Dr. Mac Culloch on the Parallel Roads of Glen Roy. 371 



admit of explanation at present ; but they do not admit it on any 

 view of the subject, and it is not essential to the truth of any hypo- 

 thesis that it should explain every fact, unless we were in posses- 

 sion of all the collateral and posterior circumstances which may 

 have modified or altered the actions' of the presumed cause. A few 

 of them indeed do admit of explanation, and will be considered 

 when the causes of the present terraces are investigated. 



It is necessary to describe the manner in which the water of 

 a lake is supposed to act in producing its shores, or in which the 

 assumed waters must have generated the lines of Glen Roy. This 

 action consists first in checking the constant and gradual descent of 

 the alluvia of the hills. The descending matters thus losing a large 

 portion of their weight by immersion in the water, and in winter 

 often rendered still more buoyant by being entangled in ice, are 

 thrown back against the face of the hill by the incessant action of 

 the superficial waves, and are thus evenly spread against its side, 

 producing an inclined shore, proportioned in breadth and declivity 

 to the various circumstances already enumerated and to the length 

 of time during which the action has been continued. This kind 

 of levelling action is easily seen in many of the highland lakes, and 

 is conspicuous in particular on the prevailing lee shore, wherever 

 such a one exists. Loch Rannoch offers a striking example of it 

 at its eastern boundary, since its situation subjects it to a very dis- 

 proportionate prevalence of westerly winds, accompanied by a 

 corresponding power in the waves which break on that margin. 



It is next necessary to consider in what respect the large terraces 

 of Glen Roy can be connected with the supposed existence of a 

 former lake occupying that valley. 



Wherever a river is found entering an existing lake, it is skirted 

 by a shore wider than the general shores of that lake. This aecu- 



