4 The Irish Naturalist. January, 



and published some drawings of the porphyritic felspars as 

 they appeared in thin sections under the microscope. The 

 most recent and most detailed account of the geology of the 

 island was published in 1898 in the Quarterly Journal of the 

 Geological Society of London, and sets out the results 

 obtained by Messrs. Gardiner and Reynolds ( 8 ) during a short 

 holiday spent on the island in 1897. This last account, beyond 

 bringing up to date certain details concerning the rocks 

 (especially the igneous ones, which the authors were the first 

 to study seriously), does not differ very materially from the 

 work of Du Noyer. Both accounts have been drawn on when 

 necessary in preparing the matter which now follows. 



The writer, in the course of several visits to the island, has 

 prepared a drift map of Lambay for the Geological Survey of 

 Ireland (Plate 3), which is now published for the first time, 

 by permission of the Director. Jn conjunction with it is a 

 solid geology map embodying the results of previous workeis, 

 with a few minor alterations and additions which were 

 necessary in view of further evidence obtained recently. 



Geological History and Physical Geology. 



Geologically speaking, Lambay is of considerable antiquity, 

 consisting, as it does, of rocks referable to the older Palaeozoic 

 formations, and though rocks more recent than the Old Red 

 Sandstone may once have buried the island beneath them, 

 no traces of any formation belonging to the period of time 

 intervening between the Devonian Period and the Glacial 

 Epoch now 7 remain. 



It is, however, very probable that rocks of Carboniferous 

 age once covered the area now occupied by the island, as 

 they have covered the geologically contemporaneous rocks at 

 Portrane, and their subsequent denudation has resulted in 

 the isolation from the mainland of this portion of a once 

 continuous mass of Silurian rocks. It is a matter for specu- 

 lation as to whether any of the formations so well preserved 

 for us under the plateau basalts of Co. Antrim were deposited 

 over the Carboniferous limestone as far south as Dublin, 

 though there is nothing inherently improbable in the idea. 

 Should such have been the case, we may regard the present 



