Professor Enwarp Eivutu— On the Nature and Origin of Beds ot Chert. 75 
Lower Limestone. Amorphous pale gray limestone, highly fossiliferous, resting on pale gray 
limestones, frequently oolitic ; below these are regularly bedded dark 
gray crystalline limestones with thin beds of shale, and nodules of 
chert. 
Lower Limestone Shale. Thin grits, shales, and limestones—fossils abundant. * 
The total thickness of the Carboniferous Limestone Series is about 2,500 feet. 
In the south-western districts of Tipperary,t Limerick, Kerry and Cork, the 
principal masses of chert occur at the top of the hmestone, immediately below the 
shales of the “Yoredale Series,” and sometimes are so abundant as almost 
completely to replace the limestone itself The rock of Cashel with its 
ecclesiastical ruins and venerable round tower rising above the plain around, is a 
conspicuous example of cherty limestone.$ 
Although it is true that lenticular bands and nodules of chert or hornstone 
occur at intervals throughout the whole mass of the Carboniferous Limestone, 
yet it is unquestionable that the principal chert bearing zone, occurs at the 
top of the formation immediately under the upper shale series. 
beds of considerable thickness occur around the base of the hilly country, which 
forms the coal basins of Castlecomer and Slievardagh. The late Professor Jukes 
and Mr. Kinahan in describing the upper limestone in Queen’s County and 
Kildare, state that—‘“the chert layers are sometimes so frequent, that they make 
the rock nearly an entire mass of chert.” At the foot of the ridge west of 
Carlow, the limestone is completely replaced by masses of grayish chert in thin 
layers, and over thirty or forty feet in thickness. The same statement applies 
to the districts of Meath, Dublin and Kildare, where the upper limestone is 
generally dark, thin bedded, and flaggy, and in the upper part is generally associated 
with beds of chert.** It is, however, in the hilly districts of Sligo, Fermanagh and 
Leitrim, that the chert beds are most conspicuous, as they sometimes almost 
entirely replace the upper limestone, which (as already described) rises into 
escarpments with precipitous faces. A remarkable mass of this kind 1s the lofty 
rock called Benachlin, which rises abruptly above the wooded slopes of Florence 
Court, in the County Fermanagh, and is well known to paleontologists for the oceur- 
rence of specimens of Pentremites preserved in chert. Knock-na-Rea, the isolated hill 
which rises from the shore of Sligo Bay is another instance. Here the whole mass 
In this position 
* These are enumerated by Mr. Baily, Ibid. p. 10. 
+ “ Explanation ” to sheet 155, by J. O’Kelly of the Geological Survey of Jreland, pp. 13 and 16. 
+ “ Byplanation ” to sheets 163, 174, &c. A very striking sketch of chert in the upper limestone N. of 
Newmarket, is given by Mr. Foot. 
§ “ Explanation” to sheet 155, p. 16. 
|| According to Mr. Kinahan the “upper cherty zone” in the district bordering the Counties of Clare 
and Galway, occurs 1,500 feet below the top of the formation, but the whole of the “‘ upper limestone ” 
is more or less cherty. ‘‘ Explanation ” sheet 124, page 11. 
g| «« Explanation” to sheet 128, of the Geological Survey Maps, p. 12. 
** O, V. Du Noyer “ Explanation” to sheet 101, p. 12. 
