190 Proceedings of the Royal Irish Academy. 



to represent marine deposits of tertiary age, and the shells therefore to have 

 been buried in situ. 



The distribution of these shelly localities is indicated in tlie accompanying 

 map (PI. IX) by the letter S. 



Two ounces of the shell-bearing clay from Eglinton were submitted by 

 the Geological Survey to Mr. Joseph Wright, f.g.s., of Belfast, who identified 

 some, seventeen species of foraminil'era, a list of which is given in the 

 Londonderry Memoir (p. 55). 



The finding of the riebeekite paisanite of Ailsa Craig, by members of the 

 Belfast Naturalists' Field Club at Limavady and at Moys,' is strong 

 confirmatory evidence of a westerly moving ice-sheet, though these erratics, 

 as others of similaiiy easterly origin, may have been slightly removed to 

 their present sites by the subsequent soutli-north glaeiation from points 

 situated higher up the valleys and more south-easterly. No pebbles of this 

 Ailsa Craig rock were discovered by the Geological Survey in the drifts west 

 of these two localities ; nor were my own diligent and deliberate searches for 

 fragments of this very conspicuous rock better rewarded. 



A question of some interest and importance is the possibility of the 

 derivation of these Antrim erratics by a rather more circuitous route from 

 an older flint-bearing drift that lay to the south of the path of the Donegal 

 ice. In my earlier efforts at reconstructing the glacial events of Inishowen, 

 I adopted this view, feeling that the elimination of a glaeiation was due to 

 the principle of economy. The arguments against such a circuitous origin 

 are, however, too weighty. Apart from the insuperable and fatal difficulty 

 encountered at the outset in the total absence of flints, etc., in the drifts of 

 the country to the south, namely, in south Londonderry and north Tyrone, 

 whence they would be supposed on this view to have been derived by the 

 northward-moving ice, the presence of the shells in the drift, and more 

 especially the distribution of this shelly drift, also the occurrence of two 

 totally distinct boulder clays in the country south of the Foyle estuary — an 

 older and lower containing only eastern material, a newer and upper contain- 

 ing only western rocks — militate against this possible source of the flints. 

 Moreover, the quite appreciable decrease of the flints, in size and numbers, as 

 traced westwards across Inishowen, though not necessarily contradictory of 

 this hypothesis, is more in harmony with the view of the transport from a 

 direct eastern source as here presented and as maintained by all the earlier 

 workers in the area. 'J'hus it would seem that the drifts were deposited by a 

 v/esterly moving iee-slieet, and were later removed from the exposed higher 

 ground and preserved only in the deeper and more sheltered glens. 



' Proc. Belfast Nat. Field Club, Ser. ii, vol. vi (1913j, pp. 581, 582, 



