102 BRITISH PALEOZOIC SPONGES. 



stone of England, whilst the overlying shales and sandstones between them and 

 the Millstone -grit are regarded by Prof. Hull 1 as corresponding to the Yoredale- 

 beds. As, however, the Sponge-beds consist of chert, closely resembling that of 

 the Yoredale series in England and North Wales, it is reasonable to conclude that 

 they may occupy a corresponding horizon, even though no well-marked line of 

 demarcation between them and the main mass of the Carboniferous Limestone has 

 up to the present been noted. 



The Sponge-beds chiefly occur as nodular masses or bands of dark, mottled, 

 compact chert, closely similar to those of Yorkshire and North Wales. 2 Microscopic 

 sections of specimens which I have lately collected from various outcrops of the 

 rock in Queen's County and Kilkenny to the south, and in Fermanagh and Sligo 

 to the north-west of Ireland, all show the presence of spicules, and distinctly prove 

 that the rock has been derived from them. 



Well-marked beds of chert, from one to three inches in thickness ('025 — *075 

 m.), are also frequently present in the dark limestones of the Calp or Middle 

 series of the Carboniferous Limestone in the neighbourhood of Dublin, and these, 

 like the higher beds, are filled with microscopic spicules. 



Owing to the irregular manner in which the nodular masses and bands of chert, 

 constituting the Sponge-beds, are intercalated in the limestones of the Upper 

 Series in Ireland, it is difficult to form an estimate of their total thickness. In 

 Queen's County and Kildare the chert layers are stated 3 by the late Professor 

 Jukes and Mr. Kinahan to be sometimes so frequent that they make the rock 

 nearly an entire mass of chert. In the ridge west of Carlow the greyish chert is 

 stated to be over 30 or 40 feet in thickness. 4 At Florence Court, near Enniskillen, 

 Professor Hull 5 estimates that the chert bands in the Upper Limestones have a 

 total thickness of perhaps 150 feet (45 m.) ; but from my own observation this 

 estimate seems considerably too high. 



1 * Scientific Trans. Eoy. Dublin Soc.,' vol. i, N. S., 1878, p. 73. 



2 In a recently published paper (' Proc. Royal Soc.,' vol. xlii, 1887, pp. 304—308) Prof. Hull, 

 E.R.S., the Director of the Irish Geological Survey, most emphatically combated a suggestion 

 made by me two years since, that the chert bands of the Irish Carboniferous Limestone were probably 

 derived from Sponge-spicules, the same as the chert beds of the Cretaceous strata of the south of 

 England (' Phil. Trans.,' 1885, pt. ii, p. 433). 



After the publication of this paper I went to Ireland and examined the chert beds in the various 

 localities from whence Prof. Hull had obtained the specimens on which he based his conclusions, 

 and I then found that there was decisive evidence that they were derived from Sponge remains as I 

 had suggested (' Geol. Mag.,' n. s., dec. iii, vol. iv, p. 44). An inspection of the microscopic sections 

 which Prof. Hull described and figured showed, as Prof. Sollas had already stated (' Ann. and Mag. 

 Nat. Hist.,' vol. vii, 1881, p. 141), that some of them were largely composed of spicules. 



3 ' Geol. Surv. Ireland, Explanation Sheet 128,' p. 12, also quoted by Prof. Hull, op. cit., p. 75. 



4 ' Scientific Trans. Eoy. Dublin Soc.,' vol. i, N. S., 1878, p. 75. 



5 * Proc. Eoyal Soc.,' vol. xlii, p. 306, Note. 



