The Scottish Naturalist. 195 



Along the base of the Highland mountains in Forfarshire, 

 &C, we meet with similar intercrossings of erratics. Thus 

 we occasionally encounter in the boulder-clays overlying the 

 Silurian regions erratics of Old Red Sandstone rocks which have 

 come from the east or south-east ; while the abundant presence 

 of erratics of Silurian origin, on the other hand, bespeak an ice- 

 flow from the west towards the low grounds. In some places 

 within the Silurian area we encounter a greyish-blue boulder- 

 clay, containing Silurian fragments only, while in other places 

 within the same area the boulder-clay becomes reddish, and is 

 charged with many boulders of Old Red Sandstone rocks. Now 

 the greyish-blue till could only have been laid down by glacier- 

 ice descending from the Silurian high grounds to Strathmore, 

 while the red boulder-clay points to a partial invasion of the 

 Silurian regions by land-ice, which had previously traversed the 

 lower-lying Old Red Sandstone areas. These apparently contra- 

 dictory movements are readily accounted for by the former 

 presence in the area of the North Sea of the great Scandinavian 

 mer de glace. My friend Dr Croll was the first to point out 

 that the glacial phenomena of Caithness and the Shetlands 

 could only be accounted for by the advance of the Scandinavian 

 ice- sheet towards our coasts, where it encountered and deflected 

 the Scottish ice-sheet out of its normal course — a sagacious 

 induction, which the admirable and exhaustive researches of my 

 colleagues, Messrs B. N. Peach and J. Home, have now firmly 

 established. The lower blue boulder-clay was evidently accumu- 

 lated at a time when the Scottish ice was able to flow more or less 

 directly east or south-east towards what is now the coast-line ; 

 while the overlying red boulder -clay points to a subsequent 

 period when the presence of the Scandinavian mer de glace was 

 sufficiently great to compel the Scottish ice out of its normal 

 course, and cause it to flow in a north-easterly direction. In 

 doing so it now and again passed from tracts of Old Red Sand- 

 stone to invade the Silurian area, and thus an overlying red 

 boulder-clay was here and there accumulated upon the surface 

 of a greyish-blue till in which not a single fragment of any Old 

 Red Sandstone rock occurs. 1 



1 The blue or grey boulcler-clay of the region referred to, I am inclined 

 to assign to an earlier epoch of the glacial period than I would the red 

 boulder-clay. The latter I take to be the moraine profonde of the last 

 ice-sheet which, as the researches of B. N. Peach and J. Home have shown, 

 flowed northwards along the seaboard of Scotland from as far south at 



