34 PROF. W. C. BROGGER OK THE [Feb. 1894, 



in magnesia and lime, and richer in alumina and alkalies, than the 

 chief rock of Solvsberget (see p. 19). As is well known, the con- 

 ditions of crystallization for pyroxene and labradorite overlap in 

 certain magmas of gabbro and diabase-composition. We find, there- 

 fore, that in one case the plagioclase has crystallized essentially 

 before the pyroxene (ophitic structure), in other cases the relation 

 has been the reverse (as in several typical gabbros). A very slight 

 alteration of the physical conditions (temperature and pressure) 

 along the cooling margin on the eastern side of Solvsberget must 

 have been sufficient here to cause an increased diffusion of the 

 plagioclase-forming compounds in the magma to take place towards 

 the contact ; while under the altered conditions the labradorite 

 would have crystallized more easily than the pyroxene. The magma 

 thus separated by a slight differentiation has then, on its subse- 

 quent consolidation, been cooled in such a manner that it has 

 assumed a porphyritic structure, and consequently has left a labrador- 

 porphyrite. This — in comparison with the main type on Solvsberget 

 — more acid rock is undoubtedly a later product of the magma, for 

 we find, at Bjerget for instance, that the labrador-porphyrite is full 

 of angular fragments of the common olivine-gabbro-diabase of 

 Solvsberget, developed as a characteristic eruptive breccia. 



In the close vicinity of Solvsberget there also occur dykes of 

 augite-porphyrite, with the same mineral composition and structure 

 as the lava-flows so abundant in the neighbourhood of Holmestrand, 

 etc., which are the oldest effusive rocks of the Christiania region, 

 and undoubtedly an outflow of the same magma as the basic rocks 

 of Gran. These augite-porphyrites show only very slight, if any, 

 difference in their chemical composition from magma 0. The above- 

 mentioned dykes are not then chemically differentiated in relation 

 to the main rock-type of Solvsberget : they merely represent good 

 examples of the influence of differences in pressure and temperature, 

 etc., on the products of crystallization. For the same magma, 

 which by cooling slowly in the boss as an abyssal mass is con- 

 solidated to eugranitic olivine-gabbro-diabase, cooling more rapidly 

 in dyke-fissures and on the surface as lava-beds, has left augite- 

 porphyrites, quite different in structure and mineral composition. 

 Examples of this kind are now well known from a multitude of 

 localities ; one need only call to mind the Monte Amiata in Tuscany, 

 so well studied by the late F. R. Williams, whose premature decease 

 we all mourn, and the magnificent monograph on the rocks of Electric 

 Peak and Sepulchre Mountain published by J. P. Iddings. In the 

 Christiania region examples of such relations are numerous. 



The differentiations in the laccolite of the Yiksfjeld show, in 

 the main, similar relations to those just described ; more acid 

 quartziferous augite-diorites are here frequent as the latest products 

 of differentiation. Time unfortunately does not allow of a detailed 

 description. 



