SoLLAS — A 3Iap to show the Distribution of EsJcers in Ireland. 801 



moraine-covered ice-cliffs. The streams at once deposit the coarser portion of 

 this, thus building up their channels and obstructing the outlets of the tunnels. 

 The blocking up of the tunnels must cause the subglacial streams to lose force, and 

 deposit sand and gravel on the bottom of the channels ; this causes the water to 

 flow at higher levels, and coming in contact with the roofs of the tunnels, enlarges 

 tliem upwards ; this, in turn, gives room for additional deposits within the ice, as 

 the alluvial cones at the extremities of the tunnels gi'ow in height. In this way 

 narrow ridges of gravel and sand, having perhaps some stratification due to 

 periodic changes in the volume of the streams, owing to seasonal changes, 

 may be formed within the ice. When the glacier melts, the gravel ridge contained 

 within it will be exposed at the surface, and as the supporting walls melt away, 

 the gravel at the top of the ridge will tend to slide down, so as to give to the 

 deposit a pseudo-anticlinal structure. Ridges of gravel deposited in tunnels 

 beneath the moraine-covered portion of the Malaspina glacier would have boulders 

 dropped upon them as the ice melts ; but where the glacier is free fi^om surface 

 debris there would be no angular material left upon the ridges when the ice 

 finally disappeared." 



Such is the author's explanation of the formation of eskers. He adds : — " The 

 process of subglacial deposition pertains especially to stagnant glaciers of the 

 Malaspina type, which are wasting away. In an advancing glacier it is evident 

 that the conditions would be different, and subglacial erosion might take the 

 place of subglacial deposition." 



Warren Upham briefly recurs to the subject of eskers in discussing the 

 " Criteria of Englacial and Subglacial Drift,''''* and maintains his previously expressed 

 views in favour of the deposition of eskers within ice-caiions. 



In an interesting account of the sand-plains, eskers, and kames of the Auburn- 

 dale district, ten miles east of Boston, Mr. W. M. Davisf shows how these different 

 features are genetically connected, and attributes the eskers to subglacial streams, 

 without denjang that in other districts they may have a superglacial origin. 



Mr. Warren UphamJ supports the view that much of the glacial drift was 

 carried within the substance of the ice, and cites Bird's Hill esker, near Winnipeg, 

 as proving that englacial drift had been carried up from a nearly level country to 

 a height of more than 500 feet in the ice-sheet. 



Professor Salisbury§ describes eskers which have been mapped by Professor 

 Culver. They are ascribed to streams flowing from the ice-sheet. 



*Tlie American Geologist, vol. viii., 1891, p. 380. 



f The Suhglacial Origin of Certain Ushers, by W. M. Davis. — Proo. Boston Soo. Nat. Hist., vol. xxv., 

 p. 477, 1890-1892. 



% On Englacial Drift, by Warren Upham : The American Geologist, vol. xii., p. 36, 1893. 

 ^Ann. Report Geol. Survey, N. J., Trenton, 1893. 



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