Dr. Mac Culloch on the Parallel Roads of Glen Roy. 341 



we find that it is divided into two glens separated by a partial rocky- 

 barrier through which the river forces its way, and that the summit 

 of this barrier is on a level with the uppermost line. But in de- 

 scribing the upper glen I showed that it bore the marks of a line 

 level with the uppermost one of lower Glen Roy, and consequently 

 both continuous with it, and produced by a common cause. It is 

 important to remark this fragment of a line, as, on the supposition of 

 a continuous water level it removes the boundary of the water from 

 lower Glen Roy to a point further east. But the next eastern 

 point which forms the present boundary of the head of upper Glen 

 Roy is the source of the Spey, and this elevation separates the 

 great eastern declivity of the water at this point, or the valley of 

 the Spey, from the western one, or that of the Roy. The measure- 

 ments made by the spirit level, as well as the observations of the 

 barometer, prove that the source of the Spey is 63 feet lower than 

 the upper line of Glen Roy. If therefore we recur to the same 

 supposition that water could now be accumulated to that level, it is 

 plain that it would flow easterly into the valley of the Spey, as well 

 as into the western outlets just enumerated.* 



If we now turn to the remaining communication of Glen Roy, 

 which is with the valley of the Spean, we shall find that this river 

 flows with no great declivity from Loch Laggan to its junction 

 with the Roy, a space of seven miles or thereabouts. The length 

 of Loch Laggan is ten miles, and it is separated at its upper part 

 from the valley of the Spey by a barrier of low rocks, and by a plain 

 of nearly four miles in length, which conducts a sluggish stream into 

 the Spey at a point about ten miles from its source. As on the 

 east side of this barrier the waters are directed to the Spey, so on 



* Plate 19. 



Vol iv. 2 x 



