THE PRE- CAMBRIAN ROCKS, BRITISH ISLES. I I 



limestones, and "younger gneiss." His belief may still prove 

 to be in some measure well founded. But at present we have 

 no means of deciding whether the quartzites and limestones of 

 the Central Highlands are the more altered equivalents of the 

 undoubtedly Cambrian strata of the north-west. - It is possible 

 that in the vast mass of metamorphosed rocks constituting the 

 wide stretch of country from the northern headlands of Aber- 

 deen to the south-western promontories of Argyllshire, there 

 may be portions of the old Lewisian gneiss, tracts of highly 

 altered Torridon sandstone, belts of true counterparts of the 

 Cambrian quartzites and limestones of Durness, and, what should 

 not be forgotten, considerable portions of some later sedimentary 

 series which may have followed these limestones, but which, by 

 the great dislocations already referred to, have disappeared from 

 the north-west of Scotland. We are gradually learning more of 

 these rocks, as the detailed mapping of them by the Geological 

 Survey advances, and when the ground on either side of the 

 Great Glen is surveyed, it may be possible to speak with more 

 certainty regarding their true geological relations. 



A glance at a geological map of the British Isles will show 

 that the metamorphic rocks of the south-western Highlands of 

 Scotland are prolonged into the north of Ireland, where they 

 spread over a region many hundred square miles in extent. 

 They retain there the same general character and present the 

 same difficult problems as to their true stratigraphical relations. 

 Quite recently, however, a new light seems to have arisen upon 

 these Irish rocks. My colleagues on the Irish Branch of the 

 Geological Survey have detected several detached areas of 

 coarse gneisses, which in many respects resemble parts of the 

 Lewisian gneiss of north-west Scotland. In some cases these 

 areas lie amidst or close to " Dalradian " rocks, but with that 

 obstinacy, which so tries the patience of the field-geologist, they 

 have persistently refused to disclose their true original position 

 with regard to these. Some fault, thrust-plane, tract of boulder- 

 clay or stretch of bog is sure to intervene along the very junc- 

 tion-line where the desired sections might have been looked for. 



