274 Scientific Proceedings, Royal Dublin Society. 



parallel markings, long in comparison with their breadth, edged 

 with brown, and white in the middle. Under the microscope they 

 present in thin slices the following characters (Plate III., fig. 2). 

 All round the edge of the fissure the iron oxide, which in the 

 surrounding parts is scattered like fine dust through the dolomite 

 crystals, becomes a continuous film, thus forming the dark border. 

 The crystals of dolomite immediately beneath the ferruginous film, 

 forming the wall of the cavity, are exceedingly well-formed rhom- 

 bohedra, giving well-defined sections showing the traces of the 

 two sets of rhombohedral cleavages. This form — as shown by 

 Renard — is quite characteristic of dolomite, and distinguishes it 

 from calcite, which only rarely occurs in primitive rhombohedra 

 (as was also pointed out by the Abbe Renard, " Des Caracteres 

 distinctifs de la Dolomite," 1879. Dr. Sorby, however, represents 

 primitive rhombohedra of calcite as occurring in several lime- 

 stones : see " Address to the Geological Society of London," 1879, 

 pi. XII., fig. 2 ; pi. XIV., fig. 1 (reprint). Dr. Sorby does not state 

 whether the crystals shown in these figures had been specially 

 examined with a view to determining the absence of magnesia.) 



The central part of the fissure is entirely filled up with calcite, 

 in large clear grains with rounded edges. Similar grains occur 

 also in the midst of the dolomite crystals which mainly compose 

 the rock, and these, like those in the fissures, are all distinguished 

 by the absence of iron oxide. Whether a film of calcite, as the 

 Abbe Renard supposes, is also present and holds together the 

 dolomite crystals, I cannot say, but I did not succeed in finding 

 any trace of it. 



It would appear from the foregoing that the fissures were pro- 

 duced during the conversion of the limestone into dolomite ; that 

 round their margins the crystals of dolomite, having more room for 

 growth, became larger and better formed than those which compose 

 the mass of the rock ; that over the surfaces of these a coating of 

 ferric oxide was formed ; and finally, owing to the passage of 

 water through the rock, holding calcium carbonate in solution, 

 calcite was deposited filling up all the chinks. 



In conclusion, I may perhaps be allowed to offer a suggestion 

 as to the cause of these curious cracks. No explanation is at all 

 obvious ; but it is evident that since the particles of the rock have 

 separated along parallel planes, that there must have been a ten- 



