440 Dr. G. J. Hinde — Organic Origin of Chert. 



theory of its inorganic origin. I also examined the Chert in the 

 Carboniferous Limestones in the vicinity of Dublin, which, though 

 coloured in some of the Geological Survey Maps as belonging to the 

 Upper Limestone series, forms part, as I was informed by Prof. 

 Hull, of the Middle division, or Calp series. 



The mode of occurrence and the physical characters of the Chert 

 are in the main similar both in the south and in the north-west 

 of Ireland. It is usually exposed on the flanks and summits of 

 some of the principal hills which have partially escaped the severe 

 glacial erosion to which the country has been subjected, and their 

 preservation may, in part at least, be attributed to the compact 

 resistant nature of the Chert and Cherty-limestones of which they 

 are composed. The Chert often occurs as nodular masses of irregular 

 form, inclosed in beds of hard bluish Limestones, and following the 

 planes of bedding, much in the same way as the flints in the Upper 

 Chalk ; but unlike the flints, these nodular masses are not sharply 

 delimited from the Limestones in which they are interbedded, but 

 there is a gradual passage from the Chert to the Limestone. 



More frequently, however, the Chert exists in definite beds, from 

 one to five inches (•025--12m,) in thickness, which intervene at 

 irregular intervals between beds of Limestone. These beds some- 

 times occur also as well-marked layers in the central portions of 

 beds of Limestone. Both the nodular aggregations and the 

 horizontally-bedded Chert usually occur in the same series of rocks ; 

 the particular mode of deposition probably depends on the extent to 

 which the sponge-remains (of which it will be shown the Chert 

 consists) are present in the respective areas. 



Owing to this irregular mode of deposition, it is difficult to arrive 

 at an estimate of the total thickness of the Irish Carboniferous 

 Chert. Nowhere have I seen a continuous series of beds like those 

 in North Wales and Yorkshire ; the Chert in Ireland was more 

 interrupted by the deposition of Limestones, but its aggregate 

 thickness may not fall much short of the beds in Yorkshire. 



On the sides of the hill of Knock-na-Eea, near Sligo, Chert 

 occurs at frequent intervals, both in beds and nodular masses 

 throughout the series of Limestone, about 800 feet (240m.) in 

 thickness, forming the upper portion of the hill, and at Ben Bulben 

 to the North of Sligo Bay, the Limestones, in the higher slopes, 

 about 600 feet (180m.) in thickness, also contain an abundance of 

 Chert. I should estimate the Chert to vary from one-tenth to one- 

 fifth of the total thickness of these beds. On the sides and summit 

 of the hill of Benachlan, Floi-ence Court, near Enniskillen,^ Prof. Hull 

 estimates that the Chert-bands in the Upper Limestone series may 

 have a thickness of 150 feet (45m.) out of a total thickness of 400 

 feet, but from my own observation I should judge that this estimate 

 is considerably too high. In the South of Ireland the upper portion 

 of the Upper Limestone series consists, according to the reports of 

 the Geological Survey,^ nearly entirely of a mass of Chert from 



1 Proc. Eoyal Soc. vol. xlii. p. 306. 



2 Explanation of Sheet 128 (1859), p. 11. 



