316 Geological Society of Ireland. 



to appreciate their differences in form. Such a study readily leads 

 to a knowledge of the different arrangements of the grains of sand or 

 silt, according to the forces acting upon them ; a knowledge very 

 material when the i-idged and furrowed surfaces of various beds (and 

 we find them amongst our oldest accumulations) are brought under 

 our notice. 



Considering it a duty on the part of the Geological Survey to aid 

 the protrress of the Geological Society of Ireland, Mr Du Noyer read 

 papers on the sections observed on the Dublin and Drogheda Rail- 

 way, and Pi'ofessor Edward Forbes made a communication respecting 

 the probable geological age and British equivalents of the Silurian 

 rocks of the hills commonly known from one in particular, — the Chair 

 of Kildare. Having, with Professor Oldham, examined, during the 

 last autumn, the succession of rocks there seen, and ascertained that 

 they constituted a thick series of older accumulations, in which vol- 

 canic ashes or detrital matter derived from igneous rocks, as well as 

 molten masses of the latter, were mingled with common sands and 

 mud, forming its lower part, I have little doubt of the true relative 

 position of the limestone, in which the organic remains are chiefly 

 found. Indeed, the whole group of these hills seems little else than 

 an old island of pala30zoic rocks of the date to which allusion has 

 been previously made, on the shores of which conglomerates and other 

 rocks of the old red sandstone were accumulated. These, again, were 

 covered up by the beds of the great carboniferous limestone, one of 

 the most marked accumulations of the British series, especially for 

 the evidence it affords of general similar conditions having existed 

 over a large area at the same period, the modifications of these con- 

 ditions being very gradual, though at the same time very marked, in 

 different parts of that area. 



It is probable that, like the Mendip Hills at a subsequent geolo- 

 gical period, this island-land became covered up, as it was depressed 

 beneath the sea, by the accumulations of the old i-ed sandstone and 

 the carboniferous limestone ; — these accumulations again to be in a 

 great measure removed by those extensive denudations which we 

 have abundant evidence to shew took place over this region. 



Reposing on the part of the series in which the igneous rocks oc- 

 cur, and beneath a thick accumulation, chiefly arenaceous, the beds 

 of limestone are found in which the fossils noticed by Professor E. 

 Forbes were discovered. Many of the fossils from the Chair of Kil- 

 dare had previously been obtained by members of the Geological So- 

 ciety of Ireland, and are to be found in the collections of Mr Griffith, 

 where they have been described by Mr M'Coy. Availing himself 

 of all the sources of information presented. Professor E. Forbes, 

 takincr zoological evidence for his guide, considers that these lime- 

 stones of the Chair of Kildare are not only referable to the Lower 

 Silurian series, but members of the lowest part of it, and equivalent 

 to the Bala limestones and their associated beds in North Wales. A 



