1865,] JAMIESON LAST CHANGES IN SCOTLAND. 169 



gone eastward down the valley of the Dee, but not southward, 

 being repelled apparently by the ice descending from the high 

 ridge of quartz -mountains that forms the boundary between the 

 counties of Aberdeen and Perth. 



As regards the midland region of Scotland, the Boulder-clay lies 

 thickest on the eastern slope of the island; in the West Highlands 

 there is comparatively little of it, the rocks being very much bared. 

 This is what might be expected from the more gradual and longer slope 

 of the east side. Over much of the low ground of the Scottish coal- 

 field also there seem to be heavy masses of it. It is frequently 

 disposed in banks of very irregular thickness, often thinning out 

 abniptly, and having occasionally an irregularly undulating or 

 hummocky surface. 



The physical quality of this boulder-earth shows it to be due to 

 some peculiar action. It may be said to consist of rough stony 

 debris intimately mixed with a very fine mud, which seems to have 

 been derived from the tear and wear of the stones. This implies 

 powerful friction, combined with the presence of water, and yet 

 an absence of any current to carry off the fine sediment. Earthy 

 stuff dropped in the sea from melting ice I should think would 

 form a different deposit ; for the water would hold the fine muddy 

 particles in suspension for a time, while the sand and stones would 

 fall at once to the bottom. I consider that its true nature and 

 origin was first indicated by Agassiz, in his communication to the 

 Geological Society of London, on the 4th Nov. 1840, and more 

 clearly developed by him in a subsequent paper in the Edinburgh 

 New Philosophical Journal for 1842*. 



§ 4. Peeiod of Depeession. 



a. Glacial-marine beds. — Eeposing on the irregular surface of the 

 boulder-earth, we find, in some of the lower grounds adjoining the 

 coast, beds of finely laminated clay and sand containing sea-shells, 

 remains of starfishes and Echini, bones of seals, stones encrusted with 

 Balani, Foraminifera, and other relics of marine life, showing that 

 the sea had occupied a considerable part of what is now dry land. 



Thick beds of this laminated marine clay frequently occupy 

 basin-shaped hoUows of very limited extent in the Boulder-clay, 

 thinning out abruptly where the ground rises, as, for example, at 

 Portobello near Edinburgh ; this mode of distribution seems to occur 

 chiefly where there is hilly ground in the neighbourhood. In the 

 low north-eastern part of Aberdeenshire the marine clay is often 

 spread in wide sheets, ranging up to a height of 300 feet above the 

 sea : at this altitude there is a bed of it 13 feet thick on the brow of 

 an eminence near the town of Turriff, eight miles inland, where it is 

 dug for making bricks and tiles. It is rare, however, to find it of 

 pure quality at this height. In most districts this fine laminated clay 



* My confidence in the opinions I have fornaed regarding the glacial pheno- 

 mena of Scotland is greatly strengthened by fmding the same views ably advo- 

 cated by Mr. Geikie in his admirable memoir on this subject, " On the Pheno- 

 mena of the Glacial Drift of Scotland," Trans. Geol. Soc. Glasgow, vol. i. part 2. 



