422 MR T. J. JEHU ON 



But cases may occur in which the lowest point of the drift-basin does not lie vertically 

 above the bottom of the old moraine-filled valley. " In such a case the stream would 

 cut down rapidly until it reached the level of the rock, and then, in the majority of 

 instances, the stream would cut sideways along the junction between drift and solid 

 rock until, when the stream reached its original position, the lake would be drained. 

 But if a subsidiary range of rock lay between the position attained by the stream 

 issuing from the lake and the position of the former valley-bottom, denudation would be 

 retarded to so great an extent that the lakelet would become much more permanent, 

 and its depth would be the difference between the height above sea-level of the 

 bottom of the old moraine-filled valley and that of the present exit." Hence it is quite 

 possible that the tarns, though really moraine-blocked, should have their exit over rock, 

 and so this in itself is no proof of the existence of a true rock-basin. To prove con- 

 clusively the existence of a true rock-basin, it becomes therefore necessary to show that 

 there is no drift-filled valley by which the water might have escaped before the filling 

 up with drift took place. An examination of the tarns of Lake-land has convinced Mr 

 Marr that in many cases, and possibly in all, a valley of this kind did exist. 



In 1896 Mr Marr published a paper in the Proc. Geol. Assoc, "On the Lake 

 Basins of Lake-land" (vol. xiv. p. 273). It is there argued that Dr Mill's detailed 

 observations on the lakes are antagonistic to the theory that the lake-basins owe their 

 formation to ice-erosion. Mr Marr remarks that " the subaqueous scenery of the lakes 

 presents several difficulties, on the supposition that they are formed by ice-erosion. Each 

 of these difficulties may not be absolutely fatal to the theory in itself, but taken 

 together, they seem to me to furnish a mass of evidence which cannot be got over. 

 On the contrary, the scenery can be readily accounted for on the supposition that the 

 lakes are due to the damming-up of river-eroded valleys, which have had a certain 

 amount of material deposited upon their floors, both before and after the process of 

 conversion into lakes." He gives instances of drift-filled valleys in England, one of 

 which occurred in the outskirts of the lake-district itself, in the Furness district, where 

 there is an indication of a buried valley at least 450 feet below sea-level. He proceeds 

 to consider separately each of the lakes surveyed by Dr Mill, and argues that in no 

 case can we exclude the possibility of a drift-barrier sufficient to account for the exist- 

 ence of that lake, and that consequently there is no proof of the existence of rock-basins 

 in Lake-land. 



*3. Wales. — No systematic survey of the fresh-water lakes of North Wales had up 

 to this time been carried out. It is true that the late Sir Andrew Ramsay, in his 

 memoir on " The Geology of North Wales," stated that the Rev. W. T. Kingsley had 

 been sounding the lakes of North Wales with a view of proving them to be true rock- 

 bound basins, but the results of this work have never been published. 



Sir Andrew Ramsay, whose name is associated with the theory of the glacial erosion 

 of lake-basins, has referred to many of the lakes of Snowdonia and Eastern Carnarvonshire 

 in his well-known work on The Old Glaciers of Sivitzerland and North Wales, where the 





