MR DAVID MILNE HOME ON THE BOULDER-CLAY OF EUROPE. 67 



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bergs, the lower portion of which penetrated the sea-bottom, pushing before 

 them boulders and pebbles, and pressing the sediment into greater compactness. 



Besides the generally mutilated condition of the sea-shells in the boulder- 

 clay, there is another circumstance, first pointed out by the late Edward Forbes, 

 which tends in the same direction. On examining the shells found in the drift- 

 beds of Wales, he observed that they belonged to different zones of life. These 

 drift-beds presented " a confused mixture of fragments of species from all depths, 

 both littoral and such as invariably live at a depth of many fathoms ; inhabitants 

 some of muddy grounds, some of sandy, some of rocky. Deep and shallow 

 water species could not have lived together, or have been thrown up on one 

 shore." His conclusion, therefore, was, that this confused and unnatural 

 mixture " indicated the action of some disturbing influence, through the agency 

 of icebergs, or a wave of translation, or of both combined." 



The beds to which this observation applied was, it is true, not boulder-clay 

 or till, but mud, gravel, and sand, " in the lowest beds of which were small and 

 large boulders of transported rocks polished and scored." * The position of these 

 drift-beds was 1360 feet above the sea. 



If icebergs acted on these drift-beds, as Edward Forbes inferred for the 

 reasons mentioned by him, icebergs could have acted in like manner on the 

 materials of boulder-clay. 



Those who look upon the boulder-clay as a land deposit, meet the fact of 

 marine shells being found in it thus : — They say that the glaciers which formed 

 the deposit reached the coast, as now in Norway and Greenland, and pushed 

 detrital matter out into the sea, where it became occupied by testacea. This 

 answer is not satisfactory, because the testacea are almost invariably found in a 

 mutilated state. These animals must have been bred and grown in a sea-bottom, 

 which, whilst they lived, was undisturbed ; and if the disturbance of their dwell- 

 ings was due to the protrusion of glaciers, this would be admitting the marine 

 origin of the deposit. Moreover, if the climate was so severe as to bring glaciers 

 to the coast, icebergs would also abound in the adjoining seas ; so that the 

 question would then be, whether the effects were more likely due to the pro- 

 trusion of glaciers, at the mouths of valleys, or to icebergs drifting in the sea 

 and grating along the bottom. On the former supposition, boulder-clay would be 

 formed only at particular spots, viz., at the mouths of valleys which reached the sea. 

 On the latter supposition, boulder-clay would be formed much more extensively. 

 The great abundance and continuity of the deposit in Northern Britain is there- 

 fore better accounted for by the iceberg than by the glacier hypothesis. 



The advocates of the glacier theory of boulder-clay have also referred to the 

 fact that terrestrial remains occur in the deposit, from which an inference 



* Notes of a Ramble through Wales, by W. S. Symonds, 1864, p. 12. 



