I860.] JAMESON DRIFT, ABEEDEENSHJEE. 355 



the internal structure of which sometimes shows that their present 

 form is not the result of denudation on what had formerly been a 

 horizontally arranged deposit, but whose inward undulations conform 

 to the exterior outline. And further, had the excavation of the drift 

 been altogether owing to fluviatile agency, we should expect to find a 

 greater concentration of large boulders along their channels, and also 

 more evidence of river-meadows (or hemghs, as they are called in 

 Scotland) at high elevations, than what we see. 



Then take a valley with a chain of lakes. These still sheets of 

 water woidd arrest all the stones and gravel brought down by the 

 stream from the valley above — in short, everything except the finer 

 impalpable mud, and, where the lake was large and deep, even most 

 of that. Now I think the signs of denudation in the neighbourhood 

 of such lakes and in the valleys beneath them are much greater than 

 we should expect on the supposition of mere river-action combined 

 with a very gradual, slow emergence, like that supposed to be going 

 on in Scandinavia at the present day. The "crag-and-tail" pheno- 

 menon before mentioned is equally inexplicable on a similar theory. 



From these and other considerations which it would be tedious to 

 dwell on longer, it therefore appears to me that the retreating action 

 of the sea during the emergence of the land from the waters of the 

 drift-period has borne a considerable part in the matter. The reces- 

 sion of this sea, caused by a strong earthquake-shock upheaving the 

 land, would give rise to an action sufficient to account for many of 

 those appearances which I have touched upon. Even the records of 

 modern times show how often (wc may say, indeed, how constantly) 

 nature acts in this way, although, I think, it is going too far to 

 suppose that the short space of time covered by the roll of history 

 has afforded a complete insight into the operations of nature in this 

 respect. 



Multitudes of such shocks probably occurred as the land gradually 

 straggled out of the sea; and the grating effect of the waters as they 

 rushed back off the land would be sufficient to scour out the narrow 

 ravines of the valleys, project the gravel out into the open spaces, and 

 give rise to those appearances of "crag-and-tail" so frequently seen. 



In the valley of the Dee this superficial water-rolled gravel is 

 very well marked as far up as [nvercauld, nearly 50 miles inland, 

 and fully L000 feet above the present sea-level. Further up the val- 

 ley the underlying drift becomes of looser texture and of a more 

 gravelly character, so that it is less easy to distinguish the two 

 deposits. 



§ '2. With regard to the boulder-drift, I had examined sections of 



it at many points between Itamhnry and liraeniar, and found it ex- 

 tending all up the valley, everywhere possessing \ erj similar features. 

 No beds of brick-clay nor any gnat thickness of laminated sand had 

 presented themselves between these two villages, which are upwards 

 of forty miles apart, the former situated at an elevation of aboul 200 

 feet, the latter aboul L130. Everywhere the drift seemed to test 

 immediate!] on the old rocks, and consisted of a hard tenacious mass 

 of grittj mud, varying in tint from bluish-grej to pale brownish- 



