Chakleswquth — Glacial Geology of North-West of Irehmtl. 217 



camptonites of Duiiree, and the felspathic grits of Gleiitogiier, indicates a 

 eneral northerly transpoi-t. 



\ 10. The Sjjerrin Mountains and the Foyle Estnary. 



The Sperrin MounJ}ains are equally extensive and in their average 

 altitude at least as high as the Barnesmore Hills; indeed, in actual altitude 

 of the highest summits, the Sperrin Mountains slightly exceed the latter. 

 The reason for their difference of behaviour, the one acting as a powerful 

 centre of dispersion, the other overridden by extraneous ice, is probably, in 

 tlie first place, due to their respective positions relative to the position of 

 maximum precipitation, and, secondly, to the fact that the Sperrin Mountains 

 lay between the eastward-moving Donegal Ice and the westward-moving 

 Scottish Ice, which by piling up the glaciers and raising the ice-surface 

 caused the stronger Donegal Ice to override the hills wliich lay in its 

 immediate track. Nor is the contrast in the behaviour of the Speriin 

 Mountains as compared with the Sligo Hills less striking. So. far as my 

 researches allow me to speak, the latter hills seem to have been able 

 completely to withstand and deflect the Donegal Ice masses, tliough the 

 distance by which they are removed from the great Barnesmoie centre is 

 only some 16 miles, as against almost double that distance fn the case of the 

 Sperrin Mountains. 



The Sperrin Mountains during tlie maximum glaciation were overridden 

 by Donegal Ice. They are thickly swathed in drift that spreads northward to 

 Lough Foyle, where at Eglinton it rests upon the Scottish boulder-clay. 

 Here more or less rounded pieces of the older reddish and calcareous boulder- 

 clay liave been torn off and enclosed in the newer deposit, proving that the 

 later glaciation from the south-west has to some extent churned up the older 

 drift. In the bottom of the Eoe Valle}', there appears to be a mixture of 

 drifts from eastern and southern sources respectively, the constituents 

 of quartz, epidiorite, quartzite, and a few small granites, together with 

 sohist and carboniferous sandstone, with occasional pieces of chalk- 

 flint, chalk, and basalt, the whole embedded in a clayey and sandy 

 matrix. These materials are also found associated together at Calihame 

 Bridge and along the lower (northern) flanks of Glenshane Mountain, and in 

 the streams, east of Bolea, where the matrix of the drift is brick-red in 

 colour, due to the disintegration- of the Triassic rocks. A similar mixture of 

 materials which at lower levels characterizes different drifts does not at higher 

 levels necessarily imply a double glaciation, for iu these higher regions 

 metamorphic rocks, basalt, and chalk crop out over adjacent pieces of councry. 

 This close association of the outcrops of rocks, the boulders of wliieh, with 



