706 Proceedings of the Royal Irish Academy. 



the reaction of spring- water, containing a large percentage of carbonate 

 of lime, with sea-water, at a very high temperature.*' Eut this theory, 

 which bears a slight degree of resemblance to that of Hunt (see post), 

 will not answer, since many dolomites, e. g. those the carboniferous 

 formation in Ireland, not only are interstratified with limestone, but 

 actually the same bed may be highly fossiliferous limestone in one 

 place, and pass into dolomite in another. Such instances are common, 

 and it is clear that such a rock could not have been deposited from 

 boiling sea-water. 



(6). Dr. T. SterryHunt apparently endeavours to strike the happy 

 medium between the Wernerists and the Plutonists, but still his 

 theory will be found not to account for the interstratification, and 

 passage into each other of fossiliferous limestones and dolomites. It 

 supposes the reaction of river waters holding in solution carbonate of 

 soda, with sea- water contained in shallow basins, and further decom- 

 positions of chloride of calcium and subsequently of sulphate of mag- 

 nesium into bicarbonates of calcium and magnesium respectively ; the 

 former being precipitated first, but that, under certain conditions, 

 a mixture of the two maybe precipitated together. " The subsequent 

 action of heat upon such magnesian sediments, either alone or mingled 

 with carbonate of lime, has changed them into magnesite or dolomite. "f 

 I am at a loss to see why Dr. Hunt's own objection to Von Morlot's 

 theory does not also apply to this. In both cases only a mixture of 

 the two carbonates is obtained in the first instance, and the element of 

 sufficient heat may be supposed as well in the one case as in the other. 

 But besides this, the whole theory fails altogether to account for the 

 carboniferous dolomites of Ireland : for the facts that it is possible 

 to produce specimens from the same bed, of fossiliferous unaltered 

 limestone, and of true dolomite, and that beds of dolomite lie above and 

 below highly fossiliferous limestone, as I shall show hereafter. More- 

 over, the number and development of the fossils with which the lime- 

 stones abound, as well as the general stratigraphical character of the 

 deposits, and the extremely capricious manner in which the dolomites 

 occur, show that they could not have been formed in a series of shallow 

 seas, unless we admit an extraordinary series of changes of level, and 

 of physical features, during the period of the formation of the carboni- 

 ferous limestone — a position which is quite untenable. 



I take the dolomites of the carboniferous limestone as a test of these 

 theories, not only because I am best acquainted with them, but be- 



* Biscliof, viii., p. 161. — Also, Ann. de Cliem. and Phys., xxiii. Also, Eeport 

 Brit. Assoc, 1849 (Birmingliam) , Transactions of Sections, p. 36, wlierean abstract 

 of his views is given. 



t Chem. and Geol. Essays, pp. 80-90. On the Chemistry of Dolomites and 

 Gjrpsnms, also pp. 91-92, 309, et innltis aliis. I should not refer so particularly 

 to this in the present instance, but that Dr. Hunt applies his theory to the formation 

 of " all magnesian limestones." 



