Alfred Harlier — Subaerial Erosion of SJiije. 485 



II. — Notes on Subaerial Erosion in the Isle of Skye. 



By Alfred Harker, M.A., F.G.S., of H.M. Geological Survey of Scotland. 

 (Commuuicated by permissiou of the Director- General.) 



CONSIDERING the prime importance attaching to the subject in 

 almost evei-y branch of geological inquiry, it is not a little 

 remarkable that so little has hitherto been done towards obtaining 

 precise measures of the actual waste of land-surfaces and comparing' 

 the rates at which the agents of destruction operate at the present 

 time in different districts. Arrangements have indeed been made' 

 during recent years to observe systematically the results of marine 

 erosion on our coast-lines, but concerning the far more important 

 subject of subaerial erosion we have exceedingly few accurate data. 

 Sir Archibald Geikie, in his recent address to the Geological 

 Section of the British Association at Dover, has called attention to 

 one aspect of the question, viz., as it affects estimates of geological 

 time, and has urged the desirability of accumulating observations, 

 which should be, so far as the conditions of the problem permit, 

 of a quantitative kind. The few remarks which follow are based 

 on notes made during four or five years' work on the Geological 

 Survey of the central part of Skye, and it is perhaps hardly 

 necessary to point out that the conclusions arrived at ai'e limited to 

 the same area. Indeed, if such observations are to have any value, 

 it must be by comparison with observations made on similar lines 

 in other districts. 



Central Skye is a district of sufficiently varied physical characters. 

 Mountains over three thousand feet high rise within two or three 

 miles of the sea. These, with their subsidiary branches, embrace 

 extensive corries, drained hy streams which have a high gradient, 

 and become in flood - time formidable torrents. Beyond the 

 mountain-tract are deeply eroded basaltic plateaux, rising in places 

 to 1,500 feet or more above sea-level. The rainfall is probably the 

 heaviest in Britain, or indeed in Europe, amounting to as much as 

 100 or even 150 inches at sea-level, and doubtless much more 

 among the mountains, while the district is swept during the greater 

 part of the year by winds which often rise to severe gales of several 

 days' duration. A newcomer to such a district might well expect 

 to find decisive evidence of the continued activity of eroding and 

 transporting agencies, but such expectation, as I shall show, would 

 not be realized. 



Skye, in common with the other islands of the Inner Hebrides, 

 has often been cited as a striking example of the visible results of 

 subaerial erosion, and it would be difficult to find in the British 

 Isles a district more instructive in this respect. Lava-flows of 

 Eocene age present their truncated edges on the hill-sides to a total 

 thickness of thousands of feet. The gabbro and granite are of later 

 date than the lavas ; yet these pi atonic rocks, consolidated doubtless 

 under a thick covering of tlie t)asaltic lavas, now form the liighest 

 ground in the district. My thesis is that the vast amount of erosion 



