﻿418 Reports and Proceedings — Geological Society of Ireland. 



I. — KoTAL GrEOLOGiCAL SociETY OF IRELAND. — March 16, 1877. 



" On the Nature and Origin of the Beds of Chert in the Upper 

 Carboniferous Limestones of Ireland." By Prof. Edward Hull, 

 M.A., F.E.S., Director Geol. Survey of Ireland. With "Chemical 

 Notes," by E. T. Hardman, F.C.S., Geol. Survey of Ireland. 



After reviewing what had been published by previous authors on 

 the origin of chert-beds, and showing that much remained to be done 

 in this department of petrology, the author proceeded to describe the 

 geological position of the principal cherty zone of the Carboniferous 

 Limestone of Ireland, showing that, while bands of chert occur at 

 intervals throughout this formation, the highest beds immediately 

 under " 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 belonging to those of marine animals 

 — such as corals, crinoids, foraminifera, and occasionally molluscs, 

 animals which build their shells or skeletons of carbonate of lime 

 rather than of silica. The siliceous paste in which these forms are 

 inclosed 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. From 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 pseudoraorph, 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 

 believing that at the close of the period during which the Carbon- 

 iferous Limestone was formed over the area of Central Ireland, the 

 sea-bed was elevated, so as to be covered with 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 surrounding 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 the process of silicifi- 



