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Proceedings of the Royal Irish Academy. 



altogether obliterated. But ttis is a constant occurrence among Irish 

 dolomites.'* I have already referred to one instance of it in the Co. Ty- 

 rone ; it is also frequent in the County Kilkenny, in many places within 

 a circle extending fi'om Gowi'an to near Ballyragget ; and I have hand 

 specimens showing the gradual alteration, the fossils being completely 

 obliterated, and the blue limestone at one side becoming perfect crys- 

 talline dolomite at the other. Large masses of dolomite are seen, 

 which, when traced out, are found to abut against and merge into 

 limestone, and in some places, as at Ballyfoyle, there will be as many 

 as twenty or more alternations of limestone and dolomite in a distance 

 of less than haK a mile ; the limestone always full of marine fossils, 

 by no means dwarfed in appearance. (See figs. 4, 5, 6, Plate 42). 



\_Note added in Press. — I have mentioned that limestones are by no 

 means homogeneous in composition, and that the cellular structure would 

 be capriciously determined by the most calcareous, and therefore most 

 soluble parts. I have lately analysed some limestones fi'om Ballyfoyle, 

 which are interlarded with and pass into dolomite. The following were 

 made from specimens of the same bed, taken a few yards apart : — 



Ay.^TSIS OF LniESTOXE FEOli: BALXYPOTLE. 



' I. II. 



Carbonate of lime, 



,, ,, magnesia, . 

 Ferric oxide and alumina. 

 Insoluble residue, . . 



87-72 

 3-80 

 2-52 

 5-80 



91-06 

 1-00 

 2-05 

 5-70 





99-84 



99-81 



1 



Specieic Gravity, 2-89. 



Specific Gravity of Dolomite. — On the principle I have advocated, 

 •viz., the removal of carbonate of lime from limestones, and the conse- 

 quent porosity of the resulting dolomite, the specific gravity ought to 

 be less than that of limestones. I am aware that Dr. Apjohn has, in 

 the paper already cited, stated the contrary ; but it must be remem- 

 bered that the determination is rendered very difficult in the case of 

 dolomite by the circumstance that, while in the mass, it is porous and 

 cellular, and must be of less specific gravity than limestones, which are 

 compact — the small pieces, which could only be weighed on our 

 balances, are usually compact. However, those I have tried certainly 

 possess a lower specific gravity than limestone. This is shown in the 

 examples given above of the Ballyfoyle dolomite and limestone.] 



* Biscbof refers also to this fact as common to dolomitic formations. 



