304 Dr Ch. Martins on the Marks of Glacial Action 



the north limb of the hill, 500 above the sea ; whilst, on the 

 Pentland Hills, there are, near Dunsire, striated rocks at 

 the height of 800 or 900 feet, and on the east of Cairnhill 

 at not less than 1400 feet* But what proof have we that, 

 since the glacial epoch, the middle region of Scotland has 

 been elevated 1800 feet % In Norway, that classic land of 

 change of level, the shelly bed — skalen-schicht— does not ex- 

 ceed, according to M. Keilhau, 565 feet,t and the most ele- 

 vated terraces reach only to 600 feet. 



The theory of floating ice, then, must necessarily be 

 supported upon a previous hypothesis^ namely, that at the 

 epoch of floating icebergs the mean level of Scotland 

 was at least 1400 feet lower than it is at present ; or, in 

 other words, that it had undergone a change of level three 

 times greater than that of Scandinavia. 



Let us now then study the mode of the formation of floating 

 icebergs, and the action which they can exercise upon the 

 rocks in situ^ when they are impelled by marine currents. 

 All being agreed that these floating ice-masses must be assi- 

 milated to those which now still detach themselves from the 

 polar glaciers, we shall, in a few words, review the mechan- 

 ism of their formation. 



When at Spitzbergen, in summer, a glacier descends in 

 a valley which opens into the sea. it stops not at the shore, 

 but continues to advance, and enters into the ocean. The por- 

 tion of it which plunges into the water gradually melts ; and 

 at low water the inferior part of the glacier rests suspended, 

 so that we can perceive an interval or space between the 

 water and the ice. It is under these circumstances that 

 portions of the glacier are detached, drop into the sea, and 

 are set at liberty by the ebbing tide. Such is the origin of 

 floating icebergs. Before these floating ice-masses can wear 

 and groove the hardest rocks, such as trap, it is necessary 

 that they carry upon their inferior surface, pebbles or grains 



* Mr Maclaren on Grooved and Striated Rocks in the Middle Region of Scot- 

 land. Edin. New Phil. Journal, vol. xlvii., p. 176. 1849. 



t Daubr^e. Bulletin Soc. Geolog. de France, vol. xiv., p. 574, 1843. 



