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



not however inconceivable that the causes which are now, by the 

 accumulation of alluvium, obliterating the existing lakes, should, 

 under some variation of ground, have heaped a barrier in the course 

 of a valley ; and generated at one period a lake which they were 

 afterwards destined to destroy, or which, accumulating strength by 

 confinement, while the opposed barrier was undergoing a slow waste, 

 should suddenly break its bounds and again desert the valley which 

 it had been previously compelled to occupy. But the difficulty of 

 removing the other barriers, which in this case must have remained 

 after the breaking down of that one, continue unsolved even on 

 this supposition ; and other causes, which we know not well where 

 to seek, must be found to explain the removal of alluvia from points 

 where they appear at present to be, on the contrary, accumulating. 



This difficulty is still augmented by examining certain phe- 

 nomena connected with the lines of Glen Roy, which seem to 

 point to a distinct and still more distant alternation of the state of 

 the land at those places where a free communication now exists 

 between them and the sea. These will be found to involve a set 

 of actions even more intricate than those by which the water was 

 originally drained from this lake. 



In describing Glen Roy I have noticed the appearances of lines 

 on the sides of the small torrents which descend from the faces of 

 the hills. They enter into these furrows to a certain depth, but are 

 not continued throughout the whole curvature. Now if we ex- 

 amine the structure and position of the rocks which form the sur- 

 face of the hill at any of these points, or consider the alluvium by 

 which they are in other places covered, we shall have no hesitation 

 in admitting, that like all similar furrows and water courses on the 

 faces of hills, they have been formed by the action of the rivers 

 which now occupy them, or have been scooped out by the descent 



