54 ROCK-METAMORPHISM. 



It will not be denied, except perhaps by a few, that earthy 

 sedimentary rocks have been mineralized over extensive regions 

 into ordinary metamorphics ; there is therefore no reason why 

 chemical changes or niethylosis may not have been effected on 

 a regional scale. Bischof, according to the extract previously 

 quoted from his ' Chemical and Physical Geology/ was evidently 

 not averse to this view ; nor would Heddle, we strongly suspect, 

 be opposed to it. 



Speaking of the euphotide at Portsoy, and the occurrence 

 there of a " very siliceous limestone in immediate contact with 

 it," Heddle proceeds " Now the frequent association of thin 

 beds of limestone with serpentine supplies very direct evidence 

 of the conversion of hornblendic and augite rocks into serpen- 

 tine. In that fact we have a ready answer to the question , 

 ' What becomes of the carbonate of lime necessarily formed 

 during such an alterative process as the above?' I will not 

 say that limestone is always to be found in such association ; we 

 do not always find limestones even where we have indubitable 

 evidence that they once existed ; for here the very thing that 

 makes can unmake or sweep away. The carbonate of lime 

 thus fashioned out of the rock forms a belt beneath the residual 

 serpentine, thicker or thinner in accordance with the original 

 thickness of the stratum of transformed rock ; also thicker or 

 thinner according to whether that rock was augitic or horn- 

 blendic ; for the former can supply considerably more lime than 

 the latter. This calcareous belt must lie beneath the parent 

 rock, sealed against any great amount of further change, unless 

 or until upheaval or denudation expose it to meteoric influences. 

 Then water, flowing either downwards or upwards, may nay, 

 in time must sweep it away in solution, leaving lime- sink or 

 collapsed void to evidence its former existence. But if the 

 limestones, so frequently associated with serpentines, are thus 

 to be assigned to the decomposition of the rock which yielded 

 these serpentines, we have a crucial test of the soundness of the 

 theory of the change, in the inquiry as to whether unchanged 

 gabbro, or other such rocks, occur in contact with lime. That 

 it never does, I will not say : but, in glancing at a sketch geo- 

 logical map which I have constructed of the district where these 

 rocks occur, I find, as regards the great belt of diorite and 

 diallagic rock which sweeps up Central Scotland, that where 



