664 
PROFESSOR PRESTWICH ON THE ORIGIN 
still exists on some of the theoretical questions.* The main physical features have, 
however, been so carefully and well noted, that they may now mostly be taken as 
admitted. 
In explanation of the general problem, I accept the lake theory of Macculloch and 
Dick-Lauder, and the glacial theory of Agassiz. I cannot, however, agree with the 
expositions of those theories, whether in respect to the mode of formation of the 
“ roads,” or in regard to the precise age of the lakes, which have been of late years 
very generally held. I therefore desire to place before the Society my grounds of 
dissent, and to submit the views which I have been led to form by a visit made last 
summer to the remarkable district of Lochaber and from the general consideration of 
the subject. I will first briefly notice the various theories that have been previously 
advanced, and mention what, as it appears to me, are the objections to them. 
§ 1. The Detrital and Marine Theories. 
The masterly papers of Macculloch t in 1817, and of Sir T. Dick-Lauder I 
in 1818, gave reasons to show that the parallel roads in Glen Roy were due to lake 
action; but, while coming to this conclusion, they both felt the great difficulty, in the 
then state of our knowledge, of conceiving by what means the lakes could have been 
dammed up. Macculloch conjectured some form of detrital barriers as a possible cause, 
but could suggest no adequate agency either for the origin or removal of such barriers. 
In 1847, Mr. David Milne Home, after a special examination of the superficial 
deposits of the district, proposed an explanation for the barriers on the “ detrital 
theory ,” of which he has since been the consistent advocate. § To account for their 
formation as well as for their removal, he supposes that after the period of the first 
great glaciation of Scotland the land was “submerged to the extent of 3,000 feet or 
more,” and that during this submergence, or as the sea retired, “ beds of clay, sand, and 
gravel, and also erratic boulders were deposited” by currents from the W.N.W. and 
N.W. ; and he points to the large accumulation of such detritus in the valleys, and the 
common occurrence of detached portions and isolated masses, together with a general 
sprinkling of similar detritus, on the hill slopes at elevations above that of the highest 
of the “parallel roads,” in support of his views. He considers “it manifest that all this 
district was formerly covered by detritus to 2,000 feet above the sea, and that it filled 
* They have been well summarised by Chambers, in ‘ Ancient Sea Margins,’ p. 95, 1848; by Jamieson, 
in 1863 {post) ; by Sir Charles Ltell, in ‘The Antiquity of Man,’ 4th edit., p. 300, 1873; by Dr. James 
Geikie m ‘ The Great Ice Age,’ t 2nd edit., p. 227, 1877; by Professor Tyndall, in Proc. Roy. Inst. Gt. 
Britain for June, 1876; and by other writers; while the admirable maps of the Ordnance Survey now 
give the accurate heights and the exact range of the “roads,” which are further described and illus¬ 
trated by the late Sir H. James in ‘ Notes on the Parallel Roads of Glen Roy,’ Ordnance Office, 1874. 
t Trans. Geol. Soc. of London, vol. iv., p. 314. 
+ Trans. Royal Soc. Edinburgh, vol. ix., p. 1. 
§ See his exhaustive memoirs “On the Parallel Roads of Lochabei’,” Trans. Royal Soc. Edinburgh, 
vol. xvi., p. 395, 1847; vol. xxvii., p. 595, 1876; and vol. xxviii., p. 93, 1877. 
