1912-13] 647 



Amongst many proofs which may be given of the conclusion 

 arrived at many years ago, more or less generally accepted since, 

 of a westerly ice-flow over Ulster,* I may instance, at the outset, 

 the occurrence of Ailsa Craig erratics in the Roe Valley. Through 

 the assiduous labours of Madame Christen and Miss Andrews 

 several specimens of the rock have been unearthed at Moys and 

 Limavady, and at Dervock. t 



To Mr. Robert Bell we are also indebted for the tracings of 

 erratics, amongst which he discovered those of Ailsa Craig at 

 Aghalee, and near Randalstown. f 



During the recent Survey of the Drift-deposits of the London- 

 derry region ample evidence was obtained for a considerable 

 inflow of ice, charged with shell-bearing clay and boulders, 

 referred to in detail in the new Survey Memoir of that region. X 

 Of this encroachment from the north or north-east ample corrobo- 

 ration has presented itself some 12 miles inland in the Roe Valley. 



Furthermore, abundant evidence, in the form of glacial stri<z and 

 wide distribution of erratics from the south, is available throughout 

 North Derry, for an extensive northerly ice-movement from the 

 accumulation along Maxwell Close's axis, already mentioned. 



It is obvious that a region which has experienced such vicis- 

 situdes — successive ice-movements in such a variety of directions — 

 must necessarily present as great an admixture of non-local rocks 

 at each point, as submergence and debris-charged ice-bergs could 

 possibly afford. There is this difference between the possible 

 outcomes of the two hypotheses which must not be overlooked, 

 namely, that, in the former case, there is the possibility of classi- 

 fying the drifts, however varied the admixtures in certain places ; 

 whereas in the hypothesis of submergence, and distribution by 

 ice-bergs, it is not easy to perceive the likelihood of that possibility. 



*The hinder part, it may be necessary to explain, being of Scottish ice, 

 and the foremost, probably of Irish ice pushed forward. 

 + See Geological Section Report ante. 

 X " The Geology of the Country around Londonderry," 1908, pp. 54-56. 



