52 Mr Thomson on the Parallel Boads of Lochaber, 



to the phenomena of the terraces themselves, and true prin- 

 ciples of the motion of glaciers, were then wanting. Had these 

 been within the reach of Agassiz, he could easily have modifi- 

 ed his explanation so as to remove all valid objections which 

 have been brought forward against it, and could have shewn 

 the invalidity of others which are still adduced, but which, I 

 think, will not be admitted by those who have duly appre- 

 ciated the principles of the viscidity of glaciers, as developed 

 in the theory of Professor Forbes. The object of Agassiz, 

 however, at that time, was probably rather to adduce the 

 Parallel Roads as confirming his grand idea of the former 

 extensive prevalence of ice in these latitudes, than to enter 

 fully into tlie details of the mode in which the roads had 

 been produced ; and in representing his supposed glaciers on 

 the map which accompanies his paper, his intention was, 

 perhaps, not so much to assert that the glaciers had acted 

 exactly in the way he indicated, as to illustrate the suppo- 

 sition that glaciers, acting in some such way, would be found, 

 in the end, fully to explain all the phenomena. Be this as 

 it may, Mr Milne succeeds in shewing that the explanation 

 by means of the supposed glaciers is inconsistent with ob- 

 served facts. He then goes on to assert, that glaciers could 

 not possibly have penetrated to the places where their pre- 

 sence would actually have been required. This statement, 

 of course, constitutes the turning point of the whole ar- 

 gument, since, if it were correct, it would overthrow the 

 glacial explanation. I hope, however, to be able, in what 

 follows, to give good reasons against its soundness ; but, in 

 the mean time, it will be necessary to advert to the facts 

 which invalidate, in its details, the explanation given by 

 Agassiz. 



Previously to the researches of Mr Milne, it had been 

 known that there exist three " summit-levels^'' or " water- 

 sheds,"' in connection with three of the Parallel Shelves ; but 

 the existence of a fourth had not been noticed, and it had 

 even been asserted by Mr Darwin, that " the middle shelf of 

 Glen Roy is not on a level with any water-shed." Mr Milne 

 has, however, found the wanting water- shed in Glen Glaster, 

 a small glen which, though branching up from Glen Roy near 



