2,12 Proceedings of the Royal Irish Academy. 



Scottish ice-flow " (page 108).. J. P. Campbell aud the Geological Survey 

 alike regard the X.-W.-S.-E. set as the newer, with which datiug of the 

 striae I am iu agreem^ent ; the fine distinctuess and incisive character of the 

 striae permit the determination of the relative age of the two sets without 

 difficulty. It would seem, therefore, that in the earlier stages, glaciers flowed 

 from the Derryveagh hills down the Gweebarra valley, producing the set of 

 striations trending roughly parallel with the latter, and that the ice proceed^ 

 ing from the hills to the south aud east under the impulse of the great 

 Barnesmore Ice, was during the maximunf stage so powerful as to cause the 

 Derry\-eagh Ice to be deflected towards the north-west. Wlien the main ice- 

 mass began to wane the Deriyveagh glaciers had either already retreated to 

 the hills, or were unable to re-assert th.emselves and stream down the 

 Gweebarra, because of the continued pressure of the ice from the south-east. 



The Bosses form a distinct topographical unit, being composed of a well- 

 jointed gi-anite. The joints have detei-mined the direction aud parallelism of 

 the countless narrow lakes, the straight stream coui-ses, the trend of the coast, 

 and the configuration of the off-lying islands. 



The lake-bestrewn surface of the Bosses is composed of countless 

 mowtoivnee granite bosses, indicating with singular regularity and constancy 

 an ice passage over the region in a direction K. 20° or 30" W. The surface 

 is dotted over with granite erratics of all sizes; the largest, the size of a 

 cottage, is situated just near CroUy Station. 



Along the inland margin of the plain, thousands of boulders have been 

 precipitated in tumultuous confusion. They also characterize this fall-line in 

 the southern areas, e.g. from Glenties to Letterilly aud along tlie south-east 

 side of the Gweebarra valley. They are to be ascribed iu part to the quarrying 

 of blocks along the margin of the plain where the steep face of the well- 

 jointed granite greatly facilitated such operations by the ice, in part to 

 morainic deposition in late-glacial times, when glaciei-s proceeded along the 

 coastal plains. 



The drift is very thin, but in its composition amply confirms the testimony 

 of the striations and rocJics moutonne'es, as to the direction of ice-flow. Thus 

 quartzites and diorites are practically absent south of Bun beg, where the 

 drift is almost exclusively gi'anitic. At Bunbeg, metamorphic erratics begin 

 to put in an appearance aud become increasingly numerous as we proceed 

 northwards over the Bosses. This distribution is due to the fact that only 

 here do we pass into the track of the erratics from the quartzite and 

 metamorphic hills of Tievealehid and Crocknafarragh. This line, marking the 

 western limit of these metamorphic erratics, is inserted on the small map 

 (fig. 1 1. I'he Bosses country has therefore sufiered considerable erosion and 



