396 MR MILNE ON THE PARALLEL ROADS OF LOCHABEK. 



Mr Darwin's theory inadmissible, and to point out the manner in which, as it 

 appears to me, that the lakes were drained, — not as supposed by Dr MacCulloch 

 and Sir Thomas Dick Lauder, by convulsions of nature, but by the gradual ope- 

 ration of ordinary causes. 



Though it is the principal object of this paper to account for the formation of 

 the Lochaber shelves, there are no views regarding them which can be suggested, 

 which have not a more general bearing, and the soundness of which may be tested 

 by evidence supplied from other sources. Former writers, accordingly, and espe- 

 cially Mr Darwin, have felt it to be necessary, after giving their explanation of 

 the parallel roads, to shew, that the principles on which it rests, are, at least, not 

 inconsistent with any established truths in other branches of geology. 



I shall not shrink from subjecting the Lake theory, which I have to submit, 

 to a similar ordeal ; and the more so, as I feel satisfied that it receives great sup- 

 port from geological considerations now held to be well established. 



As the whole details of the parallel roads have been fully described by for- 

 mer writers, I shall limit myself to points on which I have obtained new informa- 

 tion, or with regai'd 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 possibility of errors and omissions in the calcu- 

 lation. M. Bravais, in his paper on the lines of former sea-level in Finmark, sug- 

 gests, " that an accurate geodetic levelling should be applied in the case of the 

 doithtful lines in Scotland." evidently referring to Glen Roy. Mr Horneh. 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 

 thev really are parallel^ Similar doubts had been expressed by Sir R. L Mur- 

 CHisoN, Mr Horner's predecessor, in his anniversary 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, agi-eed 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. Steven- 

 son, 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 tbllowing 

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



