724 Proceedings of the Royal Irish Academy. 



It is perfectly clear that these cavities "svere produced by the action 

 of "water, no doubt acting on the more calcareous parts of the rock, 

 which of course -would not be homogeneous in composition. Such a remo- 

 val of the lime in one part might be accompanied nearly simultaneously 

 by the deposition in a previously formed cavity of some of the material 

 brought a^vay. The calcareous -water trickling do-wn the sides of such 

 a vacancy woidd have a good opportunity of evaporating, and deposit- 

 ing its fi-eight. It is possible in such cases both the percolation and 

 the evaporation of the -water -would be slo-wer, and more uniform than 

 in large caverns ; and thus largely crystalline masses of calcite -would 

 result, instead of finely crystalline stalactites. 



This cellular character of dolomitic limestone is exceedingly -well 

 sho-wn in a quariy' at Loughiy, near Cooksto-wn, County Tyrone, in 

 ■which is the following section : — 



Section at Itochhead, near Loitghry. 



Feet. Inches. 



3. Boulder clay, 5 



2. Purplish crystalline encrinital lime- 

 stone passing do-wn-wards into pui'ple 



dolomitic limestone, -with large cavities, 3 10 



1 . Sandstones and grits, 7 1 



15 11 



The upper beds, ■which are dolomitic, are eaten a-way in curious- 

 ca-vities, as sho-wn'in the sketch (fig. 1, Plate 41). These -were possibly 

 formed during the alteration of the limestone. They could hardly 

 have occurred] since, because dolomite once fonned is so insoluble. 

 The ca-vities also are coated -with calcspar. 



In the south-east of Ireland the carboniferous limestone is much 

 dolomitised, and aiiords good opportunities for the study of that 

 mineral. In some places, as in the county Carlo-w, a persistent band 

 of black dolomite extends for miles, as may be seen on glancing at the 

 Carlo-w Sheet of the Greological Survey Map.*' Here it occupies such a 

 definite position in the carboniferous series, that it might be taken ta 

 be an important di-vision of it. The rock, -where perfect, is hard, 

 compact, and sub-ciystalline, but it is wonderfully cellular, fully a 

 tliird of it being wanting. The cavities are, so far as I can judge, 

 coated only with calcspar. After many searches I was unable to find 

 a single specimen of bitter spar, or dolomite, which can only occur 

 here in a very few isolated localities, if at all. 



Professor Jukes made a certain distinction between varieties of th& 



* Sheet 137. 



