18 ON THE PARALLEL ROADS 



expended on a more gentle slope, where they will produce 

 shelves less distinctly indented indeed, but much broad- 

 er. The number of lakes which furnish examples of these 

 broad shallows within the margin, where they have easi- 

 ly inclined shores, is so great as to render it unnecessary to 

 instance any of them. This last remark, however, should be 

 kept in view, because it will be found to apply to many parts 

 of the particular description of the courses of these Lochaber 

 shelves, which it is my intention to give, and will more espe- 

 cially explain those parts of them, where an inattentive observer 

 would be apt to suppose that they disappear entirely, when, in 

 reality, they only become less distinct, by becoming wider, 

 and consequently more gradually shelving. 



After subjecting mountain lakes to this close investigation, it 

 will probably strike every one, that the consequence of the 

 sudden escape of the water from one of them, would be the 

 immediate appearance of a glen having a range of horizontal 

 shelf, traced high up on the face of its mountains, and in every 

 respect similar to those exhibited in Lochaber ; and this would 

 be either distinctly seen all around it, or would appear only 

 partially, as the nature of the different parts of the sides of the 

 hills happened to be more or less obdurate or friable, and 

 would present all the modifications of breadth, that might have 

 been occasioned by their various degrees of slope, as well as by 

 their numerous promontories, swells, hollows, or bendings. 

 Through the kindness of a friend, who was one of the party in 

 my first visit to Glen Roy, and who is well acquainted with 

 the spot I am now about to describe, I am fortunate in ha- 

 ving it in my power to produce a case exactly of this descrip- 

 tion, where there cannot be a doubt that such an escape of the 

 waters of a lake did unquestionably happen, although the ex- 

 act time when, and the actual manner how it occurred, is lost in 

 the obscurity of the dark ages. The valley to which I allude is si- 

 tuated a little above the town of Subiaco in Italy, lying about 

 forty-six miles to the eastward of Rome, and twenty-eight 



from 



