92 ROCK-METAMORPHISM. 



been impregnated with magnesia sublimed from the adjacent 

 diorites. 



Reverting to the magnesian limestones to which we have 

 restricted ourselves, our view, as already made known in one of 

 our memoirs*, is methylotic ; but as it differs in some important 

 particulars from others that have been published, we proceed to 

 give a description of it. Our objection to the opposite hypo- 

 thesis involving simultaneous precipitation of carbonates of mag- 

 nesia and lime is based on the absence of any recent deposits 

 of the kind. 



The magnesian limestones of the north of England are plen- 

 tifully fossiliferous ; and from the general facies of their orga- 

 nisms, also their wide distribution, it seems far from improba- 

 ble that they have been formed in deepish water of a wide-spread 

 oceanf. There are beds, as the Durham marl-slate and the 

 Manchester marls (the one underlying and the other overlying 

 the magnesian limestones), which are largely characterized by 

 carbonate of magnesia. 



To explain the methylosis of these different rocks the follow- 

 ing hypothesis is suggested. It is based on the assumption, 

 which has gained many adherents of late years, that during the 

 Triassic period, particularly towards its close, the widely-spread 

 ocean which had thrown down the Permian deposits became 

 reduced to mere inland seas resembling the Caspian and Lake 

 Aral. 



Our hypothesis assumes that, under these altered circum- 

 stances, the water of the Triassic seas, besides much of it dis- 

 appearing through evaporation, sank into the subjacent rocks, 

 and that the salts of this water, under the force of chemical re- 

 actions generated by new conditions, became variously combined 

 with the materials of these rocks. The abundance of chlorides of 

 sodium, magnesium, and calcium, and o sulphates of magnesia 

 and lime in sea-water readily explains the origin of the rock-salt 

 and gypsum in the lower members (red shales of Cheshire and 

 Lancashire) of the Keuper J. We are more especially concerned, 

 however, with the other deposits, resulting from the magnesium 



* Quart. Journ. Geol. Soc. vol. xxii. pp. 211, 212. 



t See ' Monograph of the Permian Fossils of England,' Palseontographical 

 Society, Introduction, pp. xvii, xviii ; also Appendix in present work. 

 I The thick beds of rock-salt at Nantwich, Cheshire, appear to be of 



