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



fore remains to be added on the subject, but I shall proceed to state 

 the very few remaining arguments on which it must rest, and op- 

 pose to them the difficulties, and the conditions now long passed 

 away, under which such an order of things must have existed. 



The absolute water level which is found to exist between the 

 corresponding lines both in Glen Roy and in those vallies which 

 communicate with it, admits of a ready solution, on the suppo- 

 sition that a lake once occupied this set of vallies ; nor can it 

 be explained on any other. As a free communication, in one 

 direction at least, still exists among them, it would even now 

 be easy to imagine the water replaced in the same situation : the 

 difficulty of confining it will be a subject for future consideration. 

 If however a lake be considered the cause, it is plain that the lines 

 in question were once the shores of this lake ; and it equally fol- 

 lows that it had existed at three different elevations, and that the 

 relative depths of these three accumulations of water may be mea- 

 sured by the relative vertical distances of these three lines from the 

 bottom of the valley. Thus the nature of the retaining obstacles 

 becomes more complicated, and adds materially to the difficulties 

 that will hereafter be examined in detail. 



It is necessary in the first place to recal to the reader's mind the 

 few facts which most directly bear on this view of the case. 



We have seen that the hills which bound Glen Roy have their 

 sides covered with a coat of alluvial matter, which in some places 

 consists of sharp substances, appearing to be merely the ruins of the 

 summits above ; while in others it is formed of rounded ones, de- 

 posited in such a manner, as to indicate a transportation from places 

 more distant ; in these the lines are formed. The same circum- 

 stances occur in the other vallies which communicate with the 

 principal one. 



