96 M. Keilhaii's Theory of Granite^ and other Rocks, 



produced by the penetration of magnesia exhalations into masses 

 of carbonate of lime. There is nothing in our previous remarks 

 which can be directed against the admissibility of this theory ; 

 but it certainly anticipates the data afforded us by chemistry. 

 It is not satisfactory to me, because it involves other extremely 

 arbitrary assumptions. It cannot be denied that in some places 

 dolomite occurs in very thin but extensive strata, and quite in 

 a regular manner between other mountain-rocks ; now how is it 

 to be explained why the sublimed matter left all the other strata 

 untouched, and only exerted its influence on the carbonate of 

 lime in thus converting it into dolomite ? In the irregular' 

 masses of dolomite, which are completely surrounded by Nep- 

 tunian strata, there is a similar but less considerable difficulty 

 to be solved. But, not to state other objections, I shall merely 

 mention what seems to me the greatest, that the whole emana- 

 tion-hypothesis reposes on Vulcanism; for if the black porphyry 

 (melaphyre) or other massive mountain-rock, with whose intru- 

 sion dolomization has been put in connection, be not pyro-gene-. 

 tic, then the occasion of such an extraordinary mode of forma- 

 tion gives way, and we are just as little able to assign another 

 cause in its stead ; and indeed it ought only to be in the case 

 of extreme necessity that we should have recourse to the theory 

 of emanations. 



In the mean time it may be remarked, that whoever, by 

 adopting the dolomite theory, admits all these suppositions, 

 must necessarily find no insuperable obstacle to admitting that 

 the materials for the formation of felspar, quartz, mica, horn- 

 blende, &c. may, in like manner, be conveyed by exhalations, in 

 so far as was required at the places in the already existing 

 masses where the development of these minerals should be as- 

 sumed to have gone on. If we assumed that these conveyers 

 of matter rose up from beneath, then we could say of the great 

 porphyry masses in our transition district that they were their 

 result, that the emanations had established themselves in the 

 layers of the sandstone, that, on the contrary, the substances 

 necessary for the formation of eurite-pcfrphyry had not been 

 elevated to that height, but had united themselves to certain 

 strata in the vicinity of the primary rocks, and so forth. 



But, in considering the subject more closely, we may leave 



