112 The Irish Naturalist. February, 1907. 



We therefore look for a Post-glacial land connection. The 

 channel which separates Lambay from the mainland is two 

 to thiee miles wide. Its depth is tolerably uniformly six to 

 seven fathoms. There is no evidence of this being a recent 

 channel. Post-glacial subsidence of sufficient amount to form 

 the channel where formerly there was dry land plays no part in 

 the annals of local geology ; nor are physical conditions such as 

 would account for its formation by marine denudation within 

 that period. It is true that along a line drawn from the Burren 

 Rocks on Lambay to Portrane coastguard station on the main- 

 land a kind of reef runs, on which the maximum depth is 5J 

 fathoms. This is piobably foimed of the hard andesitic rocks 

 which appear on each side of the channel ; in any case, it cannot 

 be looked on as the wreck of a former land bridge. The channel 

 has all the appearance of being a Pre- glacial depression, its origin 

 being possibly very much Pre-glacial. 



How, then, are we'to get our land-connection ? The suggestion 

 which seems to us to offer fewest difficulties involves the former 

 extension seaward, upon the shallow shelf which fringes the 

 present coast-line, of the boulder-clay which mantles the country 

 all along the east coast — material brought southward by the 

 Irish Sea ice, and accumulated along the edge of the land. In 

 many places, as around Clew Bay and Galway Bay, the solid 

 geology of the coast is masked below heavy drift, in the form of 

 promontories and islands. It seems quite possible that the 

 passing away of the Ice Age left the ground which is now occupied 

 by the shallow waters off the Dublin coast covered with a mantle 

 of drift, the surface of which was above sea-level, and over which 

 animals and plants might migrate freely. Post-glacial marine 

 denudation, which could hardly be called on to cut the present 

 Lambay channel out of the solid rocks which may at one time 

 have occupied it, would, it may be granted, have sufficed to 

 remove the same amount of drift, and to erode this softer material 

 back to the Pre-glacial coast-line. That many species of plants 

 and animals have, since its final isolation, reached Lambay by 

 the agency of wind, and of sea, and directly or indirectly by the 

 act of man, is of course evident. 



