4 On Drift. 



all do, the great value of the researches of our countryman Profes- 

 sor Forbes on the Alpine glaciers, they should in any degree forget 

 the debt they owe to the distinguished Swiss naturalist and his 

 countryman, who were the first to point out the effects of glaciers 

 in smoothing and striating rocks, to urge their effectiveness in the 

 transport of blocks, and to indicate phenomena of a past epoch 

 similar to those of the present time, in such a manner as to com- 

 mand the attention of geologists, and finally to lead to the adoption 

 of our present views respecting the glacial epoch. It is especially 

 to M. Agassiz, and his ardour in the pursuit of scientific truth, that 

 we owe the first knowledge of this subject in our own country. 

 His visits here, and the personal favour with which he was received 

 among us, gave him frequent opportunities of expounding his 

 views ; and I cannot refrain on this occasion from expressing the 

 delight with which I call to mind the open-hearted hospitalities 

 which he exercised in the deep recesses of the Bernese Alps, and 

 from testifying to the perfect unreserve with which he communi- 

 cated his views to those alike who favoured or opposed them. 



I have already remarked that water was formerly almost the 

 only recognised agent in the transport of erratic blocks. On the 

 introduction of the glacial theory it was superseded, and appeared 

 to be almost forgotten ; "nor does it still seem to have regained 

 what I conceive to be its just claims, in the minds of many geo- 

 logists. On the abandonment, however, of some of the unreason- 

 able claims of the glacial theories, and the distinct recognition of 

 large portions of drift as subaqueous phenomena, the importance 

 of currents as agents of transport gained more attention, though 

 there are probably many persons who yet fail to realise in their 

 own minds the enormous power which such currents may possess, 

 even without greater velocities than may be easily allowed them. 

 This power arises from the fact, which I have elsewhere demon- 

 strated, that the moving force of a current, estimated by the 

 weight of a block of any assigned form and material, increases as 

 the sixth power of the velocity of the current. It is this which 

 accounts for the circumstance that the same atmosphere which in 

 one state of motion constitutes a summer breeze, but just sufficient 

 to move the leaf or the flower, exerts at other times the almost 

 irresistible force of the storm. It is on this account, too, that, 

 reasoning from the power of ordinary currents of two or three 

 miles an hour, we are liable to miscalculate so entirely the force 

 of a rapid current. 



I consider the distinct recognition of these three agencies of 

 transport — glaciers, floating ice, and currents — as essential to the 

 final establishment of sound theoretical views on this subject, and 

 the great majority of geologists are probably prepared to recognise 

 them to a greater or less extent. It is equally essential that we 

 should be prepared to assign to each of these agencies its share in 



