112 , 



cases, as on Arthur's Seat, the Calton Hill, and TordufT to the west- 

 ward of Bonally, near Colinton, the surfaces were equally dressed and 

 scratched although the superficial strata were absent. 



In the neighbourhood of Edinburgh, the oldest member of the su- 

 perficial strata is the boulder-clay, which rests on the dressed sur- 

 faces. The boulders are chiefly of trap, rounded, and, in many in- 

 stances, scratched, and the accompanying fragments are portions of 

 the strata in the immediate neighbourhood. The author was in- 

 clined to the belief, that the boulders had acquired their form pre- 

 vious to their enclosure in the clay, through which they are irregu- 

 larly distributed, scarcely ever in contact ; and in which, even sup- 

 posing the mass in motion, they must, in a great measure, be in re- 

 lative rest. 



The author considered it probable, that the dressing had taken 

 place previous to the formation of the boulder-clay, and at the pe- 

 riod when the boulders themselves acquired their form ; and that it 

 was produced, when the rocks were under water, by the attrition 

 of stones or ice. He here adverted to the action of icebergs, and 

 considered their influence as having been exerted in fresh water, ac- 

 cording to a notion which he had long entertained, that, during the 

 formation of the older members of the superficial strata, the land 

 under consideration was covered by a lake of fresh water. He then 

 adverted to the glacial theory of Agassiz, as well fitted, if otherwise 

 probable, to account for the dressings ; because, if a sheet of ice co- 

 vered the country for a series of years, it must have experienced innu- 

 merable shiftings, and produced corresponding abrasion of the rocks, 

 hy its expansions and contractions as a solid, with every change of 

 temperature, independent of any dilatation by freezing. 



The author here took, notice of the structural character of the trap 

 rocks of the district, as exhibiting, in the crumpled surfaces of their 

 strata, and the sUckensides with which they abound, appearances 

 which may readily be confounded with dressings. 



The difficulties attending any explanation, yet proposed, of the 

 mode of formation of the boulder-clay, the author considered as very 

 great, and frankly stated that hitherto he had not been able to ar- 

 rive at any satisfactory conclusion. 



The bouldei'-clay becomes more sandy towards the upper portion 

 of the mass, and passes, by means of alternating beds, into the next 

 newest formation, the boulder gravel and sand. The gravel forms 

 the lowest portion of this mass, and frequently contains numerous 

 boulders of trap towards its lowest portion. The sand towards the 



