122 Lieut-Colonel Portlock on the Scratching of Bocks 



explanation of drift must combine the actions of all. One of 

 these, namely, the simple action of the tidal current, seems 

 scarcely to have been noticed in reference to this subject, 

 though it has, doubtless, co-operated materially in producing 

 some of the results observed. Almost every bay on our coast 

 may afford evidences of the accumulation of gravel and sand 

 by tidal currents ; and it is very curious to observe how the 

 shingle, or marine gravel, is sometimes moved over a smooth 

 surface of sand. Nor is this the only effect, as I was enabled 

 to observe, to great advantage, the action of the moving 

 gravel in producing grooves in more solid substances, as 

 exhibited in the breakwater at Southsea Castle, near Ports- 

 mouth, which has been worn in the most remarkable manner 

 in deep parallel grooves, so as to assume the appearance of 

 the most complicated mouldings. I have no doubt that a 

 similar grooving takes place in ordinary rocks when favourably 

 situated as regards the tidal current ; and 1 would, therefore, 

 suggest to every observer of such phenomena the inquiry, 

 whether the grooving, as regards its physical position, is 

 most nearly in relation to the action of a land glacier or to 

 that of tidal currents. 



Nor must fluviatile action be overlooked, as its power is 

 very great under fitting circumstances. For example, can 

 it be doubted that such a river as the St Lawrence must, 

 both by its waters and by the masses of ice it bears along 

 upon them, convey large quantities of detritic matter, and 

 arrange that matter at the bottom of the lakes in a manner 

 very similar to that observed in the drift of our plains and 

 estuaries. Other evidences of fluviatile action in pot-holes, 

 &c., as pointed out by that eminent American geologist. 

 Professor Hitchcock, should not be overlooked. On solid 

 rocks the action of rivers is small, but on the softer secondary 

 strata, and on drift, it is considerable. In these latter cases 

 they are modifying agents, removing and readjusting ancient 

 drift, so that it is to more ancient rivers, or more ancient 

 causes generally, that we must ascribe the drift itself. In 

 making our selection between these causes we must not 

 forget that a mere present difference of level between the 

 point observed and the bottom of the valley or the stream 



