Geology and Mineralogy. 81 



lias; been accumulated — about 15,000 hand specimens, 6,000 thin 

 sections, and 300 chemical analyses. The results of this work 

 were to have been embodied in volume IV, but the discovery of a 

 remarkable series of alkali rocks in the Fen district by Professor 

 Brogger's colleague, Professor V. M. Goldschmidt, in 1918 made 

 it imperative to study this district at once, because it differs so 

 strongly from the Christiania district. It is the results of this 

 study, this interim investigation as it were, that are presented in 

 this superb monograph. It need hardly be said that it represents 

 a very notable achievement, but coming as it does at the time of 

 Professor Brogger's seventieth birthday, the rernarkableness of 

 the achievement is enhanced many fold. 



The outstanding feature of the Fen area is the occurrence of 

 large intrusive masses of limestone and dolomite in association 

 with ijolitic rocks that range from highly leucocratic to highly 

 melanocratic. 



The Fen area, lying 115 km. southwest of Christiania, is 4.2 

 square kilometers in extent, and is surrounded on all sides by 

 Pre-Cambrian granite. A little less than half of the area con- 

 sists of intrusive limestone, termed sovite. The predominant 

 rocks consist of a series of plutonic pyroxene-nephelite rocks. 

 In composition they range from hypermelanocratic members — 

 jacupirangite — through members in which pyroxene predom- 

 inates (melanocratic rocks here named melteigite), through 

 members in which nephelite predominates (the leucocratic rock 

 ijolite), finally to the hyperleucocratic end member of the series 

 — urtite. All these rocks carry more or less primary calcite. In 

 connection with the systematic description of this series, the 

 occurrence of all ijolitic rocks the world over is reviewed and 

 critically discussed, and melteigite is shown to occur in various 

 alkalic provinces. 



At Fen melteigite predominates. The melteigite magma 

 exerted a truly remarkable and profound contact metasomatism 

 on the Pre-Cambrian granite, the result of this action being to 

 transform the bordering zone into a wide band of aegirite sj^enite, 

 or fenite. Fenite has also been formed by the assimilation of the 

 granite by the melteigite magma. Chemically and texturally 

 the pyrogenetic fenite often cannot be distinguished from the 

 fenite of metasomatic origin. 



Countless dikes of calcite (sovite) and a lesser number of 

 dolomite (rauhaugite) cut the melteigite, ijolite, and fenite. 

 Their occurrence in the fenite of metasomatic origin rules out 

 the possibility that the dikes may in reality be roof -pendants. 

 Professor Brogger is emphatic that the carbonate rocks are truly 

 igneous and leaves us in no doubt as to what he means by a dike, 

 namely that a dike consists of material that was injected as a 

 homogeneous fluid mass during one pulse, filling the fissure and 



Am. Jour. Sci.— Fifth Series, Vol IV, Xo. 19.— July. 1922 

 6 



