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COSMOS 



Ptone, where strata of dolomite are found to be interspersed 

 in limestone ? Where, in this case, are we to seek the con- 

 cealed channels by which the plutonic action is conveyed ? 

 Even here, it may not, however, be necessary, in conformity 

 with the old Roman adage, to believe "that much that is 

 alike in nature may have been formed in wholly different 

 ways." When we find, over widely extended parts of the 

 earth, that two phenomena are always associated together, as, 

 for instance, the occurrence of melaphyre and the transforma- 

 tion of compact limestone into a crystalline mass differing in 

 its chemical character, we are, to a certain degree, justified 

 in believing, where the second phenomenon is manifested 

 unattended by the appearance of the first, that this apparent 

 contradiction is owing to the absence, in certain cases, of 

 some of the conditions attendant upon the exciting causes. 

 Who would call in question the volcanic nature and igneous 

 fluidity of basalt, merely because there are some rare instances 

 in which basaltic veins, traversing beds of coal or strata of 

 sandstone and chalk, have not materially deprived the coal 

 of its carbon, nor broken and slacked the sandstone, nor 

 converted the chalk into granular marble? Wherever we 

 have obtained even a faint light to guide us in the obscure 

 domain of mineral formation, we ought not ungratefully to 

 disregard it, because there may be much that is still unex- 

 plained in the history of the relations of the transitions, or in 

 the isolated interposition of beds of unaltered strata. 



After having spoken of the alteration of compact carbonate 

 of lime into granular limestone and dolomite, it still remains 

 for us to mention a third mode of transformation of the same 

 mineral, which is ascribed to the emission, in the ancient 

 periods of the world, of the vapours of sulphuric acid. This 

 transformation of limestone into gypsum, is analogous to the 

 penetration of rock-salt and sulphur, the latter being depo- 

 sited from sulphuretted aqueous vapour. In the lofty Cordil- 

 leras of Quindiu, far from all volcanoes, I have observed 

 deposits of sulphur in fissures in gneiss, whilst in Sicily (at 

 Cattolica, near Girgenti) sulphur, gypsum, and rock-salt 

 belong to the most recent secondary strata, the chalk forma- 

 tions.* I have also seen, on the edjre of the crater of Vesuvius, 



* Hoffman, Oeogn. Reise, edited by vou Dechen. s. 113 119, und 

 880-386; Poggend. Annalcn der P?tysik bd xxvi. s. 41 



