168 David Milne, Esq., on Polished and Striated Rocks, 



The clay and boulders are in contact with the smoothed and 

 striated rocks. There is seldom any intervening deposit. I have 

 occasionally seen a thin layer of sand between the boulder clay and 

 stratified rocks. There are several localities pointed out in my 

 Memoir on the Lothian Coal-fields. At the part of the Victoria 

 road, about 300 yards to the north of Sampson's Ribs, Mr Nichol 

 pointed out to me a layer of sand, about an inch or two in thickness, 

 at irregular intervals, between the boulder clay and the sandstone 

 strata. These sandstone strata presented no appearances of smoothing 

 and rutting. Their original covering of sand had apparently not been 

 altogether removed and forced away by the rush of boulder clay. 

 That might happen in some places and not in others ; but wherever 

 there are rocks smoothed and striated, the boulder-clay is in imme- 

 diate contact with them. 



I do not say that this circumstance, of itself, proves that the 

 dressing of these rocks is to be attributed to the passage over them 

 of the boulder-clay, loaded with its heavy and compact greenstones 

 and felspar porphyries ; but it is a very important step towards the 

 solution of the question, to find, that instantly after these rocks were 

 smoothed and striated, they were covered by this deposit. Not only 

 is there nothing to indicate the existence of any interval of time be- 

 tween the two things, but some of the strisc are so minute, that had 

 they not been covered up almost so soon as formed, they would not 

 have been preserved. 



2. But would the passage of this boulder-clay, forced forward by 

 an enormous or powerful volume of water, hvLwe produced the smooth^ 

 ings and strice on the rocks 1 



That water, even by the friction of its own particles, will, in time, 

 wear down the hardest rock, is undeniable ; and if it is hurrying 

 along clay and gravel and heavy stones, the process of attrition will 

 be great and rapid. I think, therefore, that it is not difficult, in 

 this way, to understand how rocks (whether large boulders or hill- 

 sides) have been worn down, and smoothed and polished. 



But how are the scratches and furrows to be accounted for 1 Or- 

 dinary gravel, however great the accumulation, would not produce 

 that effect. But blocks of porphyry, basalt, felspar, and other hard 

 stones, if heavy and angular, would, I think, be graving-tools quite 

 sufficient for the purpose. Now, as already stated, there are multi- 

 tudes of angular blocks of porphyry, and other hard traps, in the 

 clay deposit covering these scratched rocks, many of which were 

 seen to be in actual contact with their surfaces when first exposed 

 by the workmen ; so that the instruments necessary for the pur- 

 pose were lying on the spot where they had been at work, and in 

 the farther execution of which they had been arrested. 



That water has been the j)olishing agent, will be the more readily 

 allowed by those who are familiar with the polished surface of rocks 

 forming the channels of mountain torrents ; and were any great de- 



