Chaulesworth — Glacial Geology of iVorih- West of Ireland. 187 



V. — The Eaelier Scottish Glaciation. 



Three move or less distinct glacial episodes are distinctly recognisable in 

 the region under review ; the first, au early invasion of the eastern parts by 

 Scottish ice, advancing from the east, followed by a severe glaciation of the 

 whole area by ice proceeding from the Donegal Highlands, and lastly a retreat 

 of the local ice and a re-advance of the eastern ice. These successive phases 

 will be described in the order of their occurrence. 



Dr. A. li. Dwerryhouse has shown tliat the Scottish ice invaded the 

 north-east of Ireland, and, transporting the rocks from those parts of Scotland 

 which lay in its path, both from the mainland and the adjacent islands, 

 proceeded in a general south-westerly direction over the counties of Antrim 

 and Londonderry. Further south, it progressed in a more southerly direction 

 to the ilourne and Carlingford Mountains, overriding these hills a-nd passing 

 out to sea. 



The whole of that country examined by him was co\^ered by the Scottish 

 ice, and the limits^ of the invasion are therefore to be sought in the region 

 dealt with in this communication. 



There are two quite distinct areas of Scottish or eastern drift in this 

 country, one situated in Inishowen and to the north of the Sperrin Mountains, 

 the other to the south of these mountains in the counties of Tyrone and 

 Mouaghan. 



The evidence for this earlier Scottish glaciation is furnished by the 

 presence in the drifts of erratics referable to Antrim or eastern sources, 

 e.g. chalk-flints, chalk, and basalt, and occasionally to some Scottish parent 

 rock, as the striae of this phase ha^'e, except in a few instances, been effaced 

 by the later ice proceeding from the south and south-west. 



Flints are very common in the drifts of all the valleys of Inishowen, and 

 may be found on almost any surface on the intervening tablelands and moors. 

 In the Bredagh river, an echinoderm was found loose in the drift. In the 

 west of the peninsula, flints up to 4 inches in diameter and associated with 

 occasional boulders of basalt were obtained iu many places in the thick drifts 

 of the Owenerk, Crana, Mill, and Owenkillew rivers. They were even 

 obserxed iu the Lough Swilly area, namely in ihe fine sections in the streams 

 draining the western flanks of Piaglitin More. 



Fragments of chalk, though generally distributed, are far less plentiful. 

 " Chloritic Chalk " was found in a few places in the east of the peninsula, e.g 

 in the Bredagh Glen, and in the stream W. of Greencastle. 



Basalt boulders and pebbles are not uncommon, especially in the drifts of 

 east Inishowen. As the tertiary basalt dykes traversing the rocks of this 



