28 



SiO a .. 



Tio; 



Al,O a 



Fe" 2 3 



MgO 



CaO.. 



Na.,0 



K.,0.. 



PROF. W 



. c. 



BRoGGER ON THE 





Table VI. 





Average 



of Analyses of 



Camptonite 







Olivine- 



$ 







Gabbro- 



Bostonite. 







Diabase. 



4645 







46-48 



4-37 







2-56 



14-18 







14-fiO 



14-27 







14-44 



6-15 







654 



1024 







11-23 



3-04 







2-87 



1-3Q 







1-38 



[Feb. 1894, 



Difference. 



-003 

 + 1-81 

 -U-32 

 -0-17 

 -0-39 

 -0-91) 

 +017 

 -0-U8 



100-00 



100-CO 



I am well aware that every calculation of this hind must agree very 

 accurately with actual fact if any importance is to be attached to it ; 

 the calculations of rock-compositions as the results of mixtures of a 

 basaltic and a trachytic magma, according to the law enunciated 

 by Bunsen some forty years ago, are in this connexion warning 

 examples. In our case the accordance is very close in all the com- 

 ponents, except the titanic oxide and the lime. With respect to 

 the former, I may remark that its percentage varies between wide 

 limits as well in the camptonites as in the olivine- gabbro-diabases. 

 In the camptonites from Msena and Egge the percentage of titanic 

 oxide is greater than usual, but in the average of the olivine-gabbro- 

 diabases the small proportion of titanic oxide in the rock from Dignses 

 (see p. 19) lowers the average percentage somewhat considerably. 

 As to the lime, it must be observed that in the olivine-gabbro-diabases 

 the indicated percentage represents the whole of the lime originally 

 saturated with silica; while, in the average of the camptonites and the 

 bostonites, the lime percentage is calculated in rocks from which, by 

 abundant carbonatizing, a portion of the lime originally present 

 has probably been carried away. 



These circumstances being taken into account, I am of opinion 

 that the accordance is sufficient ; such an agreement cannot be 

 accidental. I think, therefore, I have sufficiently proved that the 

 camptonites and bostonites in Gran have been produced by differen- 

 tiation of an original common magma, whose chemical composition 

 agreed with the average composition of the olivine-gabbro-diabases on 

 the volcanic fissure-lines between Brandberget and Dignaes. 



This differentiation can further be proved to have taken place in 

 a liquid magma, even before crystallization of any importance had 

 begun. This appears evident from the striking difference in the 

 mineral composition of the olivine-gabbro-diabases on the one 

 hand, and the camptonites and bostonites on the other. In the 

 former, among the minerals — that is, among the first crystallized 

 constituents — pyroxene, olivine, and dark biotite are prevalent, 

 brown hornblende wanting as a rule, or quite subordinate. In the 

 camptonites, on the contrary, the predominant dark mineral is 

 always brown basaltic hornblende, often amounting to more than 



