hy the passage over them of Sharp Detrital Matter. 113 



drift, and have contrasted them with similar appearances in 

 secondary sandstone rocks. It is not the time to enter into 

 an explanation of such phenomena further than to urge that 

 they prove a long continued and regular mode of deposit, 

 implying a sequence of strata. Nor do I see anything in 

 their character which can, a priori, decide in any case whe- 

 ther they preceded or followed the deposition of boulder- 

 clay. Local evidence has, however, proved that in some cases 

 the clay preceded the stratified sands and gravel ; but it is not 

 impossible that, in other cases, marine currents may have 

 swept over the surface of rocks, and hurried with them sand 

 and shingle prior to the deposition of boulder-clay. If such 

 were the fact, the surface of a rock may have been smoothed 

 and polished by the friction of the sand passing over it, and 

 subsequently grooved and scratched by the more slow move- 

 ment of glaciers or of boulder-clay. 



" I have now come to the point where I may fitly notice 

 the paper or letter of a respected fellow-member, your for- 

 mer president, Mr Robert Mallet, brought forward on the 

 11th December, in which he lays claim to priority in an 

 explanation of the mode in which rocks have been grooved 

 and scratched. Mr Mallet states that, in company with 

 Professor Oldham, he examined, in May 1844, the cuttings of 

 the Drogheda Railway made through the calpe, in the neigh- 

 bourhood of Killester, and observed numerous scratches in 

 the rock, and on the lower surfaces of boulders imbedded in 

 the clay and gravel beds above it ; and some of the scratches 

 appeared to indicate that the superincumbent clay had been 

 forced en masse up hill over inclined calpe beds ; and that 

 he concluded, from the evidence in general (and communi- 

 cated his views on the subject to Professor Phillips and the 

 Council of the Society, on the 5th June 1844), that the 

 scratches had been caused by the movement en masse of ' the 

 clay and gravel beds over the rock beneath, and that the 

 scratches upon the latter, as well as those upon the large 

 boulders reposing on the rock and imbedded in the clay, had 

 been produced by their being carried over the rock along with 

 the moving masses of clay and gravel.' On the 12th Nov. 

 1845, Mr Mallet read a paper on the subject to the Society, 



VOL. LII. NO. cm. — JANUARY 1852. H 



