244 The Scottish Naturalist. 



carried its characteristic boulders as far as Birmingham before 

 the general mer de glace had attained its greatest dimensions. 

 But when that period of maximum glaciation arrived, the Welsh 

 boulders would be unable to travel so far towards the east, and 

 the Scottish and Cumberland boulders would then cross the path 

 formerly followed by the felstone erratics from Great Arenig. 



Again, it is evident that when the mer de glace was gradually 

 decreasing similar oscillations of the ice-flow would take place, 

 but in reverse order, and thus would give rise to a second series 

 of intercrossings. Moreover, we must remember that the Glacial 

 Period was characterised by several great changes of climate. It 

 was not one continuous and prolonged period of cold conditions, 

 but consisted rather of a succession of arctic and genial climates; 

 so that the same countries were overrun at different epochs by 

 successive mers de glace, each of which would rework, denude, 

 and redistribute to a large extent the morainic materials of its 

 predecessor, and thus might well cause even greater complexity 

 in the dispersion of erratics than has yet been recognised any- 

 where in these islands. 



Mr Mackintosh refers to the occurrence of chalk-flints and 

 Lias fossils associated with northern erratics in the drift-deposits 

 of the west of England, the presence of which, he thinks, is fatal 

 to the theory of transport by land-ice. Thus, he says, chalk- 

 flints, &c, have been met with at Lillieshall (east of Wellington), 

 at Strethill (near Ironbridge), at Seisdon (between Wolverhamp- 

 ton and Bridgenorth), at Wolverhampton, near Stafford, and near 

 Bushbury. Chalk -flints have also been found as far west as 

 Malvern and Hatfield Camp, south of Ledbury. All these 

 erratics have crossed England from the east, according to Mr 

 Mackintosh and other observers. Not only so, but, as Mr 

 Mackintosh remarks, those found at Wolverhampton, Birming- 

 ham, &c, "must have crossed the course of the northern boulders 

 near its southerly termination." And since both northern and 

 eastern erratics are found associated in the same drift-deposit, it 

 seems to him " impossible to explain the intercrossing by land- 

 ice or glaciers." Now, on the contrary, those eastern erratics 

 are scattered over the very districts where I should have ex- 

 pected to find them. The observations of geologists in East 

 Anglia have shown that that region has been invaded by the 

 mer de glace of the North Sea basin. * This remarkable glacial 



1 See Mr Skertchly's description of East Anglian deposits in ' Great Ice 

 Age,' p. 358. 



