GEOLOGY 549 



perofskite, and titaniferous magnetite, as minor products of the 

 reaction. An associated melilite-biotite rock is believed to be 

 the result of the further replacement of monticellite by these 

 minerals. The residual liquid was alkalic ; and Bowen has 

 shown experimentally that nepheline reacts with diopside to 

 form melilite, a reaction analogous to that taking place in the 

 natural rock. The formation of melilite and other lime-rich 

 minerals may therefore be due to normal equilibrium processes 

 in the magma, and not to the assimilation of limestones. 



The veteran petrologist Prof. W. C. Brogger has now added 

 a fourth volume to his classic work on the alkaline igneous 

 rocks of the Kristiania region of Norway {Die Eruptivgestelne 

 des Kristianiagebietes, IV, Das Fengebiet in Telemark, Nor- 

 wegen. Vidensk. Skr., I, Math. -Nat. Klasse, No. 9, Kristiania, 

 1920, pp. 408). The area dealt with occupies only 4 square 

 kilometres, but is nevertheless replete with the most vivid 

 petrographical interest. No less than 13 new rock types are 

 described, the greatest novelty being the demonstration of the 

 existence of a series of magmatic carbonate rocks, which are 

 associated with an ijolite series containing a variable amount 

 of primary calcite. There are also nepheline-syenites and alkali- 

 syenites, with numerous hybrid types. The carbonate rocks 

 are believed to have been derived from a mass of sedimentary 

 limestone melted in depth ; and the central mass of carbonatite 

 is regarded as having floated upon the heavy silicate magma of 

 the region. The age of this series, first regarded as an outlier of 

 the Devonian Kristiania petrographic province, is now believed 

 to be late Pre-Cambrian, a view which brings its petrographical 

 characters and age relations into line with those of numerous 

 other scattered provinces throughout Scandinavia {e.g. Alno). 

 The enormous wealth of observation in this great memoir makes 

 it a veritable mine of interest for petrographers, and quite 

 precludes an adequate review in this place. 



By a curious coincidence Prof. S. J. Shand has just described 

 an extremely similar occurrence to that of the Fen district in 

 Sekukuniland (Transvaal), within the Bushveldt granite {Trans. 

 Geol. Soc. South Africa, 24, 1921, 11 1-49.) This forms a roughly 

 oval area of 7 sq. miles consisting of differentiated nepheline 

 syenites and ijolites surrounding a block of limestone, the 

 ijolite being found in a belt adjacent to the limestone. The 

 latter is regarded as a block of the Dolomite group of the Trans- 

 vaal Series brought up by the rising magma. The formation 

 of the ijolite between nepheline-syenite and limestone is shown 

 to be due to the assimilation of limestone by the original 

 nepheline-syenite magma, roughly one-third of its own weight 

 of limestone being absorbed. 



C. H. Clapp's study of the igneous rocks of Essex Co., 

 36 



