nd, 1893.— W. H. Hudleston, Esq., M.A., F.R.S., 

 President, in the Chair. 



HHHE following communications were read : — 

 -*- 1. 'The Basic Eruptive Rocks of Gran.' By W. C. Brogger, 

 Ord. Prof, of Min. and Geol. at the University of Christiania, 

 For. Memb. Geol. Soc. 



In previous communications the author has maintained that the 

 different masses of eruptive rock which occur within the sunken 

 tract of country lying between Lake Mjosen and the Langesunds- 

 fjord are genetically connected, and have succeeded each other in a 

 regular order. The oldest rocks are the most basic, the youngest 

 (except the unimportant dykes of diabase) are the most acid, and 

 between the two extremes he has found a continuous series. 



He is now preparing a detailed monograph on this series of 

 eruptive rocks, and in the present communication he gives an 

 account of the results of his work on the oldest members. 



Several bosses of basic plutonic rock, now forming a series of 

 dome-shaped hills, lie along a north-and-south fissure-line. The 

 most northerly is that of Brandberget in the parish of Gran, about 

 50 or 60 kilometres N.N.W. of Christiania, and the most southerly 

 occurs at Dignaes on Lake Tyrifjord, about 35 kilom. W.N.W. of 

 the same town. The prevailing rock in these bosses is a medium 

 or coarse-grained olivine-gabbro- diabase ; but pyroxenites, horn- 

 blendites, camptonites, labrador-porphyrites, and augite-diorites also 

 occur. Analyses of the typical rocks from three localities on 

 the north-and-south line are given, and the conclusion is reached 

 that the average basicity of the rocks forming different bosses 

 decreases from north to south. 



The contact-metamorphism is referred to ; and the presence of 

 hypersthene in the altered Ogi/gia-shalea, coupled with its absence 

 from the same shales where they have been affected by quartz- 

 syenite, leads the author to the conclusion that the chemical nature 

 of the intrusive rock does, in certain cases, produce an influence on 

 the character of the metamorphism. 



Innumerable dykes and sheets of camptonite and bostonite are 

 associated with the above-mentioned plutonic bosses. These are 

 regarded by the author as having been produced by differentiation 

 from a magma having the composition of the average olivine-gabbro- 

 diabase. Analyses are given, and it is proved that a mixture of nine 

 parts of the average camptonite and two of the average bostonite 

 would produce a magma having the composition of the average 

 olivine-gabbro-diabase. The petrographical variations, such as the 

 occurrence of pyroxenites and augite-diorites, in the plutonic masses 

 themselves are described, and attributed to differentiation under 



