On the Geological Structure of the Highlands of Scotland. 43 



deenshire Highlands is completed, it will be impossible to affirm 

 positively whether or not Laurentian beds actually reappear.* 



The two Unconformities. Bearing upon the geological history 

 of this region, there is nothing more remarkable than the occur- 

 rence of the two unconformities, — the lower, between the Lauren- 

 tian and Cambrian beds, and the upper, between the Cambrian 

 and Lower Silurian series. 



Both of these are of the most trenchant description. The 

 abrupt truncations, and the sudden change of characters in the 

 beds lying on either side of the boundary lines, suggest long 

 intervals of time and great changes in the physical conditions 

 under which deposition took place. 



In the first place, the Laurentian beds were metamorphosed, 

 contorted, elevated out of the sea-bed, and denuded, before the 

 Cambrian beds began to be spread over the uneven floor thus con- 

 structed. Professor Ramsay considers the Cambrian sandstones to 

 be of fresh water or lacustrine origin,-|- and that glacial conditions 

 were to some extent prevalent during their formation.j Alarge pro- 

 portion of the pebbles and blocks found in the Cambrian beds is 

 composed of fragments torn from the Laurentian masses. 



Of the Cambrian formation it is clear that only a fragment now 

 remains in the North West Highlands. Its base is often visible,but 

 its original upper limit never. It bears evidence of the effects of at 

 least two great denudations. The first before the deposition of 

 the Lower Silurian beds ; the second at a later period, probably 

 often repeated, and coming down to recent times. The great 

 buttresses of horizontal sandstone — against which the Lower 

 Silurian beds rest — may be regarded as the eastern margin of 

 continental land of the Lower Silurian period, embracing the 

 outer Hebrides, and an unknown region beyond.|| At that period 

 the Cambrian sandstones were tilted and the Lower Silurian beds 



* The only rock resembling Laurentian which we noticed in the central districts was a 

 remarkable massive red gneiss, at Inchbrae, seven miles from Garve, in a N. W. direction, 

 on the Ullapool road. This gneiss consists of red felspar, black mica, a little quartz and 

 epidote. As far as its composition is concerned, it might be of Laurentian age ; but in 

 position it appears to be high up in the Silurian metamorphic series. 



t Phys. Geol. and Geog., Gt. Britain, 5 Edit, p. 285. 



J Pres. Address, Brit. Assoc , Rep. 1880, p. 17. This view is founded on the dis- 

 covery by Professor Giekie of glaciated surfaces of Laurentian rock passing underneath 

 Cambrian sandstone at intervals all the way from Cape Wrath to Loch Torridon, together 

 with large blocks of gneiss in the Cambrian bed. 



II Ramsay, Phys. Geol. and Geog., p. 87. 



