26 PROF. W. C. BROGGER ON THE [Feb. 1894, 



V. Their Origin by Differentiation". 



The above-mentioned relations prove, in my opinion, that the 

 common magma, from which the basic bosses along the lines Braud- 

 berget-Solvsberget-Viksfjeldene-Digna3s are crystallized as olivine- 

 gabbro-diabascs and pyroxenites, etc., must have been the same magma 

 as that from which the innumerable dykes and sheets of camptonite 

 and bostonite are derived ; and further, that all these dykes and 

 sheets have arisen as the result of a differentiation in the original 

 magma, in such a manner that first a basic portion, corresponding 

 with the camptonitic magma, has been separated out by diffusion, 

 and, subsequently, the remaining more acid magma has furnished 

 the material for the bostonite-dykes and sheets. 



If these views be correct, the chemical analyses of the different 

 kinds of rock should give undoubted proofs of the process of 

 differentiation in the original magma; and, in my opinion, they 

 do so. 



The calculated average composition derived from the three 

 analyses of the olivine-gabbro-diabases of Brandberget. Solvsberget, 

 and Dignaes probably indicates very closely the average composition 

 of the original basic magma, which was pressed up during the 

 oldest magma-eruptions in the Christiania region, along the western 

 boundary of the sunken tract. By differentiation in a magma of 

 this composition the separate magmas of the camptonite-, and the 

 subsequent bostonite-eruptions, must have been formed. 



I have now caused a series of chemical analyses to be made of 

 camptonites and bostonites from one and the same locality, from 

 both sides of Brandberget : of camptonite from Lindberget on Lake 

 Msena, at the western base of Brandberget, and from Egge on the 

 south-eastern side; also of bostonite from the same 'cutting' as the 

 analysed camptonite. These analyses (by L. Schmelck) have yielded 

 the following result : — 



Table III. 

 IV. V. VI. 



Camptonite, Mcena. Camptonite, Egge. Bottonitc, Mcsna. 



Si0 2 40-60 42-05 56'50 



Ti0 2 4--20 560 0-85 



A1 2 3 1255 12-30 18-14 



Fe„0 3 5-47 3-81 312 



Feb 9-52 952 2"8ti 



MgO 896 4-83 1-22 



CaO 1080 11-55 3 38 



!STa 2 2-54 2-18 5-28 



K.,0 119 111 160 



CO., 2-68 268 511 



H,0 2-28 2-88 1-26 



1G0 79 98-51 99 32 



All these rocks are from sheets ; they are, on the whole, rich in 

 carbonates. Calculating the substances free from water and car- 

 bonic acid at 100 per cent, (the iron as Pe 2 3 ), we get: — 



