208 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



to believe that rocks with nearly identical mineral characters may 

 occur in more than one period, and as the geological connexion of 

 the strata is often obscure, many difficulties have still to be en- 

 countered in such investigations. 



There seems little doubt that the gneiss of Ben Nevis and the 

 Great Glen forms a continuous formation with the great mass of this 

 rock represented, in Dr. Macculloch's Geological Map and my own, 

 as running from Inverness -shire through Ross and Sutherland to the 

 northern extremity of Scotland. In a former paper I have shown 

 that there is no reason for separating this gneiss from that of the 

 north-west coast of the mainland or of the Long Island. The chief 

 distinctions arise merely from the greater predominance of intrusive 

 igneous rocks, granites and syenites, in the latter, and recur in the 

 eastern gneiss wherever these igneous masses appear. The mica-slates 

 of the south have also their representatives in these north-western 

 regions, where some of the mica-slates, like the thin, even-bedded 

 varieties on the Kyle of Tongue, contain garnets. Others, like those 

 of Ben Hope and Strath Oykell, have more resemblance to the talcose 

 varieties on Loch Creran. As the mica-slates in the south are a distinct 

 formation from the gneiss, it is probable that this may also be true 

 of those in the north, where their relations have been less studied*. 



The overlying red sandstones and conglomerates of the west 

 coast (" Cambrian" of Murchison) have no representative, so far as 

 yet ascertained, in the South-west, unless some portions of the red 

 sandstones in Cantyre belong to the same formation. The quartzites 

 and limestones that rest on them are Lower Silurian, according to 

 Mr. Salter's determination of the fossils found in them by Mr. Peach 

 and others. As I have ascribed the same age to the slates of Easdale 

 and Oban, it would follow that the connected quartzites in the 

 south-west are, to a certain extent, of the same age with those in 

 the north-west. This view of the identity of these quartzites, 

 embodied in Dr. Macculloch's Geological Map and my own, has also 

 been adopted in the recent map of Murchison and Geikie. As both 

 formations are the uppermost in the series, there is, of course, no 

 improbability in this view ; and as this Lower Silurian Period was 

 evidently of very long duration, and in other regions contains a great 

 diversity of deposits, the marked difference in some of the phenomena 

 cannot be held as invalidating it. The most remarkable of these 

 diversities are, first, the entire absence in the south of the great 

 underlying red sandstone and conglomerate group of the north. But, 

 as I pointed out in 1856, this red sandstone is even in the north a 

 very limited formation, not extending far to the east, where the 

 quartzite in many places appears resting immediately on the gneiss, 



* It is necessary to mention this diversity, as Sir R. I. Murchison appears to 

 have overlooked the statements to this effect in my former papers, and conse- 

 quently represents me as maintaining the identity of the gneiss of the west coast 

 with certain mica- or chlorite-slates. They are identical only so far as both 

 belong to the great series of metamorphic formations inferior to the red 

 sandstone and quartzite ; but, both in my papers and map, they are represented 

 as distinct formations, each with its own peculiar features, and, it may be, of 

 widely different age. 



