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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



there can be little doubt, in our opinion, that the Lewisian beds 

 formed the western or shore-side of an extensive trough of a deposit 

 of the same age, the central portions of which are covered by the 

 waters of the broad channel of the Minch. 



On the mainland, indeed, there are decisive proofs that these 

 sandstones and conglomerates are of Cambrian age, because they are 

 clearly surmounted, as shown in previous memoirs, by strata con- 

 taining Lower Silurian fossils. In the Lewis, however, where there is 

 no upward sequence, the inference that they are of the same remote 

 age must rest upon the fact of their being made up exclusively of the 

 Laurentian gneiss, and on the appearance they present of having been 

 a portion of the same great range of red and chocolate-coloured 

 sandstones which occupy the opposite headlands of Ross and Suther- 

 land. At the same time it is right to state that the sandy beds in- 

 tercalated in the conglomerates of the Stornoway headlands are so 

 infinitely less coherent than the red sandstone of the mainland (for 

 the thin courses of sand in the Lewisian beds are rarely in the form 

 of solid stone, but have rather a soft marly character) that some 

 doubt must always remain as to the true age of these insulated de- 

 posits. It is the more essential to enter this caveat, because there are 

 red conglomerates at various places along the west coast of Scotland 

 (as, for example, at Oban), the ages of which are also incapable of 

 rigorous demonstration, from the absence of any overlying deposits. 



The conglomerate of the Lewis is cut through by dykes of green- 

 stone, which have a direction from N. by W. to S. by E., and are well 

 exposed on the northern face of the headland of the Eye. Contain- 

 ing some olivine, these dykes of greenstone have produced a marked 

 effect on the conglomerate through which they cut, as seen on the 

 sides of the cliffs. Elanked on either side by a thin " Sablband " of 

 Lydian stone which separates the greenstone from the conglome- 

 rate, the latter is for a yard of a whitish colour, — the red colour of 

 the rock, as due to the presence of iron, having been driven off. 



Laurentian Gneiss of the Mainland in Ross-shire. — Descriptions 

 have previously been given of the character of the older gneiss of 

 Sutherland, where it usually forms low headlands on the sides of 

 deep bays, and is uneonformably surmoirnted by mountainous masses 

 of Cambrian Sandstone. The same rocks, extending southwards 

 along the western shore, expand largely in Ross-shire, as exhibited in 

 Gairloch and on the sides of Loch Torridon, as well as on the eastern 

 side of the long freshwater Loch Maree. On the right bank of Loch 

 Maree this gneiss contains both limestone and ironstono, which occur 

 in bands regularly interstratified, and which have the normal strike 

 of these ancient beds, as in the outer Hebrides. This strike from 

 N.W. to S.E. is therefore parallel to the great depression occupied 

 by the water of Loch Maree. In no portion of the North-western 

 Highlands is there a tract which more completely exhibits the en- 

 tire independence of this Laurentian gneiss of all those overlying 

 deposits, also termed gneiss, with which Prof. Nicol has recently 

 sought to identify it, both in his Geological Map of Scotland and 

 in his memoir read before the Geological Society. We therefore 



