J. A. Birds- - On the Isle of Man. 85 



submergence in the, sea, and due, partly to the action of the sea 

 itself by tides and waves, partly to rainfall and an accumulation of 

 snow and ice upon the land, combined with the most effective cause 

 of all, the grinding of coast-ice swept along by violent currents. 



If, referring to the diagrams at p. 83, it be asked why the same 

 kind of deposit was not formed .in the later periods, when the land 

 was at the same height, as in the first submergence, — that is, why 

 the Upper Boulder-clay is not precisely like the Lower Boulder- 

 clay, — I can only suppose that there were other conditions besides 

 the degree of elevation above the sea which would account for the 

 difference, and, among these, perhaps the chief would be the 

 enormous extent and mass of ice in the first period, forming a thick 

 continental sheet, like that of Greenland now ; while in the second 

 period the ice was much more partial, and subject to disturbance 

 which broke it up, and gave rise to "packing" and drifting. We 

 must also remember that a portion of the sands and gravel of the 

 previous deposits would be mixed with the later Boulder-clay — 

 whence came its loamy character, and a proportion of, or perhaps all, 

 the foreign rocks now found in it. 



At the end of his " History of the Isle of Man," Mr. Cumming 

 has given several sections, showing the Boulder-clay and Drift- 

 gravel, in all of which he represents the former as extending under- 

 neath the latter. 1 My belief is that in this representation the Upper 

 Boulder-clay, near the base of the mountains, has been confounded 

 with the Lower Boulder-clay exposed in the coast cliffs, as at Point 

 Oanstal, Blue Point, Turby Head, and Kirk-Michael in the north of 

 the island, and perhaps also at Hango Hill in the south. Instead, 

 therefore, of Mr. Cumming's division into a Boulder-clay and Drift- 

 gravel, I would propose another into — 



A. Newer Glacial Formations. 



Upper Boulder-clay, containing almost exclusively local 

 rocks, angular, or very slightly rolled, with occasional beds 

 of sand and gravel. 



B. Older Glacial Formations. 



1. Stratified sands and gravel, containing an abundance 

 of foreign (English, Scotch, and Irish) rocks well rolled. 



2. Lower Bowlder-clay, with patches of sand and gravel, 

 containing a small proportion of foreign rocks. 



In a word, I believe, from my own observation, that the con- 

 clusions as to the order of the glacial deposits adopted by the 

 Geological Survey for the north-west of England and for Scotland, 

 and announced by Prof. Hull 2 as holding good for the east of Ireland, 

 are true also for the Isle of Man, 



1 See " History of the Isle of Man," by Eev. J. G. Cumming, plate viii. 



2 See an article " On the General Relations of the Drift Deposits of Ireland 

 to those of Great Britain," by Edward Hull, F.R. S., etc., Director of the Geo- 

 logical Survey of Ireland, Geol. Mag. 1871, Vol. VIII. p. 294. 



