726 Proceedings of the Royal Irish Academy. 



on tlie theory put forward already, that is, the extraction of carbonate 

 of lime from the limestone rock. 



A small stream runs from the railway down to the E"ore at this 

 place. On one side of it, to the south, dolomite crops out, with its 

 usual rugged aspect. On the other side, blue fossiliferous limestone. 

 At first sight it would seem that the limestone ended abruptly against 

 the dolomite, but in reality it dips underneath it. The dolomite pre- 

 sents the usual characteristics, being highly crystalline, and full of 

 drusy cavities, with calcite. The limestone is a compact bluish rock, 

 in thick beds ; the upper beds are magnesian, although not yet dolo- 

 mitic, but they are beginning to show the drusy cavities, and are 

 undoubtedly some distance on their way in the direction of dolomite. 

 The most interesting fact, however, is, that between every individual 

 bed of the limestone is a thick layer, or rather bed of calcite, from three 

 to nine inches thick ; and this is even visible on the top of the uppermost 

 bed, which there is overlaid by a thin coating of drift clay or soil ; but 

 eventually disappears under the dolomite. (See fig. 2, Plate 41.) 



IS'ow it is perfectly evident that the calcite layers are derived from 

 the limestone beds above them. It would be difficult to prove that 

 each layer was derived solely from the immediate bed above it, but 

 this is not impossible. The calcites are of a fairly uniform thickness, 

 and the quantity abstracted from the overlying beds would be quite suf- 

 ficient to alter very materially the composition of the limestone. These 

 beds are about eighteen inches thick, and the corresponding calcites 

 three to nine inches. Taking the latter, and assuming, for argument 

 sake, that the limestone originally contained about 12 per cent, of car- 

 bonate of magnesia, the removal of sufficient calcite to form a layer nine 

 inches thick would increase the percentage of magnesia carbonate in the 

 limestone to over 20 per cent., which would nearly correspond to the 

 composition of some dolomites. 



One other point with regard to the Irish dolomites I have already 

 partly referred to — viz., that they help to supply further evidence in 

 refutation of Dr. Hunt's theory as to the origin of dolomitic rocks, that 

 is, their original deposition as sediments from an evaporating sea basin, 

 and subsequent modification by heat. I cannot do better than here 

 quote more fully Dr. Hunt's words. (See "Conclusions" of his paper 

 on the Chemistry of Dolomites and Gypsums.*) " Dolomites, mag- 

 nesites, and magnesian marls, have had their origin in sediments of 

 magnesian carbonate, formed by the evaporation of solutions of bicar- 

 bonate of magnesia. These solutions have been produced either 

 by the action of bicarbonate of lime upon solutions of sulphate of 

 magnesia, in which case gypsum is a subsidiary product, or by the 

 decomposition of solutions of sulphate or chloride of magnesium by the 

 waters of rivers or springs containing bicarbonate of soda. The sub- 

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



* Chem. & Gcol. Essays, p. 90. 



