120 Lieut.-Colonel Portlock on the Scratching of Bocks 



of very hard silicious rocks, and, being moved on by the mass 

 of the glacier, act as so many diamonds on the rock below, 

 scratching its surface, whilst the mud and ice polish it. If," 

 he adds, " the striae and grooves under existing glaciers ha 

 been laid bare, and thereby made as distinct as those of ex- 

 tinct glaciers are, the connection between the striae and the 

 action of glaciers would have been long since admitted, and 

 an explanation of the phenomena neither sought for in cur- 

 rents of water or of mud." 



I shall quote no further from M. Agassiz, and I have dwelt 

 so long on the subject merely because I consider it of high 

 importance to determine the real mode of deposit of the 

 various descriptions of drift ; and though I have observed a 

 disinclination in many geologists to dwell on this or other 

 branches of physical geology, I cannot but feel that it must 

 ever be a reproach to geology if it cannot succeed in unravel- 

 ling the difficulties which still obscure our explanations of 

 drift, an operation which has taken place ahnost at the 

 moment of man''s birth. My predecessor in your chair had 

 also occasion to refer to this subject in his address ; and with 

 his observations I partially agree, partially disagree. I cannot 

 agree with him when he considers M. Martin's argument 

 defective, in illustrating the possibility of glacier action in 

 the mountain near Loch Lomond, at an elevation of 2400 

 feet, by the discovery of such action in the Alps, 2468 feet 

 above the bottom of the valley which contained it. There 

 is evidently no analogy between the case he adduces, namely, 

 that as one rock (protogine, for example) occurs in one dis- 

 trict, the statement of its occurrence in another might be 

 thereby established. Doubtless were some strange and ap- 

 parently anomalous rocky compound said to have been found 

 in some particular district, our belief of the possibility of its 

 existence would be aided by a knowledge that it had been 

 found in some other similar district ; and in this way the 

 glacier phenomena of the Alps may reasonably be called in 

 to illustrate those of the Scotch and other mountains. I do, 

 however, agree with my learned predecessor — and I had long 

 since expressed that opinion — that it is dangerous, nay, un- 

 philosophical, to view the phenomena of all " so-called drift 



