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British representative of the oldest known stratified rocks, 

 the Laurentians of Canada. In Sutherland, they take the 

 form of a highly crystalline granitoid gneiss, forming a 

 range of low rolling hills and headlands along 

 the sea coast, and dipping manifestly under the very pic- 

 turesque detached mountains, which tower some miles 

 inland. These mountains, Coulmore, Canisp, Suilvein and 

 Cunaig, are mainly composed of a vast thickness of choco- 

 late coloured sandstone and conglomerate, entirely unfossi- 

 liferous so far as present observation goes, reposing 

 unconformabl y on the gneiss already described. The caps, 

 however, and eastern flanks of these mountains (for their 

 escarpment is to the west) consist of stratified quartzite, 

 also resting unoonformably upon the russet sandstone 

 series. This quartzite, which is simply sandstone 

 altered by igneous action, again dips under a considerable 

 thickness of limestone, little if at all altered, and exhibiting 

 all the peculiar features of limestone districts generally, 

 viz., the surface split up into a kind of pavement, the 

 streams disappearing down " swaUows " and flowing 

 through underground channels, and the propensity which 

 the mountain limestone always has to form bold escarp- 

 ments and caverns. This again is seen to dip beneath 

 another stratum of quartz rock, similar to the other. It is 

 the limestone which really gives the key to the position. 

 TJnfossiliferous as it generally is, it has in one place, near 

 Duirness on the north coast, yielded to the researches of a 

 local geologist, Mr. Peach, a number of fossils, which 

 have without hesitation been referred to the Lower Silurian 

 period, and in particular, to a low stage in that period, the 

 lower part of the Llaudeilo formation, which formed the main 

 subject of the former lecture. This being established, we re- 

 cognize in the underlying quartz rock, the Lingula Flags, 

 or " Zone Primordiale " of Barrande, and in the uncon- 

 formable chocolate sandstones and conglomerates beneath, 

 we have the Cambrian system (the Longmynd or Bottom 

 rocks of Sir R. Murchison), and in the (still unconformable) 



