18 ME. T. F. JAMIESON SUPPLEMENTAET 



of the vegetation covering it. I have no doubt therefore that the 

 men of the Koyal Engineers were often at a loss what spot to fix 

 on as representing the exact middle of the terrace, which is alto- 

 gether an arbitrary point, there being no definite boundary of either 

 the upper or lower margin. 



" It is indeed impossible," as Sir Thomas Dick Lauder long ago 

 remarked, " to perform a mathematically-accurate levelling process 

 on such rude and indefinite subjects as these shelves are." ^ Shoals 

 of debris have come down up©n them from above and gaps have 

 been torn by the mountain torrents, slips also have occasionally 

 happened ; it is indeed surprising that they are still so visible. In 

 some places mistakes evon might have been made as to what actually 

 constituted one of the Parallel Eoads, as I shall afterwards show, 

 for spurious markings occur which might readily be confounded with 

 them, and the Government surveyors were not geologists. I there- 

 fore think that Prof. Prestwich makes too much of the variations in 

 level recorded on the six-inch map when he argues that the lines 

 are not so rigidly horizontal as he fancies they ought to be if they 

 had been produced by the shore-action of a lake. The Ordnance 

 Surveyors themselves do not seem to have shared in this opinion, 

 for Col. Sir Henry James, who directed the operations, stated that 

 " there could be no doubt as to the general correspondence of the 

 levels of the terraces at difi'erent points," ^ and after the survey of 

 them was concluded he spontaneously wrote to me as follows : — 



" Ordnance Survey Office, Southampton, 

 19th November, 1873. 



" I send you to-day an impression of Sheet 63 of the one-inch map of 

 Scotland, which contains Glen Roy and part of Glen Gluoy. This sheet, in 

 outline with contours, has enabled me to lay down the Parallel Eoads and to 

 show their extent downwards. I had a contour run at a little above the height 

 of the upper road, and also one at the level of the lower : these served as feelers 

 through the country lor an extension of the roads, but nothing was found. I 

 have myself examined the ground and feel certain that you have correctly de- 

 scribed the roads and proved that Agassiz's theory was the true one." 



The Parallel Eoads have been examined carefully with good in- 

 struments by many civil engineers and scientific men, and I believe 

 all who have so examined them have been of opinion that the lines 

 represent a horizontal water-level, such as would be formed by the 

 margin of a lake either salt water or fresh. Dr. Tyndall — surely 

 a competent witness — tells us that he had an opportunity of inspect- 

 ing some of them with a theodolite, and he declares them to be " all 

 sensibly horizontal."^ I may mention that when standing on the 

 lowest line on MealDerry (see plate in Lyell's ' Antiquity of Man '),. 

 and taking a sight through a good spirit-level to the portions of the 

 same line ranging some miles up Glen Eoy, I observed that the 

 most distant parts of it sank slightly, but perceptibly, below the- 

 thread of the instrument in such a proportion as might be expected 



1 Trans. Eoy. Soc. Edin. vol. ix. (1821) pp. 6 & 7. 



2 Quart. Journ. Geol. Soc. vol. xxviii. (1872) p. 240. 



3 Proc. Eoy. Inst. vol. viii. (1876) p. 234. 



