236 The Irish Naturalist. October, 



The term *' Arctic" here inchides species which are extinct 

 in the British seas at the present day, but have an Arctic and 

 Scandinavian range. *' Northern" includes those which 

 have an Arctic and British habitat, but do not range south of 

 Britain. It will be seen from this table that while the number of 

 species collected in the various stations varies greatly — owing 

 partly to deficient collecting— there is a persistent proportion 

 of the shells exotic, with an arctic habitat at the present day. 

 Astarte borealis — a typical arctic species — occurs in every 

 deposit from sea-level up to Glendoo Wood. Lcda perjiula — 

 another tj^pical glacial species — occurs at sea-level and at 

 three localities between 1 50 and 650 ieet. When the species 

 having a northern range at the present day are added, the 

 boreal aspect of the fauna becomes very clear, and as this 

 character remains constant from sea-level to 850 feet, the con- 

 tinuity of the deposit may — provisionally at least — be assumed. 

 A great amount of work at erratics and shell-history remains 

 to be done, and this may modify this conclusion in certain 

 respects, but it is doubtful if the general broad uniformity of 

 the fauna of the high-level and low-level Drift deposits will 

 be overturned. 



As this paper deals merely with a possible correlation of 

 the high and low-level drifts by a comparison of their respec- 

 tive faunas, any remarks on the probable mode of formation 

 of the deposits in general may be out of place ; but the follow- 

 ing suggestions are offered. With the advance of the Irish 

 Sea Glacier the Boulder-clay was laid down to a height of over 

 1,200 feet on the Dublin hills, the contained .shells being car- 

 ried as erratics along with the limestone, Chalk flints, and 

 mica-schists. At the close of the Glacial period, when the 

 retreating ice-sheet would form lakes by damming back the 

 drainage from the hills, the sands and gravels were probably 

 tormed; the decaying ice-sheet supplying the water necessary 

 to erode the Boulder-clay and supply materials for the forma- 

 tion of the stratified deposits. The torrential nature of the 

 earlier stages of this period is very well shown by the boulder 

 bed of Larch Hill, where the deposit has the appearance of an 

 immense shingle bed thrown up by a river in full flood, the 

 granite boulders being probably supplied from some source 

 quite near at hand. 



National lyibrary, Dublin. 



