531 Mr. Arcliibald GeiJcie [June 7, 



The thick mass of red sandstone and conglomerate, which rests 

 directly on the Archaean gneiss, forms some of the most singular 

 scenery in the north-west of Scotland. Owing to vast denudation, 

 which began before the next gi'oup of strata was deposited, it has 

 been worn down into isolated mountains, which rise like a chain of 

 colossal pyramids along the western shores of Sutherland and Eoss. 

 The almost level lines of stratification give to these eminences a look 

 of architectural symmetry, in striking contrast with the more tumul- 

 tuous aspect of the other rocks of the region, while theii* red tone 

 of colour marks them out boldly from the wastes of grey gneiss below 

 and the crags of white quartzite beyond. From the far northern cliffs 

 of Sutherland these massive red sandstones can be followed almost 

 continuously to the southern headlands of Skye. They reaj^pear in 

 great force in the island of Eum, beyond which they are not certainly 

 traceable. A group of highly altered grits and schists, seen under 

 the great basaltic plateau of Gribuu, on the west side of the island of 

 Mull, may mark their extreme southerly limits.* The red sandstones 

 certainly do not come so far south as lona, and not a trace of them 

 has been met with in Ireland. They extend westwards across the 

 Minch, for a small portion of them skirts the eastern shore of the 

 Long Island. How far they may have stretched eastward cannot 

 now be determined, for their limits in that direction have been 

 obscured or effaced by the extraordinary series of gigantic earth- 

 movements to be afterwards referred to. There can be little doubt, 

 however, that they did not reach the district east of the line of the 

 Great Glen, though they not improbably lay in thick mass over 

 much of the country to the west of that valley. 



We cannot now trace the original limits of these red rocks, yet we 

 can hardly doubt that they never covered an area at all comparable 

 in extent to that of the rocks below and above them. They appear, 

 indeed, to have been accumulated in one or more basins, shut off from 

 free communication with the open sea, where the deposition of ferru- 

 ginous precipitates among the ordinary mechanical sediment could go 

 on during the deposition of many thousand feet of rock. Such con- 

 ditions of sedimentation were not very favourable to the existence of 

 life in the waters of these enclosed basins. Nevertheless, that the 

 waters were not entirely lifeless is shown by the discovery of organic 

 remains on two widely separated horizons among the sandstones. 

 These remains occur in grey and dark shales, the colour and compo- 

 sition of which suggest a temporary influx of water from without 

 and the cessation for a time of the deposition of the iron-oxide. At 

 the lower horizon the fossils consist of calcareous rods, the organic 

 grade of which is still in dispute ; at the higher they include some 



* My attention was called to these rocks by the Dake of Argyll, who himself 

 suggested their jwssible Cambrian age. I visited them this spring, and found 

 them to be greatly metamorphosed. They do not appear in lona, where the base 

 of the sedimentary series is found resting on the Archsean gneiss. 



