43 1 Sir G. S. Mackenzie on the Theory of 



form and relative positions similar to the glens of Lochaber, 

 but without any appearance of parallel terraces. 



The subject is extremely interesting; for there is doubtless 

 harmony between cause and effect ; and there can be no phe- 

 nomenon in nature ascertained to be the effect of a known 

 cause without its being possible to discover how that cause 

 was brought into operation in reference to the effect. The 

 phenomena of Glen Roy are probably the only ones in ex- 

 istence where the effect at once proclaims the cause, but which 

 at the same time have for a long period wrapped in obscurity 

 the modus operandi. If my conjecture shall not prove a happy 

 one, it will, I hope, have at least the effect of reviving a sub- 

 ject of interest to geologists, and leading to more exact reflec- 

 tion upon it. 



I must begin by recalling attention to the fact, that a vast 

 body of water has, at a remote period, swept over the face of 

 this part of the earth, in a direction ascertained by numerous 

 facts to have been nearly from north-west to south-east. To 

 whatever cause this great debacle may be attributed, I assume 

 the fact of its having occurred, in order to explain, if possible, 

 the phenomena of Glen Roy. Although the mode in which 

 I conceive them to have been produced is before my mind in 

 a lively manner, so that I almost imagine 1 see the operation 

 going on, I feel, it is confessed, some fear lest I should not 

 succeed in making it intelligible to others. To those who 

 have seen the parallel roads I hope my attempt will not be 

 altogether unsuccessful. 



On referring to the map accompanying Sir Thomas Lauder 

 Dick's memoir, it will be observed : 



1. That the great glen through which the Caledonian canal 

 passes has its direction N.E. and S.W., at right angles to 

 the direction in which the waters of the debacle flowed when 

 they covered the mountains. 



2. That the glens in which the phenomena of the parallel 

 roads are to be seen have their openings into this great glen 

 facing the N. W., that is, the current of the debacle. 



3. That Glen Gluoy and Glen Roy are bent, as it were, 

 from their openings, of which one is directly into the Great 

 Glen, the other into Glen Spean, which is also bent, but not 

 to such a degree. The basin of Loch Treig is bent in the 

 opposite direction. 



I must here remar]#, that until careful levels be taken, and 

 sections made, some appearances that may lead to the state- 

 ment of objections to the theory I am about to propound can- 

 not be held to affect the question. Relative levels, the ele- 

 vation of the summit levels all round the district, the com pa- 



