486 T. F. Jamieson — Changes of Level in Glacial Period. 



but the facts observed by geologists seem to show that movements of 

 some kind, even in a lateral direction, are possible at no great depth 

 beneath the surface, although the ^-reat internal mass of the earth 

 may be as rigid as certain mathematicians would have us believe. 

 Some degree of plasticity or viscosity in a stratum not far down 

 would suffice to allow of all the movements I have assumed to be 

 possible in the foregoing observations. But whether the outer crust 

 reposes on a stratum of this nature or not, it must, I should think, 

 be in a state of equilibrium, or if not it will move. If the down- 

 ward pressure is not exactly equalized by the upward resistance, 

 movement must ensue to restore the balance. Hence the level of 

 the surface must always be tending to an exact equilibrium, and 

 must respond to any change, however slight, in the load laid upon it. 

 The response no doubt may be quite infinitesimal, but is it not 

 inevitable ? We might compare the action to that of a spring of 

 enormous strength, so strong as to be practically rigid to human 

 observation. 



If the views of Adhemar and Croll regarding the Glacial period 

 are right, we may believe that in each hemisphere alternations of 

 great cold with warmer intervals took place owing to the precession 

 of the equinoxes. And if a great load of ice on the land really 

 causes a depression, we may infer that each period of glaciation 

 would be accompanied by a depression or submergence proportionate 

 in some measure to the greatness of the load of ice. Now there 

 seems to be evidence that some such train of events actually did 

 occur both in Scandinavia and Scotland, the severity of glaciation 

 finally becoming less, and the amount of submergence also. 



The Arctic Shell-beds of Scotland. 



I have for a long time been of opinion that in the Clyde district 

 and West of Scotland we have at least two distinct sets of marine 

 beds containing northern shells, viz., (1) an older set which attains 

 to much higber elevations (in one case up to fully 500 feet) and has 

 been more or less destroyed by a subsequent development of glaciers 

 and land-ice ; (2) a newer set confined to lower levels and which 

 has never been subjected to the same destructive action. In the 

 latter series the species are much more numerous, the shells much 

 more plentiful and in much better preservation, many being 

 evidently in the spot where the animals lived and died — rows of 

 Mya truncata, for example, in the position of life, with pieces of the 

 siphuncular tubes still remaining. It is from this series that 

 Mr. Smith, of Jordanhill, got most of his specimens. These two 

 series of shell-beds, I believe, were separated by a long period of 

 land-ice which heavily covered all the west side of Scotland and 

 spread more lightly over the eastern side. I think, therefore, they 

 ought not to be confounded together as has been generally the case, 

 seeing that a great lapse of time and an important change of 

 conditions intervened between them. In the older set the shells are 

 generally more or less crushed and are often covered by masses of 

 boulder-clay, showing that severe glacial conditions had come on 



