448 Alfred Bell — Post-glacial Drifts of Ireland. 



in their relations to each other, then the conditions mider which 

 those faunas existed were not alike, although the physical structure 

 may be the same, and the deposits close to, or even dovetailing into 

 one another. Assuming these premises to be correct, it follows that 

 if the fauna of any given locality is well or fairly known, then the 

 place of such deposit in the geologic scale may be found by com- 

 parison with other deposits whose place has been definitely fixed. 



The coasts of Ireland are more or less mountainous, the interior 

 country or central plain being nearly flat. Carboniferous Limestone 

 forms the floor of this plain, a line drawn from Sligo to Di'ogheda 

 Bays indicating its northern boundary, and another line from Galway 

 to Drogheda Bay its southern limits. The whole of this plain (as 

 well as the entire mountainous districts) is much glaciated, rock- 

 scoring abounding where the fundamental rock is freshly exposed, 

 and is covered more or less with debris, the greater portion being 

 local in its derivation ; a detritus produced at a later period than the 

 ridges of true Boulder-clay, which, by their parallel arrangement 

 with the striations in their vicinity, are evidently due to the same 

 cause, i.e. Ice. These are more plentiful in the central parts of the 

 island, West Connaught and Mayo, Longford, Cavan, Westmeath, 

 and Dublin, and thence northwards (but much less plentiful) in the 

 direction of Lough Foyle. A second series commences at the ex- 

 treme west of Co. Clare, passing northwards into Galway by Loughrea, 

 deflecting southwards into Tipj)erary. 



The effects of glaciation are not equally visible over all the 

 mountain heights, some being smooth and rounded at their very 

 tops, while others beyond a certain line bristle with jagged rock- 

 masses and pinnacles, all below being smooth and bare except for 

 transported rocks. In Connaught the glaciation extends to above 

 2,000 feet, but in the Dublin- Wexford Hills summits of 3,000 

 feet are rounded in their outlines. With one exception above 2,400 

 feet the Cork and Kerry mountains are peaked and rugged, covered 

 with the coarsest angular debris, the result of atmospheric action ; 

 but Mount Mangerton, singular to say, placed in the very midst of 

 the district in question, is completely rounded, its summit being at 

 an elevation of 2,700 feet. 



The greater part of the country, with the exceptions presently to 

 be mentioned, is covered, as already stated, with drift, local in its 

 origin, but modified by the intermixture of rocks, never very large 

 in size, derived from the mountain barriers of the plain. 



There is no internal evidence as to the exact age of the limestone, 

 drift, since organic remains are rarely present in it. The drift is 

 of two kinds, a top gravel of clean sand and rounded pebbles with 

 little clay, reposing upon a stiff clay full of rounded or angular 

 masses — the "manure gravel "in part of some authors. The top 

 gravel is often piled up into eskers many miles in length. 



Perched upon the top of the eskers, scattered over the low grounds 

 or placed high up the mountain-sides, are many large "blocs- 

 perches," some weighing above 100 tons. In the central districts 

 they have floated from the Galway mountains southward over the 



