Mr Milne on the Parallel Roads of Lochaber. 341 



scribed by former writers, I shall limit myself to points on which I 

 have obtained new information, or with regard to which doubts have 

 been expressed. 



1. One of the points of the class last referred to, is the absolute 

 horizontality of the shelves. Mr Darwin, referring to Sir Thomas 

 Dick Lauder's observations on this point (p. 76.), hints at the possi- 

 bility of errors and omissions in the calculation. M. Bravais, in his 

 paper on the lines of former sea-level in Finmark, suggests, *' that 

 an accurate geodetic levelling should be applied in the case of the 

 doubtful lines in Scotland,'' evidently refering to Glen Roy. Mr 

 Horner, the president of the Geological Society, in his last year's 

 address, observes ; *' Mr Darwin's explanation of the parallel roads 

 of Glen Roy, that they are ancient sea-beaches, appears to be now 

 generally accepted ; and it would be most interesting, if it were 

 ascertained by exact levellings, such as those of M. Bravais, whether 

 they really are 'parallel.''^ Similar doubts had been expressed by 

 Sir R. I. Murchison, Mr Horner"'s predecessor, in his annivei*sary 

 address of 1843 ; in support of which, he refers to the concurrent 

 opinion of M. de Beaumont and Professor Phillips. 



In accordance with the doubts expressed by these authorities, the 

 Geological Section of the British Association, at their last meeting, 

 agreed on an address to Her Majesty's Government, requesting them 

 to cause the parallel roads of Lochaber to be examined by the officers 

 of the Ordnance Survey, to ascertain their supposed horizontality. 



I have no doubt that the result of this official survey, if made, 

 will be to establish the absolute horizontality of the shelves. In 

 August 1844, Mr D. Stevenson, at my request, was so obliging as 

 to examine them, and the conclusion at which he arrived, is explained 

 in a letter to me, from which I make the following extracts : — " I 

 have had a number of levels taken, the particulars of which I shall 

 give you afterwards. The result^ I think, leaves no doubt as to the 

 perfect horizontality of the ' roads.'' The glen is much more ex- 

 tensive, both as regards length and breadth, than I anticipated, and 

 the height of the roads above its bottom is also very considerable, 

 and any thing like a series of cross sections, referred to the same 

 datum, would be a work of very great magnitude ; a month, I should 

 say, would not complete it. The whole we have been able to do, 

 therefore, is to test the uniformity of the levels of the different 

 roads, by viewing them with a good instrument from several points, 

 as was done by Sir Thomas Dick Lauder ; and, in addition to this, 

 a section was made along the middle road, where it is pretty well de- 

 fined from Glen Turret downwards, for a distance of nearly 3^ 

 miles, and throughout that stretch, the road was found to be per- 

 fectly honzontal.'^ ... "If I had seen that any thing furtlier could 

 be done, I would have left my assistants for a few days longer ; they 

 wore there a week." 



