On the nature and origin of the beds of chert in the upper carboniferous limestones of Ireland

Abstract

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 marine animals (such as corals, crinoids, foraminifers, 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.