112 Lieut.-Colonel Portlock on the Scratching of Rocks 



portion of it remains within reach of tidal action. As the 

 level of the water rises, the waves beat upon the base of the 

 clay cliff, and gradually undermine it, until at length the top 

 falls in, and for a time the base is protected by the frag- 

 ments. When, however, they have been washed away, the 

 work of destruction is renewed, and slice after slice of the 

 cliff or bank is thus removed, until, at length, the base retires 

 beyond the action of the higher tides, and the cliff, exposed 

 only to ordinary atmospheric agencies, finally crumbles into 

 a slope fitted for the preservation of statical equilibrium. In 

 the progress of this wear it is usual to find the boulders and 

 larger gravel of the clay heaped up as water-worn shingle at 

 the base of the cliff', the finer matter or clay having been 

 washed away. It was at the base of a bluff headland of this 

 boulder-clay, which is only exposed to the action of the waves 

 in very high tides or in storms, and is, in part, protected by 

 a bank of water-worn shingle, which had once been imbed- 

 ded in it, that I observed a portion of the rock laid bare, 

 which, far from exhibiting the jagged edges of the strata 

 habitually exposed to the wear and tear of the waves, 

 was smooth and rounded in its surface, and further marked 

 by fine sharp scratches, varying in their parallelism accord- 

 ing to the relative positions of the slopes of the rock and the 

 consequent direction of the scratches. It further appeared to 

 me that these scratches passed under the clay; and I there- 

 fore assumed, as probable, that they had been made by sub- 

 stances moving with or imbedded in the clay. It will be 

 understood that I endeavoured merely to state a fact, and 

 not to describe historically the phenomena with which that 

 fact would be naturally connected. It is right, however, that 

 I should now observe, that in my Geological Report on 

 Tyrone and Londonderry, published in 1843, I have dis- 

 tinctly pointed out that the phenomena of drift are such as 

 cannot be explained by any one movement of water, whether 

 diluvial or fluviatile or marine, but are the results of actions 

 often varied in their direction and amount, in a manner very 

 similar to that which can be traced in more ancient deposits. 

 In the illustrations to that work I have given examples both 

 of contorted strata and of cross or false stratification in 



