Tramactumn. 49 



of Scotland were moulded previous to the Carboniferous Era, 

 a statement sufficiently substantiated by the fact that many 

 of the hollows cut out from the Silurian rocks are occupied 

 by strata of Carboniferous Age, a circumstance ■which 

 could not have occurred in any other way. Among the 

 valle3'^s thus produced was the one which now forms the 

 Middle Ward of Nithsdale, in which Thornhill is situated. 

 During the earlier parts of this era (the Carboniferous) this 

 valley, with the rest of the country, was submerged under a 

 sea in which strata belonging to the Carboniferous Limestone 

 Series were deposited. In the latter part of the same era 

 the country was re-elevated, and presented the principal sur- 

 face outlines which we now behold, and these outlines so far 

 as they relate to Middle Nithsdale, I have already described. 

 There is, however, a very important factor which must not 

 be lost sight of in the consideration of this subject. I allude 

 to the vast amount of denudation which must have taken 

 place since Permian times, and which must have materially 

 contributed in lowering the height of hills which surround 

 the valley. It would be altogether out of the question to 

 attempt to form any conception of the amount of this erosion, 

 but if during one era whole valleys can be excavated on a 

 large scale, surely during many eras greater results must 

 have been produced. This is all the more probable when we 

 consider that, since the Carboniferous Epoch, Scotland has 

 been oftener above than below water, and consequently made 

 all the more liable to be denuded by the usual agents — 

 water and ice. We will not attempt to guess at the amount 

 taken off the hills ; but I think we cannot go far wrong if 

 we merely use indefinite terms and say a few thousand feet. 

 Now, adding this unknown but undoubtedly large amount to 

 the present very great height of the Lowthers and the 

 smaller ranges which diverge from them to form the valley 

 of the Nith, we would then have most favourable surface 

 condition for the accumulation and descent of a mass of ice ; 

 because, as you are aware, glaciers are an accumulation of 

 snow (formed on mountains, if in the temperate region, or 



