50 Mr Thomson on the Parallel Boads of Lochaher. 



and the second by Sir George S. Mackenzie. The new and 

 very interesting discoveries which have lately been made by 

 Mr Milne in his researelies among the hills, are brought for- 

 ward by both writers in confirmation of their respective theo- 

 ries. These discoveries, however, when taken in connection 

 with the highly important principles of the motion of glaciers 

 recently developed by Professor Forbes, appear to me to be 

 far more strongly confirmatory of the leading features of the 

 explanation given by Agassiz ; at the same time that they 

 enable me to develop this explanation more fully, modifying 

 and correcting it in some degree, so as to make it accord 

 with the new facts and principles, and thns putting it in a 

 form in which, to me at least, it appears so satisfactory as 

 to leave scarcely the slightest doubt of the agency of ice in 

 the formation of the Parallel Roads. 



Mr Milne's paper may be regarded as consisting pri- 

 marily of two parts, — the object of the one being to prove 

 that the terraces are the beaches of lakes whicli have been 

 maintained among the hills by barriers occupying tlie lower 

 parts of the glens ; and the object of the other, to shew that 

 these ban'iers consisted of earthy detritus, and to explain 

 the way in which he thinks they may have been formed, and 

 subsequently removed. 



His explanation differs from those given by Dr MacCulloch 

 and Sir Thomas Dick Lauder in 1817 and 1818, principally 

 in his attributing the removal of the barriers not to any vio- 

 lent convulsions of nature, but to the gradual operation of 

 existing causes. These, if I fully understand his statements, 

 he supposes to be the erosive action of the waters of the lakes 

 themselves, combined with that of rivers and sti'eams. On tliis 

 subject he says — " My explanation of the Lochaber shelves 

 depends entirely on the supposition that the valleys were, in 

 the lower parts of them, filled up with detrital matter ca- 

 pable of being gradually worn down and washed away." Sir 

 George S. Mackenzie, although there is much of his reason- 

 ing which I do not consider satisfactory, appears to succeed 

 completely in confuting tlie explanation given by Mr Milne, 

 so far as it depends on the supposed existence of earthy bar- 

 riers. On the other hand, Mr Milne proves, I think beyond 



