THE 



LONDON and EDINBURGH 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



DECEMBER 1835. 



LI I. On the Theory of the Parallel Roads of Glen Roy, By 

 Sir G. S. Mackenzie, Bart. F.R.SS. L. $ E. %c. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



r I ''HE memoir by Sir Thomas Lauder Dick, published in 

 -■• the ninth volume of the Edinburgh Transactions, has left 

 the theory of the parallel roads in Glen Roy, &c. incomplete 

 in only one particular, that of finding a barrier to form lakes. 

 After assuming that a barrier had existed, the nature of what 

 it consisted of presents a difficulty, and the mode of its re- 

 moval another ; for not the slightest trace of a barrier of any 

 sort is to be found. After a conversation with Sir David 

 Brewster on this subject in the autumn, when a party to visit 

 Glen Roy was proposed, a thought came across me while re- 

 flecting on the difficulties of the case, which removed all re- 

 regret that I had never examined these phenomena; and 

 I have deferred my visit, in order that I might have an oppor- 

 tunity of submitting to others the completion of the theory as 

 propounded by Sir Thomas Lauder Dick, and of considering 

 such objections to it as may be made ; so that I may be able, 

 when I do visit the spot, to observe to what extent they may 

 be well founded. Another reason for my submitting at pre- 

 sent what has occurred to me as the true origin of what has 

 remained so long a geological puzzle, is that others may ex- 

 amine various glens and valleys in Scotland with the view to 

 ascertain (as I shall not probably have it in my own power to 

 do so soon) whether any of them present conditions in their 

 Third Series. Vol. 7. No. 42. Dec. 1835. 3 K 



