12 ON THE PARALLEL ROADS 



deserves particular notice : the nature of the soil in each val- 

 ley, is materially different above and below its highest line of 

 shelf; all above this natural division, being found to resemble 

 the bare moorish soil covering any other mountain; whilst 

 large depositions of alluvial clay, sand, rounded pebbles, and 

 gravel, present themselves every where below that point, and 

 this, more particularly towards the mouths of the different val- 

 leys. Marine exuviae, however, are nowhere to be met with. 

 Perhaps the most interesting circumstance regarding these 

 shelves, is, that wherever an isolated little hill happens to rise 

 from the bottom of the valley to a height above that of the 

 level of any shelf, a delineation runs round the little hill, at 

 a level corresponding to that of the shelf on the mountains of 

 the side of the glen. 



Such, then, is the general description of the character and 

 appearance of these shelves. 



Tlicory which may account for the formation of Shelves running 

 horizontally along the sides of Mountains confining Glens. 



In the total obscurity in which the origin of these singular 

 shelves is involved, some have been inclined to regard them as 

 productions of art, others as the work of Nature. Of these two 

 opinions, I confess I cannot hesitate in rejecting the first. The 

 immensely arduous nature of the undertaking, arising not on- 

 ly from its extent, but from the numbers of accurate measure- 

 ments and levellings it must have required, — the impossibility 

 of the supposition, that the engineers of those early times to 

 which they must be referred, could have known how to make 

 the exact allowance for the curvature of the earth, and this, 



too, 



