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



nor then in existence. We have therefore two distinct alternations 

 of a formation, with one intermediate removal, of alluvia ; within 

 a period subsequent to that at which the present general distribution 

 of hill and valley were made, and therefore in times comparatively 

 recent. 



In considering the conditions requisite to the maintenance of 

 a lake in Glen Roy, I have already been obliged to anticipate 

 some of the observations which would more properly have been 

 introduced in thi6 place. They are those which relate to the posi- 

 tion of the supposed barriers, and which necessarily therefore 

 involve the magnitude of the original lake. But the details re- 

 lating to the eastern boundary including Loch Spey and Loch 

 Laggan require to be more particularized. It has been seen that 

 the boundary of the lake of Glen Roy towards the east could not 

 have been further westward -than Loch Spey. In inquiring next 

 where it is likely to have been situated, we have seen that there is 

 a communication between the mouths of Glen Roy and Glen 

 Spean, and that a free level would exist in the present state of 

 things between the lowest line of Glen Roy and Loch Laggan, since 

 it can be traced within a few miles of the foot of this lake. It ap- 

 pears therefore probable, that no barrier existed at the western end 

 of Loch Laggan so as to dam Glen Spean immediately above the 

 present marked line and between this point and the foot of that 

 lake. This probability amounts almost to certainty, when we com- 

 pare the elevation of the supposed barrier necessary for the pur- 

 pose of retaining the water of the common lake of Glen Roy with- 

 in the limits of the highest line, with the actual barrier which now 

 bounds Loch Laggan to the east. It is evident that if the waters 

 of Loch Laggan were now raised suddenly by any power to the 

 height of a barrier so supposed, they would flow over their eastern 



