METHYLOTIC ORIGIN OF DOLOMITES. 91 



CHAPTER XV. 



DOLOMITES AND DOLOMITIC ROCKS HAVE UNDERGONE 

 METHYLOSIS. 



DOLOMITES, in our opinion, are in all cases products of methy- 

 losis; but the phenomena have been effected in two different ways 

 one by substitution of their original basic and acidic compo- 

 nents, as in the dolomitic hemithrenes of the Canadian Archseans, 

 and the other by replacement, more or less, of only their original 

 basic carbonate of lime. Although the rocks we propose to con- 

 sider in the present Chapter are of sedimentary origin, it must 

 be understood that we do not deny the probability of certain 

 igneous and' silacid masses (veins and dykes in the Canadian 

 Archaeans), through methylosis, having been dolomitized. 



But the rocks we are more directly engaged with are the mag- 

 nesian limestones which, chiefly belonging to various systems of 

 the primary and secondary groups, are spread over extensive areas 

 of Europe and North America, and beneath or associated with 

 which nothiug is present except sedimentary deposits in an 

 unaltered condition. Our typical example is the Permian 

 magnesian limestone of the north of England. 



Various opinions have been propounded as to the origin of 

 the magnesian carbonate present in this limestone and others 

 analogous to it. There are some, as Apjohn, T. Richardson, 

 Hunt, and Ramsay, who believe that this compound is an 

 original constituent; while others, as Virlet, Scouler, Haid- 

 inger, Von Morlet, Bischof, Johnston, Sorby, and Hardman, 

 make it to be a superadded ingredient : the last opinion is 

 methylotic. But let the different opinions of both classes be 

 examined, and it will be found that all are at variance as to 

 the modus operandi which has developed the magnesian con- 

 stituent. 



Von Buch's celebrated theory of dolomitization (applied, 

 however, to rocks we have excluded) advocates that the lime- 

 stones forming the dolomite mountains of the Tyrol have 



