METHYLOTIC ORIGIN OF DOLOMITES. 93 



chloride and sulphate. As both are highly soluble compounds, 

 and have no particular affinity for the basic substances of ordi- 

 nary Triassic rocks (chiefly sandstones), they would penetrate 

 the latter without being precipitated or producing reactions. 

 But on coming into contact with the underlying calcareous marls 

 and limestones of the Permian system, mutual decomposition 

 would ensue. A portion of the lime of these rocks would be 

 taken up by the chlorine or sulphuric acid (originally in com- 

 bination with the magnesium of the Triassic sea-water) and 

 carried off in solution ; while the base that had been in union 

 with these acids would combine with the liberated carbonic acid 

 to form carbonate of magnesia *. 



The latter compound, by virtue of its affinities, would become 

 combined or incorporated with the remainder of the base of the 

 marl or limestone t ; and thus these rocks would become dolo- 

 mitic or converted into dolomite J. 



Another kind of methyl osis, however, supervened in some 

 localities. The Permian magnesian limestones in the north of 

 England are for the most part compact, earthy, or micro- 

 crystalline; but in the neighbourhood of Sunderland they 

 are singularly characterized by imbedded coralloidal, globose, 

 mamillated, and other forms, whose secondary or superinduced 

 origin, though first proved by Sedgwick, cannot be said to be 

 generally admitted. Consisting for the most part of carbonate of 

 lime in a crystalloidal condition, and containing only a few per 

 cent, of carbonate of magnesia, it is a fair inference that these 

 varied forms are a local development, caused by some demag- 

 nesiating agent which penetrated the depositional partings and 



subaerial origin, in which case they may have resulted from the evaporation 

 of the salt water of lagoons. 



* It may be admitted that carbonic acid would also be present in the 

 water, and thus assist in the reaction. 



t Owing to the varying percentage of carbonate of magnesia in these rocks, 

 it is not an undisputed point that this compound is in chemical combination 

 with the carbonate of lime. 



t The paste of the Bristol dolomitic conglomerate may have been derived 

 from a mechanically abraded Permian magnesian limestone j but the pebbles 

 contained in it have clearly been detached from adjacent beds of Carbo- 

 niferous limestone &c. The drift of Galway is essentially a calcareous deposit 

 derived, by glacial degradation, from the carboniferous limestone of the 

 district, and has consequently been mechanically formed. 



