360 Dr. Mac Cul loch on the Parallel Roads of Glen Roy. 



ceive. As the varying dimensions of the valley cannot be easily 

 computed, it is unnecessary to examine the question with hydro- 

 statical accuracy : a general inquiry as to the probable effects will be 

 sufficient. 



A general notion of the difference of capacity of the different 

 parts of Glen Roy, may be acquired from comparing its vertical 

 and lateral dimensions at its extremities ; f and for the sake of sim- 

 plicity I shall limit myself to this single case, although it is obvi- 

 ous that many more complicated considerations must enter into 

 the account. It will also be most simple to assume the lowermost 

 line as the groundwork of this investigation, as we here get a mini- 

 mum ratio. This cuts the rocky bottom of the valley near its up- 

 permost extremity.* A point therefore exists in the valley where its 

 vertical dimension, considered from its bottom to the summit of the 

 lowermost line is nothing, its breadth being in that place about half a 

 mile. If now we examine its lower extremity, or that part of the 

 joint valley of the Roy and Spean, where the last point of the same 

 line is visible, we find it situated at a considerable height (not 

 less than 800 feet) above the bottom of the valley, while its breadth 

 is in this place five miles. The simple statement of this fact renders 

 it superfluous to say that a body of water could not flow through 

 a valley of this form, with a level surface ; a condition required to 

 produce the line in question. It is plain that the same objection 

 applies to the case of the two upper ones, although with somewhat 

 less force, according to the ratios of the several inequalities at their 

 upper and lower extremities. 



As connected with this last argument we may now ask, by what 

 system of operations, on the same hypothesis, the lines were traced 



+ Vide Plates 18, 21, 22. 

 * Vide the tabulated admeasurements (note to page 326) where it will be seen that the 

 bottom of the Talley at its upper part is 11 feet above the lowest line. — Vide also. Section 

 (6), PI. (22). 



