1905-1906.] 323 



also vastly simplified, resolving themselves chiefly into the 

 transport of erratics as indications of the directions of ice-flows ; 

 the presence or absence of marine organisms' remaining an 

 important factor in differentiating between ice-sheets of inland 

 and transmarine origin. 



Let us pass in imagination backwards over an unknown but 

 vast number of centuries to a period when Arctic conditions 

 prevailed over Northern Europe, and mighty confluent fields of 

 ice slowly travelled onward, traversing the North Channel, the 

 Clyde, and the Irish Sea, grinding out deep troughs in the sea 

 floor where such an obstacle as Rathlin Island obstructed the 

 lower layers, enveloping it, Ailsa Craig, and the Isle of Man, 

 overflowing eastward into England, and entering Ireland west- 

 ward over the noble promontory of Fair Head, the first in our 

 list of localities recorded in this summary. 



The iceworn summit of Fair Head is strewn with erratics 

 from many points of the compass, and strolling further south 

 we observe many large blocks resting on the cliffs that encircle 

 Murlough Bay. Noticing one of schist we ponder on its origin. 

 Was it torn from a bed of similar rock on the shore many 

 hundred feet below, or was it borne on ice 20 miles across the 

 sea from similar rocks in Cantyre, whose fragments are scattered 

 over Rathlin ? Glancing at the tiny handful of pebbles 

 gathered at random, over Fair Head, which include a small, 

 rounded fragment of Ailsa Craig, which lies due east, one from 

 a point near Torr Head to> the S.E., a bit of chert, old red 

 conglomerate, and a peculiar smooth-looking green felsite from 

 Scotland or North Antrim, we see the significance of studying 

 the distribution of erratics and the important deductions to be 

 drawn from their presence on these Ballycastle headlands 



Our summary includes 36 localities, and we will follow the 

 course of the intrusive ice southwards, first considering the dis- 

 tribution of erratics from Ailsa Craig which are much worn, 

 indicating severe ice action, even some specimens dredged by 

 Mr. Welch 40 fathoms deep off Rathlin were ground completely 

 smooth. It occurred at 26 out of our 36 localities, has been 

 found round our shores by Mr. Welch at Portrush in sandhills, 



