72 rROCEEBINGS OF THE GEOLOGICAL SOCIEir. 



successive volcanic episodes. The pebbles may therefore be the 

 relics of eruptions that took place long after the period of the 

 dykes. 



II. THE you:n"gee, schists.— DALEADIAN. 



Between the Archaean rocks and everything of younger date there 

 lies a complete stratigraphical break, representing, doubtless, an 

 enormous lapse of time, which does not appear to be represented 

 within our area by any geological formation. The reality and sig- 

 nificance of this discordance are shown by the strongly-marked 

 lithological contrast between the fundamental gneisses and all less 

 ancient rocks, by the striking unconformability of even the oldest 

 formations upon the gneiss, and by the fragments in the overlying 

 conglomerates in these formations, which prove the gneiss to have 

 acquired its present characteristic structures before the conglomerates 

 were derived from it. 



Of the rock-groups which were laid down between Archaean 

 time and the oldest known portions of our Palaeozoic formations, the 

 only one of which the stratigraphical relations are abundantly clear 

 is the Torridon sandstone. This important series of strata, at least 

 8000 or 10,000 feet thick, lies un conformably on the Lewisian gneiss, 

 and is in turn covered unconformably by the fossiliferous quartzites 

 and limestones of Durness. It was paralleled by Murchison with 

 the Cambrian rocks of Wales ; but it must be more ancient than 

 at least the younger part of the British Cambrian system. The 

 Durness limestones were classed by the same acute observer as 

 Lower Silurian, and Salter pointed out the remarkable American 

 aspect of their fossil fauna. But they have not yet been satisfac- 

 torily placed on any definite palaeontological horizon among the older 

 Palaeozoic rocks of England or Wales. Most probably they are older 

 than the Arenig group. In any view the high antiquity of the 

 Torridon sandstone cannot be disputed. But for my present pur- 

 pose this ancient formation may be passed over, for it has yielded no 

 evidence of contemporaneous volcanic rocks. 



To the east of the line of the Great Glen the Scottish Highlands 

 display a vast succession of crystalline schists, the true stratigraphical 

 relations of which to the Lewisian gneiss have still to be determined, 

 but which, taken as a whole, no one now seriously doubts must be 

 greatly younger than that ancient rock. Murchison first suggested 

 that the quartzites and limestones found in this newer series are 



