The Glacial Theory of Prof. Agassiz. 351 



The ice, in consequence of its tendency to dilate, and its numerous 

 fissures, accommodates itself to the sinuosities of the rocks which con- 

 fine it, cutting off the smaller projections, and rounding and polishing 

 the larger, which assume the form of domes, and were termed roches 

 moutonnees by Saussure. Agassiz's eighth plate gives some fine exam- 

 ples of these rounded swells. Owing to the immense pressure, the 

 included pebbles of conglomerates, and the hardest veins in veined 

 rocks, are cut away to the very same level with the softer parts which 

 envelop them. 



Thus, one of the marks by which the ancient existence of glaciers 

 can be detected in situations where they are no longer seen, is the pol- 

 ished, striated, or grooved appearance of the rocks. Sometimes it is 

 very distinct, but in many cases it is not visible, because the surface of 

 most rocks wastes away by disintegration or decomposition, unless it is 

 well protected by a covering of clay or turf. The most satisfactory 

 specimen near Edinburgh, is in the quarry on the south side of Black- 

 ford Hill, at a place laid open a few years ago, where the rock leans 

 forward, forming a sort of vault. The surface of the chnkstone here, 

 for a space of ten or twelve feet in length, is smoothed, and marked 

 by stria or scratches in a direction approximating to horizontal. We 

 accompanied M. Agassiz to the spot about two months ago ; he had 

 expressed doubts as to some other supposed marks of glacial action 

 near this city, but on seeing those at Blackford Quarry, he instantly 

 exclaimed — " That is the work of the ice." On the top of Salisbury 

 Crags, at a quarry about two hundred yards from their south extremity, 

 the polishing is very well seen at intervals over a space of twelve or 

 fifteen feet just at the edge of the precipice ; and strice, running east 

 and west, will also be discovered here by an eye accustomed to observe 

 them, though they are much less distinct than at Blackford Hill. In 

 quarrying the Crags at this spot, the rock had been cut back about one 

 hundred and twenty feet from what was originally the edge of the pre- 

 cipice, and this part, which had been well protected by the turf, was 

 only exposed about 1822 or 1823. We have little doubt that similar 

 appearances would be presented if other parts of the greenstone, equally 

 distant from the edge of the precipice, were newly laid bare. Parts of the 

 north end of the Castle-rock are also curiously polished, and the groovings 

 on the western slope of Corstorphine Hill, described many years ago by 

 Sir James Hall, are well known. We have observed similar marks of 

 abrasion at Craigleith Quarry, Craigmillar Hill, and elsewhere. 



These marks of abrasion, both on rocks in situ, and on boulders 

 found in the soil, have been usually attributed to the action of the cur- 

 rents of water, rolling along stones and gravel, an explanation felt not 

 to be satisfactory, but adopted for want of a better. It is admitted that 



