Dr. Mac Culloch on the Parallel Roads of Glen Roy. 375 



As I have proved in another part of this paper, that the level of 

 the upper line of Glen Roy is higher than those of many vallies 

 which would at present afford passage to the supposed waters of 

 Glen Roy into the sea, it follows that water could not now stand at 

 that level, unless these apertures were obstructed to at least an higher 

 elevation. The determination of the position of these imaginary 

 barriers, is consequently the next point to be considered ; as well 

 as that of their number, since possibly two of these openings might 

 be closed by a single obstruction. I must therefore proceed to ex- 

 amine more particularly into the conditions required for the forma- 

 tion of such a lake as that which I have supposed to be the cause 

 of the lines in Glen Roy. 



I have shown that the uppermost one is of such a height, that 

 water standing at that level would now flow out by Loch Spey and 

 Loch Laggan, through the valley of the Spey into the eastern, and 

 by Loch Eil, Loch Shiel, and Loch Ness, into the western sea. 

 The two lower lines lying below the barriers of Loch Spey and 

 Loch Laggan, it would, under similar circumstances, find its way 

 through the three latter openings only. The condition of the sur- 

 rounding land must therefore have so far differed at that time from 

 its present state, that various dams or barriers must have existed in 

 the course of these openings. 



In attempting to investigate their places it is proper to commence 

 by assuming the least difficulties, assigning no more causes than are 

 strictly necessary to the production of the desired effect. I have 

 shown elsewhere, that the conditions of the present barriers existing 

 at the source of Loch Spey, and to the east of Loch Laggan, are 

 such as to give no reason to imagine that they have once been higher. 

 I have also shown, that by the removal of the supposed barriers 

 to a point below Dalchully, one obstruction would have answered 



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