THE EVOLUTION OF IGNEOUS ROCKS. 159 



felspar, we should expect the different partial magmas to 

 have given birth to different varieties of felspar. There 

 are, however, in other districts, examples of plutonic rocks 

 apparently derived from a common source, but differing 

 widely in mineralogical as well as in chemical composition. 

 In such cases it must be supposed that differentiation was, 

 at least in the main, anterior to and independent of 

 crystallisation. 



This and other very interesting points are illustrated in 

 a recently published paper of Brogger 8 on the rocks of 

 Gran near Christiania. Intruded in Devonian times among 

 the older strata of the Christiania district, there occurs a 

 group of igneous rocks ranging from very basic to acid, but 

 having in common certain peculiarities, such as an unusually 

 high content of soda, which would alone afford strong pre- 

 sumptive evidence of consanguinity. The field-relations of 

 the rocks are equally convincing on this point, and further 

 bring out the fact that the succession in time of the several 

 types is a definite one, viz., an order of increasing acidity. 

 The whole group of rocks has been interpreted by Brogger 

 in former memoirs as the products of successive intrusions 

 from a large subterranean magma-reservoir, in which differ- 

 entiation had already taken place. Such magma must have 

 underlain a large tract of country, and may itself have 

 originated by differentiation on a regional scale from a 

 hypothetical magma of much wider extent. This paper 

 deals only with the earliest and most basic product of 

 differentiation of the general magma of the Christiania 

 district, and the author shows that this in turn underwent 

 differentiation within the limited area described in the 

 vicinity of Gran. The rocks in question occur along a 

 tract some thirty-three miles long, following a line of 

 fissure which runs roughly north and south. The dominant 

 type is described as an olivine-gabbro-diabase, but it is 

 pointed out that its chemical, and in part its minera- 

 logical, composition varies in a definite manner in the 

 different exposures, the silica-percentage increasing from 

 43*65 in the north to 49*25 in the south. This is taken to 

 indicate a differentiation of the basic magma on a rather 



