GLACIEllS IN GREAT BRITAIN AND IRELAND. 145 



parallel to what I have elsewhere shown to be the neces- 

 sary course of a tenaceous mass of ice urged by gravity 

 down a valley, as anywhere in the Alps."* 



Before taking leave of this district, I may observe that 

 Professor Agassiz is of opinion that the parallel roads of 

 Glenroy, near the foot of Ben Nevis, are attributable to a 

 lateral glacier having been projected across the valley, 

 near Bridge K,oy, and another across the valley of Glen 

 Speane. 



By this means glacier lakes were formed, along whose 

 margin the stratified terraces of gravel were produced 

 which are now seen to line the flanks of the valley at a 

 perfectly horizontal level through several leagues. The 

 subsequent melting of the glaciers has entirely obliterated 

 any traces of the agent by means of which the waters were 

 pent up.f Mr. Darwin, however, takes a different view of 

 the subject, considering that the parallel roads are marine 

 terraces, formed during the submergence of the land to a 

 depth of 1250 feet, their present elevation. J 



Having thus made a rapid circuit of the Scottish High- 

 lands, I must now hasten to a close. I think, however, it 

 is only due to so great authorities as Professor Agassiz 

 and Dr. Buckland, that I should state that they appear to 

 have considered "not only that glaciers once existed in 

 the British Islands, but that large sheets of ice {nappes) 

 covered all the surface of the districts surrounding the 

 Highland groups." This opinion is founded on the wide 

 extent to which unstratified gravels, perched blocks and 

 polished surfaces in situ are distributed over the districts 

 adjacent to the centres of distribution. I believe, however, 

 that it is now generally allowed that floating ice, or rather 

 svjimming ice, has played a more important part in produc- 



* Edinburgh Neio PMlosopMcalJ oiirnal, vol. xi. p. 76. 

 t Proceedings of the Geological Society, vol. iii. p. 333. 

 X Lyell's Elements of Oeology, 5tli edition, p. 88. 



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