356 



W. Williams — The Megaceros in Ireland. 



would hardly measure more than 100 acres or so, the catchment- 

 basin for the reception of rainfall was very limited. 



Diagram-section of Ballybetagh Bog, nine miles south-east of Dublin. 



P=Peat. 



4. Greyish clay, mineral debris from granitic hills brought down by frost, 



ice, and rain, during the last cold period. 

 3. Brownish clay, with remains of Megaceros. 



2. Yellowish-grey clay, almost wholly composed of vegetable matter. 

 1. Fine tenacious clay (reconstructed Boulder -clay), without stones, brought 



down by rain-wash from the hills, after the great Glacial Period. 

 ^= Boulder-clay resting on granite, thickness unknown. 



We shall now describe the appearance of the clays. On making 

 an excavation we first pass through a layer of peat, about two feet 

 thick, containing occasionally prostrate trunks of oak and alder. We 

 then enter on a bed of greyish clay, which is marked No. 4 in the 

 section ; this bed is about thirty inches thick. Beneath this we come 

 to a bed of brownish or snuff-coloured clay (No. 3), in which we 

 fi.nd Megaceros remains. This clay differs in a marked manner 

 from No. 4, which intervenes between it and the peat, inasmuch 

 as it has all the appearance of a true lake sediment, consisting mostly 

 of vegetable matter. Under No. 3 bed we come to No. 2 clay, 

 which seems to be composed entirely of vegetable matter, as if a 

 heavy crop of grass had been mown and sunk to the bottom of the 

 lake, and had there been subjected to considerable pressure till it 

 became a compact mass, having a yellowish-grey colour, and cutting 

 almost like cheese. Lastly, we come to clay No. 1, which forms 

 the substratum of the entire lake-basin. This clay is exceedingly 

 tenacious, and it was in this clay, as T conceive, that the Megaceros 

 got mired ; it is such as is used by brick-makers. Near the margin 

 it thins out, but deepens to a considerable thickness near the centre 

 of the lake, and rests upon true Boulder-clay. 



Having thus briefly described the argillaceous deposits observed 

 at Ballybetagh, and having stated that a general resemblance exists 

 between them and those of lake-beds in other parts of Ireland, it 

 might be well to notice in what respect they differ. One marked 

 difference is that Ballybetagh lies in a granitic district, and all the 

 mineral matter is granitic, having been brought down mechanically 

 from the surrounding hills. Marls are usually found in the Carboni- 

 ferous limestone districts, and result from a chemical dissolution of 

 the limestone rocks by carbonic acid brought down with the rain- 

 water in solution, and afterwards carried into the lakes there. Parting 

 with a portion of its carbonic acid, the lime falls to the bottom, and, 



