Vol. 50.] BASIC ERUPTIVE ROCKS OF GRAN. 2l 



As an example of partly laccolitic bosses we will take the case of 

 Solvsberget. 



Solvsberget is a roof-shaped hill, about 1*5 kilometre (1 mile) in 

 length from north to south, and about 1 kilometre (| mile) in width 

 from west to east ; the top is elevated about 250 metres (813 feet) 

 above the surrounding country ; the eastern side is very abrupt, the 

 western side more gradually sloping. In its northern and southern 

 parts Solvsberget is formed of Silurian (Ordovician) strata of etages 

 4 a a and 4 a J3 (shales with Ogygia dllatata, Briinn, and Ampyx- 

 liinestone) ; the main strike is west to east, or west-north-west to 

 east-south-east, the dip generally showing angles varying from 50 c to 

 80°. The central part of the hill is occupied by the eruptive rocks ; 

 on tbe map it will be seen that their surface, in horizontal projection, 

 bas a boomerang- like shape ; in the south-western part the strata 

 are vertically traversed by the plutonic rock ; in the middle of the 

 ' boomerang ' the eruptive rock appears to be conformably injected 

 between the shales ; but in the eastern part the eruptive surface 

 suddenly sinks to a lower level, and seems there partly to be of a 

 laccolitic character. The whole surface occupied by the plutonic 

 rocks is only 03 square kilometre (75 acres;. 



III. The Effects of Contact-metamorphism by the Olivine- 

 (jabbro-Diabase. 



In all occurrences of these plutonic rocks we find a typical abyssal 

 contact-metamorphism, the peripheric extent of which depends on 

 the size of the plutonic masses in the different localities. At 

 Solvsberget the alteration of the Silurian strata is perceptible at 

 distances of 200 or even 300 metres (220 or 330 yards) from the 

 boundary on the eastern side of the hill, where the strikes of the 

 stratified rocks and of the eruptive boss run in nearly the same 

 direction ; in the south-east the observed alterations terminate much 

 nearer to the eruptive rock. 



The unaltered rocks of Solvsberget are tbe common, black to dark 

 grey, argillaceous shales of etage 4 a a (Oi/i/r/ia-shales), with a few 

 interbedded lenticular masses of limestone. By the contact-meta- 

 morphism the shales, as usual, are altered into dark violet hornstoues, 

 shimmeriug, as we get closer to the boundary, more and more from 

 innumerable small scales of mica, the diameter of which close to the 

 boundary attains several millimetres. In the next zone of alteration 

 the hornstone is macroscopically crystalline, with a grain of medium 

 size, often possessing a very remarkable porphyritic structure, clue 

 to crystals of plagioclase more than 5 millimetres (4- inch) long. The 

 limestone-lenses are altered into calcareous hornstone (kalksilikat- 

 hornfeh). 



The contact-metamorphism along the boundary of the olivine- 

 gabbro-diabases of Gran is mainly of interest, because the micro- 

 scopical observations on the altered rocks make it very probable 

 that the previous supposition, as to the independence of the 

 alteration in regard to the composition of the plutonic rock itself, 



