166 REPORT — 1844. 



structure of hornblende, but together with the hornblende there always ap- 

 pears another mineral, which is generally brown garnet, and the crj-stals of 

 this mineral frequently appear on the surface of the metamorphous crystals 

 of pyroxene, but never protrude beyond it. Also in this case there exists a 

 space between these different crystals filled up with carbonate of lime, which 

 in all the Arendal minerals is the last-formed substance that tills up all the 

 space left by the other minerals. Although hornblende and garnet are the 

 most frequent minerals resulting from the change of the pyroxene, yet they 

 are not the only ones that appear. In fact hornblende seems in most cases 

 to be one of the new minerals ; but garnet is now and then wanting, and in- 

 stead of it magnetical iron ore, epidote, and perhaps even other minerals 

 occur. The specimen in which the pyroxene is changed into hornblende 

 and magnetical iron ore, is a very curious one, one half of it being covered 

 with unaltered pyroxene having a smooth shining surface, the other halt 

 of it is equally covered with crystals of the same size and appearance ; but 

 they are uneven and dull on the surface, and on closer examination it is 

 easily discovered that the internal structure of hornblende may be seen in 

 every one of the altered crystals, while at the same time a number of small 

 grains of magnetical iron ore have spread themselves through the whole 

 mass. The great variety in the minerals produced from the metamorphosis of 

 the black pyroxene depends evidently upon its very variable composition and 

 its numerous constituent parts, which, according to the laws of isomorphism, 

 may replace each other. 



It is evident that these altered crystals have not been completely melted, 

 since the whole external form, depending upon the former state of combina- 

 tion, is still left. On the other hand, it is likewise evident that there must 

 have been a kind of fluidity in the interior of these crystals, else the new- 

 formed minerals could not have assumed their peculiar form. Considering 

 the rounded edges and the clay-like appearance in the exterior of the altered 

 crystals, very little doubt can remain, that the agent which produced these 

 changes was heat, and that the whole phaenomenon belongs to that class of 

 chemical changes which philosophers call cementation, and by which, with- 

 out a change in the external form, changes take place in the interior which 

 depend upon another arrangement of the particles ; as for instance in the 

 alteration which glass undergoes by being changed into the porcelain of 

 Reaumur. 



I shall presently show, that the alum slate of Scandinavia, by a completely 

 similar alteration of the different stages which easily may be traced, has been 

 changed into gneiss, and that, if we except the carbonaceous matter, no sub- 

 stance has been carried away and none has been joined with the slate ; so 

 that the whole change merely consists in a different arrangement of the par- 

 ticles, which by way of cementation have formed new minerals that did not 

 exist before. 



2. Much more frequent are those metamorphoses where new substances 

 have entered into combination with those that were present in the beds of 

 sedimentary origin, and where at least other substances have sometimes been 

 expelled or have disappeared. The metamorphosis belonging to this kind, 

 whichis most clear and evident, is the alteration of common limestone, car- 

 bonate of lime, into anhydrate or anhydrous sulphate of lime, where the car- 

 bonic acid has been expelled by sulphuric acid, which in most instances pro- 

 ceeded from the interior of the 'earth. The greater part of the ancient Scan- 

 dinavian gneiss has evidently been formed by such an action where the granite 

 as an eruptive mass has carried vapours of potash with it, which have pene- 

 trated the surrounding and heated slates. At the first instance it may appear 



