f 



Hardmax — On the Carbon iforous Dolomites of Ireland. 707 



cause while they are of extremely frequent and extensive occurrence in 

 Ireland, they are also found under perhaps the most favourable con- 

 ditions for the determining" any points with regard to either Plutonism 

 or physical phenomena. 



On the subject of Irish dolomites, two valuable papers are extant. 

 Many years ago Dr. Scouler communicated his views on that subject 

 to the Geological Society of Dublin ;* his opinion being that dolomite 

 was produced by a metamorphism of the original limestone, and, follow- 

 ing Viilet, he considered the change to be readily accounted for by the 

 infiltration of water charged with carbonate of magnesia ; which 

 water would at the same time remove some of the carbonate of lime. 

 But an important point in his paper is that he considers dolomite to 

 occur usually near some source of magnesia — either near an igneou^s 

 or ancient palseozoic rock, or close to a break in the strata, where a 

 thermal spring might have existed. This is a point which I shall 

 presently dwell upon, as many dolomites occur under circumstances 

 which do not agree with any of these conditions, and where the supply 

 •of magnesia is far below that of Kme. 



The reading of the above paper led Dr. Apjohn to make several 

 ■analyses of Irish dolomites, which he has published in the same 

 journal. f The conclusions he came to as to the origin of dolomites 

 appear to be that they are original formations ; first, because they are 

 often fossiliferous, and, secondly, because their composition is definite. 

 At the same time he suggests that some dolomites may have been 

 produced by the solvent action of carbonated water on limestones con- 

 taining some magnesia, in removing carbonate of lime, until at last 

 the rock would consist of the two carbonates in the con-ect propor- 

 tion. | 



Probably the most comprehensive account yet published of the 

 dolomite question is that given by Bischof , who, in his classical work on 

 chemical geology, has discussed nearly all the foregoing views, together 

 with many of those held by other writers. He dismisses as impro- 

 bable all those which call in the aid of volcanic or Plutonic agencies, 

 and shows that the action of water by infiltration through limestone 

 <;an alone explain the processes of dolomitization ; that is, either by 



* Observations on Beds of Dolomite Avliichocc-nr in connexion with the Carboni- 

 ferous Limestones indifferent parts of Ireland. By .Jolni Scouler, M.D., &c., Jour. 

 Geol. Soc. Dub., vol. i., pp. 382-5. 



+ Analyses of some Irish Dolomites. By James Apjohn, M.D., &c. Jom*. Geol. 

 Soc. Dub., voli., pp. 369 et seq. 



X I am inchned to adopt a modification of the above hypothesis, viz., 

 that the greater part of the carbonate of magnesia was originally secreted along 

 ■with the carbonate of lime, but that dolomite is a true metamorphic rock — the 

 alterations being due to the extraction of the surplus of carbonate of Kme. Some 

 so-called dolomites having the crystalline structure and the obliteration of fossils 

 en regie, are by no means of definite composition; they usually contain a consider- 

 able percentage of uncombined carbonate of lime, which dissolves out in weak 

 acid. 



