Hardman — On the Carboniferous Dolomites of Ireland. 725 



dolomites of the south, of Ireland, ^. e., dolomites of original deposition, 

 and dolomites produced by alteration of the original rock, with the 

 former of which the Carlow rock appears to have been classed, while 

 that of Kilkenny is supposed to be metamorphic. I rather think, how- 

 eyer, that they both are metamorphic, only in different degrees. 



The Kilkenny magnesian limestones are true dolomites both in 

 appearance and composition. They contain from 30 to 44 per cent, of 

 carbonate of magnesia ; they are usually very crystalline, of a light 

 yellow to a pearly grey colour, and do not effervesce when treated 

 with acid, except occasionally in the interstices between the crystals, 

 owiug to infiltrated carbonate of lime. They are remarkable for the 

 same cellular or cavernous structure which I have noticed in all the 

 other dolomites of Ireland ; the vacancies being coated, or filled entirely 

 with calcspar. In some places the material has been removed in an 

 exceedingly curious manner, the vacant spaces running parallel to 

 each other, and forming apparently lines of bedding, which, however, 

 they are not, as the true bedding is often also visible in such instances. 

 Sometimes they give the similitude of false or current bedding, as is 

 shown in the sketch (fig. 3, Plate 41). 



As a rule, the bedding is obliterated, and in weathering the rock 

 assumes at the surface of the ground, or wheresoever else exposed, a 

 ruggedly pointed aspect, as if dipping vertically, along the liaes of 

 joints; and this often gives rise to picturesque hillocks or escarpments ; 

 the dolomite remaining, while the more easily dissolved limestone is 

 eaten away to a lower and more uniform level, contrary to the general 

 idea which assumes that dolomite is more soluble than limestone, be- 

 cause under certain influences it disintegrates more rapidly.* 



IS^ear the city of Kilkenny very extensive, masses of dolomite occur, 

 which could only have been formed by the metamorphism of the 

 original limestone ; and in one locality — Riverview, l-t miles west of 

 the town — there is a very fair opportunity of studying the mode of its 

 production. And it will be seen that this can be reasonably explained 



* A well-known instance of this is shown by the decay of the stones used in the 

 present Houses of Parliament, being magnesian limestones from the Mansfield "Wood- 

 house quarries, and from Anston, Yorkshire. (See Building and Ornamental Stones, 

 Prof. Hull, p. 200.) Under the influence of the vitiated atmosphere of London, some of 

 this stone soon commenced to crumble away. It must be remembered that the disinte- 

 gration of dolomite is not due always to the dissolution of the whole rock, but is most 

 often merely the result of the solution of the carbonate of lime, which cements the 

 crystals of true dolomite together. Any one who has observed the process of weather- 

 ing of dolomites wiU remember that the minute crystals of which the rock is 

 composed merely fall away from each other, resulting in a loose sand-like mass, 

 but they do not readily decompose. 



According to Ansted, the most durable magnesian limestones, for building pur- 

 poses, are those containing nearly equal parts of carbonate of lime and of magnesia 

 in a state of perfect combination — that is, true dolomite. — See Ansted's Geological 

 Science (Orr's Circle of the Sciences), p. 208. 



