ON THE PARALLEL ROADS 



whence returning downwards on the other side, it produces, 

 when opposed to that part of it running upwards, the appear- 

 ance of two twin shelves, when, in reahty, they are discovered 

 by examination to be one continuous Hne. There is, of 

 course, every where, a perfect sameness in the perpendicu- 

 lar height of one range above that which is beneath it. But 

 a deception is produced to the eye, with regard to the appa- 

 rent relation of the whole shelves, to the valleys where they lie. 

 For, from the inclination of the bottom of the valleys, being 

 opposed to the perfect horizontality of the shelves, the whole 

 of the latter have the deceitful appearance of sinking on the 

 sides of the hills, as they run in a dii'ection towards the sour- 

 ces of the streams ; and of rising, as they approach the open- 

 ings of the several glens. Although the perfect linear hori- 

 zontality of these shelves, and the correspondence as to level, 

 of each particular part of a shelf, on the two sides of the same 

 glen, has been always admitted, upon simple ocular inspection, 

 yet it is by no means easy to put this important matter beyond 

 doubt. It is indeed impossible to perform a mathematically 

 accurate levelling process, on such rude and indefinite subjects 

 as these shelves are. For although they appear very distinct- 

 ly, and even sharply marked, when viewed from the glen below, 

 or from some distance, yet when the observer climbs up to in- 

 spect them more narrowly, he always finds it impracticable to 

 discover their precise limits, and they are then indeed so very 

 indistinct, that he may even be actually treading on a shelf, 

 without being in the least aware that he is doing so. This 

 shows how very imperfect they are as to form, and how very 

 little they deviate from the ordinary contour of the hills along 

 which they are traced ; for whilst their outward edge is very 

 much rounded off, they are united interiorly to the acclivity of 

 the mountain above them, by a highly inclined slope, so as to 



make 



