130 



THE GEOLOGIST. 



Ness and Loch Lochy, while these lakes have been disjoined from the 

 sea by the large alluvial plains that now extend from them at each 

 end, along the courses of the Lochy and the Ness, The operations 

 required in constructing the Caledonian Canal have ascertained the 

 reality and extent of these alluvia, while daily observation shows 

 that they are in many places at least receiving an augmentation, 

 that has a tendency at some far distant period to obliterate the lakes 

 and convert the whole into one prolonged strath, of which the future 

 summit will be Loch Oich or some point in its vicinity. If, indeed, 

 we examine the changes which the lakes of Scotland are now under- 

 going, we shall find that they are receiving accumulations of alluvial 

 matter at all the points where they are fed by the surrounding 

 streams, while a comparatively small quantity of this alluvium is car- 

 ried towards the sea. The result of this operation is to obliterate 

 them and to convert them into alluvial valleys or straths. Instances 

 of this revolution, more or less perfected, are numerous ; while no 

 case of the obliteration of a lake by drainage, similar to that of Glen 

 Boy, can be pointed out." 



But it is at this point of his argument that Macculloch encounters 

 the most serious objection to his theory ; not that that theory, as far 

 as the ancient existence of a lake in the Glen Boy region is con- 

 cerned, is erroneous ; not that he was wrong in attributing to the 

 action of the standing water of that lake, the erosion of the "parallel 

 roads " of that and the neighbouring glens ; but the difficulty existed 

 in finding some natural way of constructing and in accounting by 

 known physical causes for the total demolition of the barriers, con- 

 sidered as barriers of earth, whether of rock in situ or of accumu- 

 lated alluvia. He feels this difficulty acutely, it is evident, for al- 

 though he may be said to have completed his subject, he still goes 

 on, through a dozen quarto pages, to comment on the difficulties of 

 this position and to offer explanations. "It is not, however, incon- 

 ceivable," he says, "that the causes which are now, by the accumula- 

 tion of alluvium, obliterating the existing lakes, should, under some 

 variation of ground, have heaped a barrier on the course of a valley, 

 and generated at one period a lake which they were afterwards de- 

 stined to destroy, or which, accumulating strength by confinement 

 while the opposed barrier was undergoing a slow waste, should sud- 

 denly break its bounds and again desert the valley which it had been 

 previously compelled to occupy" 



This, however, is an argument which carries its own conviction of 



