J lie Irish Naturalist. 



foreign matter in each, they would suggest to our minds, not an 

 utter want of connexion between the two as regards origin, 

 but rather some such relation as that which I have just 

 referred to. 



The sketches given by Mr. Farrington in the May number 

 do not exaggerate the steepness of the walls of our local mag- 

 nesian deposits. He supposes that the spaces now occupied 

 by these were either carved out by the action of rapid streams, 

 or opened by terrestrial disturbances. As regards the first 

 supposition, we are met with the difficulty that there is no in- 

 stance known of a natural open water-channel which could 

 form the counterpart of these hollows in point of steepness of 

 sides ; while their abrupt termination at both ends introduces 

 a fatal objection to the cailon theory of their origin. On the 

 other hand, the idea of earth-movements giving rise to widely- 

 gaping fissures which remained open long enough to be filled 

 by the necessarily slow process of chemical precipitation, will 

 hardly appear a plausible one to an}^ observer of actual rock- 

 forms. 



On Mr. Farrington's view of their origin, the magnesian 

 deposits ought to show distinct traces of bedding other than 

 that of the limestone around. It was perhaps his desire to 

 account for the absence of anything of the kind, which led him 

 to conclude, on what seems very slender evidence, that the 

 dolomite had been always subjected to the action of heat. It 

 would indeed be remarkable if the heat had rendered it 

 crystalline without seriously affecting the limestone in im- 

 mediate contact with it. But the mere fact of dolomite 

 resembling saccharine marble does nottend to prove the action 

 of heat at all. The structure of dolomite is usually distinctly 

 crystalline ; and the difference between it and limestone in 

 this respect is occasionally relied upon as a rough means of 

 discrimination. The presence of iron pyrites in the dolomite 

 suggests rather the reducing action of percolating water 

 charged with organic matter than the action of heat. 



When we come to the history of our southern land-surface 

 we reach he climax of difficult3^ The Permian theory requires 

 that denudation should have been so rapid during the early 

 part of the Permian period as to strip off" the Coal-measures and 

 much of the Carboniferous limestone, leaving ample time for 

 a series of crust-movements which extended over hundreds of 



