178 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 11, 



remanie by glacier-action, the confusion becomes still more con- 

 founded. Such obscure portions must be deciphered by the aid of 

 the clear evidence afforded by more favourable localities. It seems 

 indeed to be the character of glacial deposits in general, whether 

 formed on land or ia water, to have a confused arrangement, so that 

 the character of a section changes at almost every step ; and this 

 may help to distinguish them from ordinary marine beds, whose sec- 

 tions have regular features over wide areas. 



e. Cause of the submergence. — It is worthy of remark that in 

 Scandinavia and North America, as well as in Scotland, we have 

 evidence of a depression of the land following close upon the pre- 

 sence of the great ice-covering ; and, singular to say, the height to 

 which marine fossils have been found in all these countries is very 

 nearly the same. It has occurred to me that the enormous weight 

 of ice thrown upon the land may have had something to do with this 

 depression. Agassiz considers the ice to have been a mile thick in 

 some parts of America ; and everything points to a great thickness 

 in Scandinavia and North Britain. We don't know what is the 

 state of the matter on which the solid crust of the earth reposes. If 

 it is in a state of fusion, a depression might take place from a 

 cause of this kind, and then the melting of the ice would account 

 for the rising of the land, which seems to have followed upon the 

 decrease of the glaciers. 



§ 5. Emeegekce of Land and final eetreat of the Glaciers. 



a. Valley -Gravel. — Along the course of all our larger river- valleys, 

 as in those of the Spey, the Dee and Don, the Tay, and others, we 

 find extensive beds and terraces of rolled gravel, which seem to be 

 of later date than the laminated clay with Arctic shells, seeing that 

 in the lower parts of the valleys the gravel overlies this clay. The 

 more recent origin of the gravel is further proved by its sometimes 

 containing rolkd lumps or nodules of the laminated clay, showing 

 that the latter must have suffered some denudation. 



In the absence of all fossils it is often impossible to distinguish 

 freshwater gravel from that which is marine ; for water arranges 

 sand and pebbles in the same way whether it be salt or fresh. 

 False bedding, as Mr. Sorby has pointed out, will sometimes help 

 us to trace the effect of tidal action ; although it is well to bear in 

 mind that back eddies often occur along the sides of a river, so that 

 oblique laminae pointing in reverse directions may occur even in 

 freshwater beds. I am therefore of opinion that it is only when this 

 feature is well developed that we can rely upon it as a test. 



Beds of gravel are by no means uncommon in the marine glacial 

 deposits, and in some of the lower districts I have occasionally ob- 

 served this " oscillating current structure," as Mr. Sorby terms it, 

 very well developed. At Ladybank railway- station in Fife, I have 

 noticed some good examples of it in a large side-cutting, also in 

 some sand-pits at Old Aberdeen. In the great shoals of gravel, 

 however, which overspread the bottom of the valleys in the more 

 hilly districts, I have never observed any decided instance of this 



