20 PROF. W. C. BROGGER ON THE [Feb. 1 894, 



According to an approximate calculation from the above analyses, 

 the mineral composition per cent, may be estimated as follows : — 



Table II. 

 I. 



Bran dber get. 



Felspars about 12 



Pyroxene 09 



Olivine 5 



Biotite 3 



Iron Ores 7 



Apatite, serpentine, 1 a 



chlorite, etc. ... J 



II. 



ITI. 



Solvsberget, 



XHffnees, 



about 40 

 20 

 12i 



5" 



about 04 



10 



9 



8^ 



6, 1 



1 



2 



100 100 100 



The average composition of the three principal varieties of olivine- 

 gabbro-diabase, the iron being calculated as jFe 2 3 (all analyses 

 calculated free from water, and finally equalized by reducing them 

 to 100 per cent.), is as follows : — 



SiO, 40-48 



Ti0 2 250 



A1.,0" 3 1450 



Fe 2 3 1444 



M?0 0-54 



CaO 11-23 



Na.,0 287 



K,0 1-38 



100-00 

 The olivine-gabbro-diabase of Solvsberget, as will be thus seen, 

 differs only slightly in composition from the average rock. 



The rocks in the northern occurrences of these oldest eruptions in 

 the Christiania region are then, as we have seen, mainly pyroxenic 

 rocks of a basic character, while the southern localities along 

 the same fissure show chiefly felspathic rocks of somewhat greater 

 acidity. Probably the average composition of all the masses of rock 

 in Brandberget differs only slightly from the above-calculated average ; 

 on the other hand, it is certain that the average composition of the 

 rocks from Dignses is more acid than this average. We have, there- 

 fore, here a remarkable example of the differentiation of a molten 

 magma in a regular manner in horizontal direction along connected 

 fissure-lines. Nor is it the only case of its kind in the Christiania 

 region. 



With respect to the geological appearance of these abyssal rocks, 

 I will simply remark that they are only partially of a laccolitic 

 character; as a rule they, are enveloped as vertical bosses, the 

 contact-plane often cutting the^djacent Silurian strata. 



The small exposure south of Bilden is, on the whole, an inclined 

 laccolitic sheet, the rocks in the same chiefly showing a porphyritic 

 structure and the composition of the camptonite-group, and being 

 only to a slight extent crystallized as pyroxenites. On the 

 "Viksfjeldene also the laccolitic character is evident. 



