96 The Irish Naturalist. May, 1911. 



Irish Mammals. — A Correction. 



Mr. R, Warri'ii writes to correct a bli]) in his note on the Wild Cat 

 published in last month's issue (supra, p. 80). The Arctic " Marmot " 

 mentioned should, of course, have been the Arctic Lemming {Cunicu/us 

 torquatm). 



GEOLOGY. 



A Post-Glacial Lake Deposit. 



Last summer I had an opportunity of investigating a shell marl which 

 occurs in a hollow to the south of the " white limestone " escarpment at 

 Megaberry, near Moira. My attention w^as hrst drawn to this by Mr. 

 John Hull who had discovered it whilst engaged in draining operations 

 in that part of his farm. 



As the area is now intersected by ditches, and the deposit situated 

 at no great distance from the surface, it was easy to make holes with 

 a spade at different points along these drains and so find the limits of 

 the lake. 



Thirty-five holes wete thus dug, and from the tabulated results the 

 following more generalised statements can be made. The upper deposit 

 varies Irom. 2 to 3 feet in depth, and consists of a dark clay yielding 

 towards the surface to a dark peaty soil. Below this in the centre of 

 the area is the marl deposit. This varies from ^ inch to 10 inches. 

 Towards the margin the marl is replaced by flint and limestone gravel. 

 Blue clay of from 2 to 6 inches in thickness is usually found below the 

 marl, but is absent where the gravel occurs. The lowest layer is a red 

 clay, which in most places is of good quality and suitable for brick making. 

 At several points this is replaced by a red loam. 



The shells found in the marl are common freshwater types — Valvata 

 [cfistatci ?), Limnaea peregya, and Pisidiiim {glaber P) were obtained. 



The general sequence of events would seem to be as follows : — After 

 the removal of the ice which covered the whole of the North of Ireland 

 boulder-clay was left in irregular mounds. Arranged in the form of a 

 semicircle with the limestone escarpment as diameter a natural reservoir 

 was formed. Into this the fine clay was washed down. Then came 

 a period in which molluscs invaded our lakelet. Their broken and 

 comminuted shells mixed with clay formed the deposit of marl. While 

 this was going on the lake sought an outlet where a rift occurred in the 

 curve of mounds at the south-east, and eventually, OAving to the deepening 

 of the bed of_ this issuing stream, the impounded lake escaped to one of 

 the feeders of the Lagan. After this the accumulation of hill-wash still 

 continued, giving us a further deposit of clay, and then bog plants laid 

 hold of the surface and peat was formed. Last of all artificial drainage 

 has greatly- improved the land, the marsh has disappeared and fertility 

 is once more established. 



ISAAC^SVVAIN. 



University College, Cork. " 



