METAMORPHOSED FUCOID SCHISTS IN SCANDINAVIA. 167 



inconceivable that granite being overloaded with an acid (silica, which ap- 

 pears in form of quartz) could give out vapours of potash, which is a base ; 

 but this depends upon the peculiar nature of silica, which at high tempera- 

 tures require less base to be dissolved than at a lower heat. I am disposed 

 to think that granite when melted is one single compound, which on cooling 

 is alone separated into the different minerals which compose it. In the 

 melted state it may give out much of its volatile bases, potash and soda, until 

 a compound remains, which for that temperature will not allow any more pot- 

 ash to be volatilized. 



If it thus is the case, that granite at a high heat may give out vapours of 

 potash and soda, these vapours will penetrate the surrounding slates, and will 

 form alkaline silicates, which at a sufficient heat will crystallize and combine 

 according to the degree of temperature either to form granite or gneiss. 

 Further off from the source of the alkaline vapours, where less potash and 

 soda penetrate, very little felspar will be formed, the whole potash being 

 converted into mica, which frequently is white, the iron entering into com- 

 bination with alumina and silica to form garnet, which in mica slate is the 

 representative of the felspar of the gneiss. Still further off from the granite, 

 not even mica slate will be formed, a sufficient quantity being wanting ; and 

 the last stage of these metamorphoses will be a micaceous, hardened clay slate. 

 Although granite generally carries vapours of potash with it, yet this is not 

 always the case ; and there exist not a few instances of protrusions of granite 

 where the clay slate has not been converted into gneiss, but changed into 

 other rocks with no portion or a trifling quantity only of alkaline substances. 



The whole mass of intrusive rocks of the trap family which are overloaded 

 with iron does not seem ever to have carried alkaline vapours with it, but 

 its peculiar produce is not unfrequently boracic aeid. Chemical affinities 

 will not allow vapours of potash or soda to escape from a compound con- 

 taining great quantities of the silicates of the oxides of iron, because potash 

 and soda would combine with silica and alumina from felspar, and separate a 

 combination of the oxides of iron in the form of magnetical iron ore. In fact, 

 this seems to be the history of some of the most interesting layers of magne- 

 tical iron ore. It appears therefore to me of importance to distinguish in 

 geological descriptions between euritic intruding rocks, which principally com- 

 prehend granite and euritic porphyry, from trappcean intruding rocks, com- 

 prehending the large femily of greenstones, basalts, &c. ; their chemical effect 

 upon the surrounding rocks being often very different. 



Having given these general views of the chemical part of metamorphosis, 

 I will go back again to the changes which portions of the Scandinavian alum 

 slate has undergone, where it comes into contact with certain intrusive rocks. 

 I had the great pleasure to make these observations in company with Mr. 

 Murchison, to whose genius and zeal we owe such very important geological 

 works, and I shall therefore not dwell much upon the geological phasnomena, 

 but principally comment upon the chemical nature of the altered rocks. 



Along the foot of Egeberg to the east of Christiania, the alum slate is not 

 separated from the older gneiss by a bed of sandstone which generally sepa- 

 rates the older gneiss from alum slate ( Viggersund in Norway, Westergothland 

 in Sweden, and Bornholm)*. 



The first state of change which this black shining alum slate undergoes 

 does not occur in the neighbourhood of Christiania, but is very frequent in 

 Hadeland and some parts of Ringerige ; it is black, very anthracitical, and 



* Near the church of Opsloe the alum slate has been quarried in former times for an alum 

 manufactory, and it is there unchanged ; its composition has been mentioned before. 



