400 Scientific Geology. 



Theoretical Considerations. 



Since gneiss is composed of the same simple minerals as granite, 

 it is natural to infer that both must have had a similar origin. And 

 especially are we led to such a conclusion, when we see in granitic 

 gneiss a gradual passage from the one rock into the other. That 

 granite has resulted from heat, instead of aqueous deposition, seems 

 to me to be so well established, that the opinion that imputes to it 

 such an origin, ought no longer to be regarded as hypothesis, 

 but as legitimate theory. Thus it is regarded by nearly all the 

 ablest geologist of Europe ; and in the proper place I hope to 

 show that our granite presents equally strong evidence of such an 

 origin as that on the eastern continent. At present I shall assume 

 that theory to be the correct one, which supposes granite to have re- 

 sulted from the melting down of other rocks ; the fused mass having 

 cooled so slowly as to present a confused crystalization. It is at least 

 a probable supposition, that the rock out of which it was produced, 

 was of mechanical origin, and consequently stratified. Now if the 

 central heat was not sufficient entirely to melt this stratified rock, yet 

 it would be powerfully affected a considerable distance upward from 

 the molten mass. The first in immediate contact with the melted por- 

 tion, would be partially fused, and hence give origin to granitic gneiss. 

 Another portion might be converted in porphyritic gneiss ; an- 

 other into lamellar ; another into schistose ; &c. All the lock we 

 may suppose so near the fluid granite, and so long in contact with it, 

 before cooling, that a crystalline would succeed to a mechanical ar- 

 rangement of all its ingredients, without losing the stratified dispo- 

 sition. 



The facility with which this hypothesis explains the graduation of 

 gneiss into granite, arid the crystalline and especially the porphyritic 

 structure of the gneiss, is the principal argument in its support. Such 

 effects we know might have resulted from heat : but they could not 

 have resulted from watery solution. 



As to the mechanical rock from which gneiss was produced, ac- 

 cording to this theory, we are left only to conjecture. Most probably 

 it was a coarse micaceous sandstone, which resulted from the disin- 

 tegration of granite, previously existing. When hornblende slate al- 

 ternates with the gneiss, we have only to suppose that formerly clay 

 slate was interstratified with the sandstone : and where there is an in- 

 terlamination of mica slate and gneiss, we have only to suppose that 



