322 Dr. Mac Culloch on the Parallel Roads of Glen Roy. 



angle, or at the inner and outer edge of the line, that no precise 

 limit for the measure of breadth can be assigned ; different modes 

 of measurement may therefore produce differences of many feet.* 

 It is sufficient if they agree generally, and in general sixty feet may 

 be assumed as an average breadth : by far the largest portion cf all 

 the lines will be found to conform to this measurement. 



Great terraces are now visible on the right. These are not pre- 

 cisely on the same level with that which I before mentioned as 

 corresponding to the course of a third and lower line, but they do 

 not differ materially from it. 



I shall not describe the various rivers which enter the glen, the 

 principal ones being marked in the plan, but may mention that in 

 this upper part of the valley, both before and at the junction of 

 Glen Turit with Glen Roy, they are generally accompanied by 

 their own lateral terraces. 



On the left hand going down the glen many marks or fragments 

 of lines are seen between the principal ones ; but these are short, 

 and are remarkable for many obscurities and deficiences. In a few 

 places there are errors of level to be seen in the lines. Examining 

 these however there appears no doubt of their having been pro- 

 duced by partial subsidences of the whole alluvial face ; and this is 

 confirmed by the appearance of one great slide on the left, which 

 has descended many feet, and which the imagination can readily 

 replace. It is necessary to be cautious in examining these instances 

 of errors of level, as the laws of perspective are apt to lead to 

 mistake when the lines pass curved surfaces elevated high above the 

 horizon. 



Where the faces of the hills have been furrowed by the long 

 continued action of descending torrents, the lines enter these hollows 



* Plate 18, Profile No. 8. 



