SoLLAS — A Map to show the Distribution of Eskers in Ireland. 821 



which the direction of ice-flow may be inferred. This is given by the bogs. It 

 will be found that, in a district where the direction of the ice-movement has already 

 been discovered from observations on drumlins and striae, the smaller bogs present 

 individually an elongated form running in the same direction as the ice-flow, or 

 they form a network, the meshes of which are all mostly elongated in the same 

 direction. The bogs were originally lakes in the boulder clay, and when these 

 occur in the form of isolated elongated troughs, they may be regarded as the 

 reverse of drumlins ; when, on the other hand, they form a network of waterways, 

 this may be owing to the existence of numerous drumlins rising above the level of 

 the water. In either case, whether indicating an elongated trough or the presence 

 of drumlins, the form of the bog becomes a valuable aid in the discovery of the 

 dii'ection of ice-movement by mere inspection of an ordinary geological map. On 

 reference to Plate lxix., it will be seen that in the case of the Ballyhaunis 

 system all the evidence concurs in testifying to a movement of the ice from 

 the south-east to the north-west, while the esker chains, for the greater part of 

 their course, run manifestly at right angles to this, or from south-west to north- 

 east. This would suggest that in their case the water channels in the ice were 

 determined partly by crevasses. We have already seen that these eskers run 

 against the slope of the ground, and it may be that the apparent convergence of 

 the chain northwards, as described above, is to be interpreted as a divergence 

 southwards, and that the water drained from the Ballyhaunis ice towards Lough 

 Corrib and Gralway Bay. 



If now we turn to the Midland system we shall find that associated with 

 a general absence of indications of the direction of ice-flow is a general absence of 

 elongation in the form of the wide areas occupied by bog. From this it would 

 appear that the movement of the ice in the centre of the country was sluggish ; 

 it probably accumulated here in an immense pool, and^ possibly persisted as a 

 piedmont glacier up to the closing stages of the Glacial Period. In the case of the 

 Portumna sub-system the flow of the ice, as shown by drumlins, bogs, striae, and 

 transport of material, was in the same direction as the run of the eskers, i.e. on 

 the whole from west to east. That the subglacial streams flowed for a considerable 

 distance in this direction is suggested by the occurrence already mentioned 

 of Connemara marble and Galway granite in the esker of Parsonstown. The 

 Portumna belt runs transversely to this east-and-west movement, and like the 

 eskers of Ballyhaunis may have been determined by crevasses. 



TEANS. EOT. DUB. SOC, K.S. VOL. V., PAET Xm. 6 A 



