202 DR. W. T. BLANFORD Otf THE APPARENT RESULT [Feb. 1900, 



of the Scottish Highlands vary enormously, from the 8000 or 10,000 

 postulated by Sir Joseph Prestwich 1 to the 80,000 years calculated 

 on certain astronomical data by James Croll, 2 and the 200,000 or 

 more estimated by other geologists. 



If I am right in regarding the small ravines on the slopes near 

 Laggan as entirely of post-Glacial origin, there ought, I think, to be 

 no insuperable difficulty in ascertaining, after a certain number of 

 years, the unknown quantity in the equation which represents 

 the time that has elapsed since the disappearance of the glaciers. 

 It is practicable to measure the amount that has been removed in 

 any of the channels that furrow the surface of the slope, and all 

 that is necessary is to determine the unknown quantity, which is 

 evidently the average annual denudation in each stream-course. It 

 would be necessary for the purpose of observation to obtain a 

 careful plan of part of the hillside accurately contoured, so as to 

 show both the breadth and depth of the channels on a given area. 

 The best place would perhaps be the hillside along Ceann Loch (the 

 uppermost part of Loch Lochy), beginning on the north-east at the 

 stream called on the map Allt nan Sithean. This is the area in 

 which stream -denudation is greatest and evidently more rapid than 

 elsewhere. It would be advisable also to have photographs made of 

 each stream-course so far as may be practicable. A comparison of the 

 plan and photographs with the actual surface of the hillside a quarter 

 of a century hence, or perhaps even sooner, would give some idea of 

 the rate of denudation, though probably a much longer period would 

 be needful for a correct determination of the time that has been 

 required to excavate the channels. Possibly a rough estimate could 

 be obtained by determining the rate at which the area occupied by 

 ravines, now probably not exceeding one fourth of the whole area, 

 increases at the cost of the intervals between the ravines, and this 

 could be determined by the photographs alone. But it is probable 

 that the rate of denudation is too slow for this plan to give useful 

 results. 



In calculating the time, it will be necessary to assume that 

 denudation has proceeded with regularity since the disappearance 

 of the glaciers ; but this of course is not certain, because rainfall 

 may have varied, and so may the amount of snow which accumulated 

 in the winter, and aided denudation by flooding the streams when 

 melting. Of course, too, the changes from severe frost to mild weather, 

 a most potent agent in disintegration, may have been more numerous. 

 But I c in not think it probable that, in a country where frost and 

 6now still play an active part in the disintegration and denudation 

 of rocks, any changes of climate that may have taken place since 

 the close of the Glacial Epoch can have sufficiently retarded or 

 accelerated the action of ordinary subaerial erosion to affect seriously 

 the calculation of the time that has elapsed. 



There is oDe concluding word that must be added. It may, as 



1 Quart, Journ. Geol. Soc. vol. xliii (1887) p. 407. 



2 ' Climate & Time,' 1875, p= 341. 



