164 On the Beds of Chert in the Limestones of Ireland. "Apr. 26, 



position of the principal cherty zone of the Carlxniiferous Limestone of 

 Ireland, showing that, while bands of chert occur at intervals through- 

 out this formation, the highest beds immediately under i; The Yoredale 

 Shales " are especially rich in chert, and are frequently entirely replaced 

 by this mineral. In these beds coralline, crinoidal, and other marine 

 forms were frequently to be recognized by the naked eye. Thin slices 

 for microscopic examination, taken from various localities, extending from 

 Sligo to Carlow, also showed that even the most dense and compact 

 masses of chert exhibit, under favourable circumstances, forms belong- 

 ing to those marine animals (such as corals, crinoids, foraminifera, 

 and occasionally mollusks) which build their shells or skeletons of 

 carbonate of lime rather than of silica. The siliceous paste in which 

 these forms are enclosed was found to be in a gelatinous state ; and the 

 forms were only to be distinguished by difference in depth of shade from 

 the paste, the shells or skeletons having disappeared. The chemical 

 analyses of these specimens by Mr. E. T. Hardman, F.C.S., tended to 

 show that the chert-beds contain various proportions of carbonate of lime 

 as well as other minerals, so that a gradation from siliceous limestone 

 into pure chert might be traced. 



Prom a review of the whole circumstances, it appeared that the origin 

 of the chert-beds was to be attributed to the replacement of the original 

 limestone or calcareous "ooze," due to organic agency, by silica, and that 

 the rock is truly a pseudomorph, a view held by several observers. 



The manner in which this replacement had been brought .about was 

 then touched upon. It was shown that there was reason for beheving 

 that at the close of the period during which the Carboniferous Limestone 

 was formed over the area of Central Ireland, the sea-bed was elevated, 

 so as to be covered by the waters of a shallow sea, exposed to the sun's 

 rays, and of a warmer temperature than when at a greater depth. The 

 waters appear to have been charged with a more than usual supply of 

 silica in solution, derived (as Mr. Hardman suggests) from the sur- 

 rounding lands, formed, for the most part, of highly siliceous materials. 

 As silica is less soluble than carbonate of lime, chemical replacement 

 would naturally take place, the carbonate of lime being dissolved out 

 and its place taken by the silica. The warm condition of the sea-water, 

 its exposure to sunlight, the porous character of the coralline, crinoidal, 

 and other forms, and the soft and "oozy" condition of the foraminiferal 

 mud would give easy access to the sea-waters, and a process of silicifi- 

 cation would take place analogous to that described by Dr. Martin 

 Duncan, F.E.S., as having occurred in the "West Indies. 



The paper was accompanied by chemical analyses and photographic 

 figures of some of the thin slices, slightly magnified. 



