102 BRITISH PALAOZOIC SPONGES. 
stone of England, whilst the overlying shales and sandstones between them and 
the Millstone-grit are regarded by Prof. Hull’ 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.” 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* 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. At Florence Court, near Enniskillen, 
Professor Hull’ 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. Roy. Dublin Soe., vol. i, N. S., 1878, p. 73. 
* In a recently published paper (‘ Proc. Royal Soe.,’ vol. xlii, 1887, pp. 304—808) Prof. Hull, 
F.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., dee. 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. 
* «Scientific Trans. Roy. Dublin Soc.,’ vol. i, N. 8., 1878, p. 75. 
5 «Proce. Royal Soe.,’ vol. xlii, p. 306, Note. 
