Charleswoeth — Glacial Geolog/j of NorUi-West of Irelund. 213 



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degradation, its surface having been doubtless lowered, not so much by 

 abrasion as by the plucking of the granite blocks, which now lie profusely 

 scatterod over the surface, and doubtless over the adjacent floor of the Atlantic. 



The thousands of lakes and lakelets have undoubtedly tlieir origin in 

 glacial erosion, 1 acting along master-joints of the granite ; even the origin of 

 those dotted over the surface of the moorland parts of the region is to be 

 sought in this erosion, as drift is extremely thin. 



The southern and eastern strips of Aran Island, which are composed of 

 granite, exhibit tlie characteristic features of the Eosses, in particular the 

 persisteiice and constancy of direction of the valleys, and the smoothed and 

 glaciated rock surfaces. Huge granite boulders, derived from the granitic 

 tracts of the island and of the mainland, lie scattered in countless numbers 

 over the remaining quartzite part of the islarrd. Some of these boulders are 

 of very great size, even up to 35 feet in length, the largest perhaps occurring 

 near Ballintra in tlie south-east of the island. This north-west transport of 

 the erratics is confirmed by the direction given by the striae, the rochcs 

 moutoniidcs, and. the banking of drift against the south and south-east slopes 

 of the hills. These have caused the lower layers of the ice to suffer some 

 deflection, so that along the southern face of the hills the currents swept prac- 

 tically east-west, and on their eastern face slightly east of the general trend. 



The numerous and large boulders of granite \vhich are scattered over the 

 quartzite hill of Tievealehid, prove most conclusively ice-transport down the 

 depression of Duulewy and Lough ISTacung, and the overriding of the whole of 

 the mountainous mass of Tievealehid and the adjacent hills by ice from the 

 Glendowan Mountains. 



The large islands of Inishbofin, Inishdooey, and Inishbeg lying off the 

 coast were overridden by a northward-moving ice-sheet. In places favourable 

 to the deposition of drift, this has accumulated to considerable depth, as much 

 as 30 feet having been observed on these islands. 



At the western end of Muckish, striae were observed on th.e lower slopes 

 going N. 60'^ W. and even N". 30° W., indicating a swinging of the lower 

 currents of the ice out of the Eay valley, where they were no longer liemmed 

 in between Muckisli and Crocknalaragagh. When the ice had fully escaped 

 from the constraining influence of the hills and had swept out into the open 

 plain it flowed almost due north, as is shown by tlie striae, bearing S.-N. 

 and N. 10° W., and by the distribution of boulders of " porphyry " that occur 

 only to the north of the outcrop. 



■ The.se joint lakes produced by glacial ero.sion are encountered iu other granitic areas, 

 e.g. L. Errig and L. Fad in the granite country between the Gweebarra and Shallogau 

 valleys, Kindrum L., and the Glenmacannive L. (one-and-a-half miles N.-E. of Glenties), 

 though the latter is in part held up by a slight accumulation of drift at its lower end. 



