188 Proceedings of the Royal Irish Academy. 



peninsula are indistinguishable from those of like age in the counties of 

 Antrim and Londonderry, it might be supposed that the occurrence of basalt 

 erratics and boulders in the drifts of Inishowen could not be regarded as clear 

 proof of transportation by ice from the east ; but the great rarity of these 

 dykes in Inishowen itself (only four occur in the whole peninsula of over 

 300 square miles, though the possibility of the existence of others concealed 

 by drift and peat may not be lost sight of), together with the undoubted flints 

 and chalk fragments of the drifts, would seem very strongly to suggest an 

 Antrim or TiOndonderry origin for these basalt erratics. 



Though these eastern erratics are universally distributed throughout 

 Inishowen, they show a distinct tendency to decrease in size and numbers, as 

 the drifts are traced westwards from the Foyle to the Swilly — a decrease the 

 more distinctly noticeable if sections in the extreme east and west are 

 compared. This distribution indicates a western transportation. This 

 feature in the distribution is still readily recognizable, despite the ploughing 

 action of the later ice, which must have swept along the earlier-formed 

 deposits and modified the distribution of their erratic ingredients. 



The most south-westerly occurrence of this drift is at the Burnfoot 

 brickfield in the "Pennyburn depression." It is described in the Geological 

 Survey Memoir,^ which records the finding of a well-preserved gri/phaea 

 incurva from the Lias of the north-east.- A very fine section in drift of 

 similar composition is also to be seen in the Low Moor Eoad quarry, 

 Londonderry City, and is described in the same Memoir (p. 38). 



The Scottish drifts of Inishowen, as indeed wherever they occur within 

 the limits of the country investigated in this work, have been churned up 

 and greatly modified by the later ice-sheet from Donegal, which has added its 

 quota of erratics to the boulder-clay. In the area lying immediately to the 

 south of the Foyle estuary, however, a pure, unmixed eastern boulder-clay is 

 still preserved in places. Its occurrence is restricted to the lower parts of 

 the valleys where it was more or less shielded from the ploughing action of 

 the later northward-moving ice. In all the localities where this boulder-clay 

 is to be found it possesses similar characteristics, being a highly calcareous 

 and argillaceous deposit, derived largely from the disintegration of chalk and 

 basalt. The enclosed pebbles and fragments are chiefly of these two rocks, 

 with great numbers of chalk-flints and a few pieces of mica-schist. It is 

 exposed in the banks of the Sheskin river at Eglintin, and in the lower part 

 of a steep face in the banks of the same river, some three quarters of a 

 mile south of Faughanvale Bridge ; in the Gresteel Burn, where the sections 



' Geology of the Country around Londonderry, 1908, p. 29. 



