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



the west they are directed to Loch Laggan. The barrier itself 

 gives no rise to waters, as it consists of a narrow ledge of rocks ; 

 nor does it appear at any time to have been liable to suffer from 

 the course of rivers. This is a species of judgment which can be 

 formed without perhaps any great risk of error, by inspecting the 

 positions and shapes of rocks and mountains, but it is a judgment 

 of which the grounds can scarcely be explained by drawings or 

 descriptions. The observation itself is of importance in examining 

 the general theories of the appearances in Glen Roy. Circumstances 

 prevented me from ascertaining the actual height of this barrier 

 above the sea, and the nature of the ground does not admit of any 

 direct levelling to it from Glen Roy, without a series of most 

 intricate and tedious operations, which would in fact be unnecessary 

 in examining the question. But by comparing its level with that 

 of the Spey at the point where the before mentioned sluggish 

 stream joins that river, and by computing the elevation of that 

 point from Garvamore of which the height was ascertained, its 

 altitude is readily estimated to a sufficient degree of accuracy for 

 the present purpose. 



Comparing these heighte therefore it will be found that the barrier 

 of Loch Laggan to the east is depressed 369 feet below Loch Spey 

 and 432 feet below the uppermost line of Glen Roy. Supposing 

 therefore that the water stood at the highest elevation in Glen Roy 

 in the present state of the earth, it would run into the Spey not only 

 by the channel of Loch Spey, but by that of Loch Laggan also.* 



Such are the communications which Glen Roy has with the 

 surrounding country, and through that with the sea. I have de- 

 scribed them with all the minuteness in my power, as they are of 

 the first importance in the investigation of the causes which are to 



* Plate 19, 





