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



In the parishes of Gran, Hole, Modum, etc., in the Christiania 

 region, these rock-varieties are exceedingly abundant ; they appear 

 partly as vertical dykes, generally with a north-and-south or north- 

 north-east and south-south-west strike, parallel to the main strike of 

 the fissures, on which the bosses of olivine-gabbro-diabase and 

 other basic abyssal rocks are situated, along the lines Brandberget- 

 Solvsberget and Buhammeren-Dignses. In one part they are also 

 arranged as intrusive sheets introduced between the Silurian strata, 

 chiefly close to the surface of the Archaean schists, especially in 

 etages 1 and 2, also in 3 and 4, and less often in the higher etages. 

 Although the surface in Gran is somewhat obscured, several 

 hundred occurrences, of dykes as well as sheets of camptonite and 

 bostonite, have been observed. As an example of the frequency of 

 the dykes, I may mention that in one area alone (along the 

 Melbostad and Helgum road) I counted more than 50 dykes in 

 lg kilometre (1 mile); the total thickness of these dykes I measured 

 to be 70 metres (227 feet), that is 1 metre of dyke-mass in every 20. 

 The thickness of the dykes is commonly | to 2 metres (1| to 6g feet) ; 

 it seldom attains 5 or 10 metres (16 or 32 feet). 



The thickness of the sheets is usually also 1 to 2 metres, less 

 often 10 metres or more ; however, a great number of intrusive 

 sheets often follow upon each other, which is the reason why the 

 total thickness in several localities may amount to 20 and even 30 

 metres (65| to about 100 feet). 



In the vicinitj 7 of Brandberget the magma has in preference been 

 intruded as sheets between the planes of the stratified rocks ; in the 

 neighbourhood of Solvsberget, on the contrary, vertical dykes are 

 more prevalent ; perhaps this circumstance can be explained by the 

 fact that at Solvsberget we find only higher etages (such as etage 4) 

 represented. 



The camptonites and the bostonites are very intimately con- 

 nected with each other, and also with the above-mentioned boss- 

 rocks. These connexions are proved by the following facts : — 



Firstly, as regards the relation between the bostonites and the 

 camptonites, we may observe that they everywhere appear together 

 in close companionship. Thus, we find innumerable examples of 

 vertical camptonite-dykes associated with immediately adjacent 

 parallel dykes of bostonite ; often the same dyke-fissure contains 

 both bostonite and camptonite, and with equal frequency it happens 

 that both rocks appear together as intrusive sheets. 



In many bostonite-sheets and dykes we find phenocrysts of brown 

 hornblende, which is the chief mineral among the phenocrysts of the 

 camptonites. I have moreover observed several examples of dykes 

 or sheets in which the centre is bostonite, the sides camptonite, or 

 vice versa. Finally, I have also, but less frequently, observed inter- 

 mediate kinds between camptonite and bostonite. 



Secondly, as to the relation between the eugranitic boss-rocks, on 

 the one hand, and the dyke- and sheet-rocks on the other, we may 

 remark that in Gran and the surrounding parishes the bostonites 

 and camptonites are represented in many hundred dykes and sheets 



