KiNAHAN — On Some of the Irish Crystalline Iron Ores. 313 



or there are hollows and pot-lioles in the latter, or the pisolitic ore 

 may be absent, the roof resting on the bole, or the latter may be 

 absent/ 



From the facts that have been stated, it seems evident that the 

 iron ore seam must be considerably younger than the overlying 

 beds, and that it occupies a shrinkage fissure, this fissure being due, 

 probably, to the drying and contraction of the lithomarge, the 

 latter shrinking away from the steatic bottom of the dolerite ; while 

 toward the margins of the basins the lignite, in general, shrank 

 away from the alumite, as between them a thin bed of iron ore 

 occurs in places.^ Into this fissure the iron ore, in part, as sug- 

 gested by Mr. Argall, found its way in solution ; but at the same 

 time it was probably considerably augmented by the iron leached 

 from the lithomarge during its change into alumite. 



To me it would appear that the above is the most plausible way 

 of accounting for the occurrence of the iron ore seam, as it lies uncon- 

 formably on the " pavement," while the " stop dykes" are evidently 

 younger than the latter, while they are older than the seam. It is 

 also worthy of note that these " stop dykes" act very similarly to 

 the cross courses, heaves, and the like, that cross a standing lode, 

 as in such cases the seam is much thicker and richer at one side of 

 the cross course than at the other. This thickening of a stranding 

 vein at one side of cross courses has not, as yet, been satisfactorily 

 explained ; but probably it is due to a somewhat similar cause 

 to that which thickens the horizontal Antrim lodes. 



If the above theory as to the formation and growth of the 

 horizontal iron lode is correct, it can easily be imagined how iron 

 ores may have accumulated in other places. They may have come 

 clown in solution from the surface, or water may have leached rocks 

 as it passed down through them, the ore subsequently being de- 

 posited in fissures and other vacancies. 



Here we may draw attention to the gossan of the Ovoca mines, 



' From a conyersation witli Walter Jameson, Esq., the resident director of the 

 Eglintoun Chemical Company's "Works, Glenarm, &c., I am led to believe that his 

 theory as to the formation of the alumite and associate heds is very similar to, if not 

 identical with, that given above. 



2 The late W. Jory Henwood, in his description of the Cornish and other mines, 

 has satisfactorily proved that the "lay and lay," or horizontal interstratified lodes, 

 occupy cavities due to the irregxilar vertical shrinkage of beds, or parts of beds. 



