W. Williams — The Megaceros in Ireland. 357 



mixing with the organic matter, forms marl of different degrees of 

 purity, owing to the quantity of clayey matter mixed with it. In 

 some instances the solution is taken up by the freshwater mollusca 

 to form their shells, which, after their death, accumulate in the 

 lakes and becomes shell-marl. In Kildare and Meath the latter is 

 more abundant than in the Limerick lake deposits. 



Let us endeavour to trace the history of the silting up of an Irish 

 lake, taking Ballybetagh as an example. Going back to the close 

 of the great Glacial Epoch, let us examine the state of things that 

 then existed, taking this as our starting-point. 



It is an admitted fact among geologists that at the time of the 

 great Glacial Epoch, or as Dr. Geikie calls it " the Great Ice Age," 

 the whole surface of the Northern hemisphere was covered with 

 immense glaciers, which, like those of the Arctic regions at the 

 present day, moved over the land, but were even more extensive. 

 Some writers affirm that they must have been miles deep at the pole, 

 but thinned off down to the 40th degree of latitude. There is evidence 

 that they flowed over the " Three Eock Mountains," 1200 ft. high, 

 only two miles from the bog that has been described. This tre- 

 mendous moving force ground down the rocks of the country, leaving 

 scorings on them which can be seen at the present day. To this 

 grinding force we are indebted for the mineral ingredients of our 

 soils, consisting as they do of pulverized rocks. 



In the case of Ballybetagh a glacier, or a part of the ice-flow, 

 seems to have passed over a hill that lies to the north-west of the 

 valley, and, descending it, displaced the matter that previously 

 occupied the hollow and pushed it on towards the south-east, leaving 

 at its exit a bank or dam of Boulder-clay, so that the waters of the 

 lake were kept at a higher level than that of the bog at present. 

 The old water-line can still be traced along the hill-sides. Here 

 we have a lake formed ; the surrounding slopes, after the climate 

 became milder, and the glaciers were melted away, were covered 

 with a coating of tenacious Boulder-clay, full of erratic and loose 

 stones. 



During the thaw the climate was intensely humid. The valleys 

 were filled with turbid water, and the rainfall, acting on a land- 

 surface perfectly denuded of vegetation, brought down the fine 

 particles of the Boulder-clay into the lake, leaving the gravel on 

 the hills, so that in the lake-bed we have a sort of geological 

 paradox, Boulder-clay without Boulders.^ This process, no doubt 

 going on for a long period, fully accounts for the deep bed of tough 

 clay No. 1. It seems to have been the clay in which the Megaceros 

 got mired and trapped, similar natural traps being spread over the 

 country wherever these lakes occur. 



Clay No. 2 can hardly with propriety be called a clay. It is, as 

 stated, a bed of pure vegetable remains, that has been ages under 

 pressure. As the climate improved, and the cold of the Glacial 

 period had passed away, vegetation commenced in the bottom of the 



^ This woiild properly be described as reconstructed Boulder-clay. 



