James Geikie — On Changes of Climate. 551 



grained ingredients predominating. In certain localities they have 

 yielded Arctic shells of species identical with those which occur so 

 abundantly in the later Glacial deposits of our maritime regions. It 

 is quite certain, therefore, that some inter-Glacial beds are of marine 

 origin. Examples of such marine shell-bearing beds occur at 

 Airdrie overlain by Boulder-clay ; more recently similar deposits 

 have been met with in the Till near Greenock ; and other localities 

 might be named. But some inter-Glacial beds are no less certainly 

 of freshwater origin. A few years ago I described in this Magazine ^ 

 a very interesting section at Crofthead, which showed most clearly 

 a set of lacustrine beds resting upon and covered by the Till. From 

 these beds the skull of Bos primigenius and other mammalian remains 

 were obtained. Thin layers of peat occurred here and there in the 

 sandy silt. I need only remind the geologist that other mammalian 

 remains, such as the Mammoth and the Eeindeer, have been met with, 

 either below or in the Till of other districts in Scotland. 



It may be as well to remark, in passing, that we can seldom be 

 certain that beds of sand and gravel underlying the Till are neces- 

 sarily of pre-Glacial age. We can only say that they are older than 

 the mass of Till by which they are covered. Pre-existing masses of 

 Boulder-clay may have been removed before the beds referred to 

 were laid down. Unless they occur in those deep buried ravines 

 which have intersected the path of the ice-sheet, I should be very 

 doubtful of their pre-Glacial age ; and even in such sheltered nooks 

 we cannot be at all sure that their deposition preceded that of the 

 true Glacial beds. For in the intervals between the accumulation of 

 successive masses of Till, the erosion in ravines and valleys by water 

 action must have been excessive. Thus, bearing in mind the many 

 successive descents of the glaciers from the high grounds to the sea, 

 and the intervening periods of aqueous erosion, it is exceedingly un- 

 likely that any pre-Glacial accumulations whatever have been pre- 

 served in Scotland. I would therefore include all the deposits that 

 underlie or are intercalated with the Boulder-clay under the same 

 head. But, as will be seen in the sequel, it is of no consequence, so 

 far as the views which I support are concerned, whether the under- 

 lying deposits be considered of pre-Glacial or of inter-Glacial age. 



The freshwater origin of many of the intercalated beds can be 

 made out even in the absence of fossils. The materials of which 

 these beds consist are arranged precisely as we should expect them 

 to have been by streams and rivers — the "lie" of the gravel-stones, 

 and the dip of the bedding, showing that the water which pushed 

 the detritus along must have flowed in a direction down the valleys 

 in which these deposits occur. Besides such distinctly fluviatile 

 accumulations, we also find in the upper reaches of our valleys 

 laminated brick-clays resting in little basin-shaped hollows of the 

 Till, and clearly covered over by the same deposit. In these may 

 sometimes be detected thin lines of peat, like those of the Crofthead 

 section, and the general appearance of the deposits themselves be- 

 tokens a lacustrine origin. In the lower reaches of our broad open 



1 Vol. V. p. 393. 



