536 THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



By an exhaustive study of the chemical characters of these 

 rocks Broggar has shown that certain dykes are complementary 

 to other dykes. 



This very interesting series intrudes metamorphosed rocks of 

 Silurian age. Although the intruded sedimentary rocks have 

 been greatly altered, the line of junction between them and the 

 intrusives is well denned. 



The whole mass is surrounded by and occupies a subsidence 

 area in an elevated Archaean massive. The elevation of the 

 massive and the intrusion of the alkaline rocks took place later 

 than the Silurian. 



2. Alno Area, Sweden* — The nepheline syenites occupy the 

 north-east and north parts of the island of Alno. The most 

 remarkable feature of this mass is the association with thefoyaite 

 of a mass of limestone containing nepheline-syenite minerals. 

 The limestone shows no sign of metamorphism, but is clearly a 

 differentiation-product of the foyaitic magma. It contains nephe- 

 line in pegmatitic intergrowth with calcite, segirine and felspar. 

 All that author's observations tend to show that the limestone 

 has been fused up or dissolved in a foyaitic magma too poor in 

 silicic acid to decompose it, and on cooling it has crystallised out 

 of the magma the same as other minerals. 



There are numerous differentiation-products ranging from 

 normal syenite and nepheline, pyroxene-syenite to a basic hyperite 

 and jacupiraugite. The d} T kes given off by the mass and cutting 

 it comprise melilite basalt (alnoite), tinguaite, and a remarkable 

 group of dykes consisting principally of calcite and zeolites, the 

 dyke-form of the massive limestone into which the foyaite passes. 



The whole mass intrudes Archsean gneiss, and as it exhibits 

 no evidence of dynamic action it can be said to be Post-Archsean, 

 There is no real passage of the nepheline-syenite into the sur- 

 rounding gneiss, yet the line of junction is hard to fix. Against 

 the gneiss the syenite becomes devoid of nepheline and contains 



* A. G. Hogbom, " The Nepheline Syenite of Alno," Geol. Foren. Stock- 

 holm, Forhandl. xvii. 2 and 3, 1895; also Min. Mag. xi. p. 250. 



