82 Professor Epwarp Hutt—On the Nature and Origin of Beds of Chert. 
ance of organic forms; there are also opaque cubical forms of doubtful origin. 
(Analysis XI.) 
No. 15. Light gray chert, with crinoidal remains preserved in silica of a dark 
shade, beds nodular ; shows a nearly colourless paste containing numerous circular 
forms, with sometimes a central darker point, in all probability sections of crinoidal 
stems; with polarized light, these are seen to be filled with silica sometimes 
showing a radiating structure, corresponding to the serments of crinoidal ossicles. 
(Analysis XII.) 
Periods at which the Chert beds were formed.—Bands and nodules of chert occur 
(as we have seen) at various stages throughout the Carboniferous Limestone of 
Ireland, but chiefly in the uppermost beds, which (except in the county Clare) are very 
generally silicified. Asregards the period of this last great silicification, Mr. Hard- 
man and myself have arrived at the same opinion, that it took place during, 
and after, the formation of the limestone itself, and before that of the over- 
lying Yoredale beds. It is to be observed in evidence of this view that the 
silicification does not extend into the joints, fissures, and faults, which were origin- 
ated after the rock had been consolidated. The silicification has taken place in the 
mass of the limestone itself, either following rudely the planes of bedding, or forming 
wedge-shaped masses and large pockets. It is probable that immediately after its 
formation, the limestone, except where coralline, was in a soft and pasty condition. 
Microscopic sections show, that the more dense and compact portions are largely 
composed of foraminifera; and such beds probably existed in the form of 
“oceanic ooze,” like that of the central Atlantic of the present day. The coralline 
and crinoidal beds, after their formation, would probably be open and porous. Thus, 
the whole calcareous mass to a variable depth would appear to have been somewhat 
accessible to the percolation of sea waters charged with silica and other compounds. 
The sea-bed became shallower.—Throughout the period of the Carboniferous Lime- 
stone, the sea bottom was deep, the waters were usually clear and free from sediment, 
and land of a continental character producing rivers was distant from the central 
districts of Ireland. The limestone was built up by organic agency under such 
favourable conditions,* but the change from these clear water conditions to those of 
the succeeding period in which the Yoredale shales and mudstones were deposited, 
must have been due to terrestrial movements. 
Amongst the results of these movements altering the physical geography of this 
part of the world, may be confidently stated the shallowing of the sea-bed over 
the limestone areas, also the production of slight discordancies in the stratification 
of the limestone to the overlying shale series. Mr. Hardman from numerous 
observations of the relations of these two formations around the borders of the 
Leinster Coal field, has come to the conclusion that the shales are loca!ly uncon- 
*Some of the most dense and unfossiliferous looking limestones, when examined microscopically, 
exhibit foraminiferal structures. 
