6 The Irish Naturalist. January, 



have been slightly modified by glacial drift deposits and minor 

 excavation along the courses of the streamlets which drain 

 them. Drainage to the south has produced a fairly well 

 marked trough in the Thorn Chase Valley on the south side 

 of the island. Below the 150 ft. contour on the western third 

 of Lambay, the ground flattens down more gently, forming a 

 smooth saddle-shaped ridge north of the castle, and becoming 

 nearly quite horizontal at the S-W. corner of the island. 



With regard to this last-mentioned locality, a good deal of 

 the ground below the 50 ft. contour presents some features re- 

 calling those of a raised beach. Its elevation is, however, 

 somewhat greater than the modern or Post-glacial raised 

 beach with its deposits of blown-sand, shingle, and shells. 

 The relation of the two, however, is not dissimilar to that of 

 the Pre-glacial and Post-glacial raised beaches of the south 

 of Ireland, so graphically described by Messrs. Wright and 

 Muff ( y ), and seeing that these observers have detected traces of 

 this formation as far north as Bray Head, it is here tentatively 

 suggested, with a full appreciation of the slender evidence on 

 which it rests, that at least a portion of the level platform 

 north of Talbot's Bay, and now covered with Boulder-clay, 

 may probably represent this Pre-glacial raised shore. 



The modifications in the land sculpture of the island pro- 

 duced by the passage of the ice sheet of the Glacial Period 

 appear to have been slight, and confined almost entirely to 

 the filling up and smoothing over of the hollows and irre- 

 gularities in the rock floor produced by ages of atmospheric 

 denudation. 



The maximum effects were produced on the lower ground 

 to the west of the island, where the drift occurs in some 

 quantity, though the whole of Lanibay was almost cer- 

 tainly over-ridden by the main ice-sheet at the period of 

 maximum glaciation, as was also the adjoining higher ground 

 which forms the peninsula of Howth; this is only inferred 

 from the presence of large erratic blocks at high elevations, 

 few other traces now remaining as confirmatory evidence. The 

 geological history, then, of Lanibay opens with the deposition 

 of sands and clays in a gradually shallowing sea in Silurian 

 times. Following the formation of a coralline limestone 

 on these rocks came a period of volcanic activity, with 



