i893. 



THE SEQUENCE OF PLUTONIC ROCKS. 295 



any universal concentric zones. The distinguishing character of the 

 local basin from which these rocks were derived was its extraordinary 

 richness in soda. The author attributes the petrological sequence to 

 the differentiation of an originally homogeneous magma, and refers 

 especially to Soret's principle and to the eflfects of successive 

 crystallisation as possible causes of this differentiation. 



The facts above described may be roughly grouped together by 

 the following hypothesis. A magma-basin existed beneath the 

 Christiania district. During the Cambrian, Ordovician, Silurian, 

 and part at least of the Devonian periods, a sea covered the area, 

 and sedimentary deposits attaining a thickness of about 4,000 ft. 

 were laid down. Then the first faulting commenced, and the district 

 began to subside into the underlying magma-basin. Basic rocks 

 were intruded and extruded. The faulting and subsidence were 

 continued at intervals, and magmas of increasing acidity were 

 successively squeezed out until the great granitic laccoHtes were 

 formed. Then, finally, numerous cracks were formed, and basic 

 material injected into them. 



The phenomena seen along the south-west margin of the 

 laurvikite district, that is, in the immediate neighbourhood of the 

 great boundary faults of the Langesundfjord, have such an impor- 

 tant bearing on the origin of certain structures in gneisses and schists 

 that some reference to them seems desirable, although they do not 

 bear on the subjects we are especially considering. 



Along the north-eastern margin of the district in question, where 

 the consolidation of the laurvikite-magma was not accompanied by 

 movement, a massive rock, closely resembling the rhomben-porphyry 

 of the sheets, was produced. Along the south-western margin the 

 rhomben-porphyry type of rock is entirely absent. The contact 

 phenomena are of a much more complicated character. The coarse- 

 grained massive laurvikite is here intimately associated with a finer 

 grained rock, which is usually striped or banded, and sometimes even 

 schistose. For this rock, which is somewhat richer in nepheline and 

 sodalite, and possesses a more uniformly granular structure than the 

 laurvikite, the author proposes the general term ditroite. 



In many places, and especially in the islands at the entrance 

 of the Langesundfjord, the fine-grained banded ditroite and the 

 coarse-grained laurvikite are blended in such a way as to suggest 

 that they are merely " schliere " of slightly different composition. 

 This, however, is not the case, for closer examination proves that 

 the laurvikite was solid when the ditroite was intruded. The folia- 

 tion of the ditroite is a kind of fluidal structure. Differential move- 

 ment arranged the darker constituents in stripes parallel to the 

 bounding surfaces, and fractured the larger constituents, especially 

 the felspar and nephehne. Moreover, the angles of these con- 

 stituents were often rounded off by the same movement, so that after 

 final consolidation a rock with a typical primary augen-structure was 



