THE GEOLOGIST. 



69- 



beautifully crystallised, and possess both their green colour and their 

 usual transparency. 



From these investigations, M. Daubree feels justified in concluding 

 that a great number of silicates — perhaps all — found in the primitive 

 crystalline rocks, have been formed by the influence of water at a high 

 temperature ; which temperature is, however, very inferior to that of 

 the point of fusion of these silicates. He seems to think that the 

 granite rocks themselves very probably owe their formation and 

 crystalline appearance to similar causes. We must content ourselves 

 here by indicating the result of experiment, and be silent as regards 

 discussion. If the experiments just related, and which are being 

 continued by M. Daubree, are repeated with success, his conclusions 

 will appear certainly very natural. The most striking feature in his 

 paper is, without doubt, the formation, in the above circumstances, of 

 feldspar — a rock so universally spread throughout nature, and which 

 plays so important a part as constituent of almost all the primary or 

 eruptive rocks. Although crystals of feldspar have been discovered in 

 scoriae, and, by Heine, in the refuse of a furnace for copper fusing,^ 

 it appears from Humboldt's statement f that they have never before 

 been purposely formed : — "Nor have chemists," says he, ''ever succeeded 

 in artifi^cially producing either feldspar or horneblend." It may be 

 well to add that feldspar is daily formed under our eyes in the lava of 

 volcanos. Since M. Daubree's observations have come to light, this 

 must doubtless be attributed to the abundant quantity of water incor- 

 porated in lava until it becomes completely solidified. 



M. Kuhlmann, of Lille, has published a long memoir on siliceous 

 infiltration or petrifaction, and on epigenesis. J In this work the 

 learned author, who has already done so much for the advancement of 

 chemical and geological science, has presented us with nothing very 

 new. He explains the curious phenomenon of petrifaction by the 

 action of carbonic acid, or of carbonate of ammonia, on the silicates of 

 potash and soda invariably found in spring water. A similar theory 

 has been already professed and published by others, and, I believe, 

 by M. Kuhlmann himself. 



The phenomenon of petrifaction is certainly one of the most curious 

 processes in. nature. Is it not wonderful to find the soft stem of 



** They were analysed by Kersten. f Cosmos, Vol. I. 



J Comptes Rendus, Nov. 9, 1857. 



