METAMORPHIC EOCKS. 253 



notice the conversion of limestone into dolomite, does not 

 attribute it to the introduction of a certain portion of talc 

 derived from the black porphyry, but considers it a modifi- 

 cation of the limestone, contemporaneous with the projection 

 of the erupted rock through wide fissures filled with va- 

 pours. But in certain localities, beds of dolomite are found 

 interposed between limestone strata, and it is yet to be ex- 

 plained how the transformation can have taken place without 

 the presence of an erupted rock, and where we are to look 

 for the concealed channels of the plutonic action. We 

 ought not to resort, however, even in this case, to the old 

 Eoman adage, "that much that is alike in nature has 

 been formed in different ways :" since over widely ex- 

 tended parts of the earth, we have seen two phenomena 

 associated, the protrusion of a certain igneous rock, 

 and the transformation of compact limestone into a 

 crystalline mass possessing new chemical properties, we 

 may well suppose that, in the few cases in which the latter 

 phenomenon alone is visible, future observation will remove 

 the difficulty, and will shew that the apparent anomalies 

 are due to the conditions under which the general causes of 

 metamorphism have acted in the particular cases. We can- 

 not doubt the volcanic nature and igneous fluidity of basalt, 

 because some rare instances occur ot basaltic dykes tra- 

 versing a bed of coal, without reducing it to charcoal, 

 of sandstone without producing the usual effect of heat, 

 or of chalk without converting it into granular marble. If 

 we have, as yet obtained only an imperfect light to guide 

 us in the obscure domain of mineral formations, it would 

 surely be unwise to abandon it, because there are some 

 points in the liistory of the transformation of rocks, and 



