an Agent of Geological Change. 341 



It is true that some of these depressions have been claimed 

 by Prof. James Greikie as the work of glaciers, but there 

 seems to me insufficient grounds for that assumption. In the 

 first place, before it could be admitted, we should have to prove 

 that these deeps are rock-basins ; which, as the surrounding- 

 bottoms are all, or nearly all, mud and sand, it would be diffi- 

 cult to do. 



They may or may not be rock-basins ; but even if we admit 

 that glaciers could, under certain conditions, scoop out de- 

 pressions in a rocky floor in such positions (a question on which 

 there exists much difference of opinion), these channels or 

 depressions could not have been cleared of the deposits which 

 formerly filled them, or afterwards have been kept open, except 

 by the action of sea-currents. 



Thus it is seen that, while the form of the channels and 

 coast-lines affect the direction and velocity of the tidal cur- 

 rents, the tidal flow reacts under favourable conditions and 

 increases the depths of the channels. Thus the actual erosive 

 influence on a rocky bottom may be small ; on a bottom of 

 softer materials it may be large, sweeping away the looser 

 sands, gravels, or clays until it reaches the rock, or until its 

 force is expended and an equilibrium of velocity and depth be 

 attained. It is at all events remarkable that in all the deep 

 pools I have mentioned, some of the deepest soundings disclose 

 rock while the shallower ones do not. 



This seems to me irreconcilable with the glacial theory of 

 the origin of these " deeps." 



It will not follow from the principles I have attempted to 

 enunciate that the deepest channels are always situated where 

 there is the swiftest current. The velocity of the current, on 

 the contrary, is often increased by the sudden shoaling of the 

 water, so that the most dangerous and impetuous tides may be 

 caused by rocky ledges which resist erosive action. 



The deeps, I take it, are the result of the steady movement 

 of a great body of water, having usually a bottom-velocity 

 probably not much less than that of the surface, and in some 

 cases more ; and the depth of erosion is to a large extent 

 determined by the nature of the materials upon which the 

 moving waters act. 



The Tide as a Distribute)'. — The foregoing examples have 

 all been taken from a coast where the contours and islands 

 give great diversity and strength to the currents, and where 

 the erosive action is consequently at its greatest. But, as a 

 distributer of material, the tide performs perhaps a greater 

 function. In less restricted areas it is not so forcible in action, 

 but spreading over a larger area the amount of material dealt 



Phil. Mag. S. 5. Vol. 25. No. 156. May 1888. 2 A 



