18 



Prof. J. Prestwicli. On the Origin 



[May 1, 



variable as the problem has to deal with, the parallelism of the 

 "roads" with the horizon, which has been remarked on by all 

 observers from Macculloch downwards, could not have been main- 

 tained For the detritus of the hill sides varies, however slightly, 

 in the relative proportions of rock fragments and soil, while the slopes 

 above and below the "roads" also vary in their angle, so that, 

 as these conditions varied, so would the momentum of the sliding 

 mass vary, whilst the resisting force of inertia would remain the 

 same. The consequence would necessarily be, that the slide would 

 continue at some places to a lower level than at others, and the line 

 of the " roads " could not be throughout parallel with the horizon. 



There is no doubt that, to all appearances, the "roads" are perfectly 

 level, and such was the opinion of the author when standing on them 

 and while working with the excellent 1-inch map of the Survey ; but 

 afterwards, on referring to the elaborate 6-inch maps, he found the 

 levels there given clearly show that the "roads" are neither at the 

 exact height of the cols nor are they perfectly parallel with the 

 horizon, after allowing for the variable inclination of the "roads," 

 and for the observations being made in their centre. 



Instead of a perfect level water line, the " roads " are really slightly 

 waved, the difference between the highest and lowest point being in 

 the four "roads" taken in descending order, 15, 11, 15, and 12 feet; and 

 while the level of the higher " roads " is in most instances below those 

 of the several cols of escape, that of the lower " road " is in all instances 

 above it. Thus in Glen Gluoy, the " road " which is never more than 

 1 foot above the. level of the Turret Col, is in places 14 feet below it. 

 The Glen Roy "roads," Nos. 2 and 3, rise 2 and 4 feet above the col 

 of the Spey, and sink 7 and 13 feet below it, whereas No. 4 "Road " 

 is never less either in Glen Roy or Glen Spean than 2 feet, and rises 

 at places to 10 and 14 feet, above the Pass of Makoul. Further, as the 

 curves formed differ for each "road," the variations cannot be due to 

 a common cause, such as subsequent movements of the ground, but 

 must be owing to differing conditions in each case local. JSTor do the 

 levels on the two sides of the valleys correspond , they often vary as 

 much as 7 or 8 feet. It is therefore not possible to reconcile these 

 variations with the hypothesis of the "roads" being lines of water 

 level due to shore action ; nor is the very variable inclination of the 

 " roads " themselves compatible with that view, for on a strand where 

 the slopes do not differ more than in these glens, there could not be 

 that irregularity of surface which exists in the "roads." 



In the instances of the higher "roads," the lower level of the curve 

 is possibly due to the steeper slopes, whilst the fact that the lower 

 "road," No. 4, remains above the level of the Pass of Makoul, may be 

 due to the circumstance either of the slopes being less, or more 

 probably to the fact of the lake having been so very much larger, the 



