270 M. Keilhau's Theory of Granite and other Rocks. 



dally in the primary outer crust ; but above all, iron-pyrites, 

 which is distributed through the whole lower part of the tran- 

 sition series ; it is on this account that alum-slate occurs here, 

 instead of the usual clay- slate, inasmuch as the alum-slate is 

 nothing else than a mere modification of the ordinary strata 

 produced by contact with the primary rocks, and which ha5> 

 been indirectly or directly impregnated with iron-pyrites (and 

 potash ?) by means of this contact.* 



The actions which produced alterations in the strata in con- 

 tact with the boundary of the granite masses, and the forma- 

 tion of the new mineral products occurring there, are of them- 

 selves of subordinate importance,t compared with the results 

 of the alterations and formations which we-have last mentioned ; 

 but nevertheless, the contact phenomena of the granite boun- 

 daries are the most self-evident. The conversion of fine la- 

 mellar soft clay-slate into a coarse lamellar siliceous slate, and 

 that wonderful passage of limestone into a more or less per- 

 fectly white crystalline marble, are remarkable phenomena ; 

 but the extension of such actions to the distance of more than 

 an English mile (l-6th of a Norwegian mile), from the granite 

 boundary, is in the highest degree striking ; so also is the occur- 

 rence at the same boundary of numerous masses of ore, of gar- 

 net, and other remarkable minerals. The silicification, and the 

 tendency to crystallize, which are in general observable at these 

 boundaries, shew that the actions here were so far perfectly 

 analogous to that by which granite itself was formed ; but the 

 substances actually formed at the contact, viz. ores, seem to re- 

 quire the application of the idea that the new materials have been 

 brought from a distance to certain points, where we find them 

 collected, — a process that may still be going on, as the forma- 

 tion of these mineral masses seems so certainly to be dependent 

 on the contact of rocks of a different nature, and whose contact 

 jnoreover is permanent. 



It is a remarkable fact, and one which has not yet been nar- 

 rated, that the usual changes of transition-strata at their boun- 

 dary with granite, have not taken place where the primary 



* Forchammer found potash in the alum-slate of tlie north. 

 '\ Bee the observations on alum-slarte in the next page. 



