408 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JunC 16, 



mica- slate differs considerably in its mineralogical characters from the 

 rock so-named in the more central parts of the Highlands. Occasion- 

 ally, as in the hills north of Campbeltown, it is a fine lustrous mica- 

 slate, especially on the planes of lamination, but even then on a cross 

 fracture it appears very arenaceous. More often it resembles a grey 

 micaceous sandstone, and many hand-specimens might readily be mis- 

 taken for such a rock, or for varieties of greywacke. This is the 

 general character of the mica-slate in Ben GoUion south of Campbel- 

 town. In that mountain it is generally a reddish or yellowish grey 

 laminated rock, consisting of fine quartz or felspar sand, with larger 

 grains of quartz and very fine scales of a mica-like mineral, but which 

 is not, as it seems to me, a true mica. In other cases the mica is re- 

 placed by a mineral more resembling talc, but mixed in such minute 

 scales with the other constituents that I have not been able to deter- 

 mine its true nature. In other varieties fine folia of haematite or 

 specular iron take the place of the mica. Indeed, I am inclined to 

 think that true mica-slates are rather the exceptions in this district, 

 and that probably some other name should be adopted for these 

 rocks. In Ben GoUion also some of the beds show, when weathered, 

 a distinct double lamination, intersecting at an acute angle, and more 

 resembling the cleavage so common in the Silurian rocks than any 

 structure I have ever observed in true crystalline strata. 



In the mountains near the Mull many beds in the mica-slate form 

 a kind of semicrystalline gneiss, bearing the same relation to this 

 rock that the mica-slate does to that of the West Highlands. In this 

 southern district the mica-slate is highly ferruginous, and imparts a 

 strong chalybeate taste to the springs rising in it. The chief distinc- 

 tions of the mica-slate from unaltered sandstones or grey wackes are the 

 distinct lamination of the component minerals, the frequent aggrega- 

 tion of the quartz in large lenticular masses or nodules, and the con- 

 tortion or twisting of the beds. This contortion is, however, rarely 

 so violent as in the mica-slate of Loch Goyle and Loch Long, and sel- 

 dom interferes with the observation of the dip and direction of the 

 beds where a sufficient surface of the rock is exposed. 



Associated with the mica-slate, in several localities, beds of lime- 

 stone are found in considerable abundance. It is usually of a very 

 dark or black colour, highly crystalline, and of a very large coarse 

 grain. It is seen in most abundance in Knock-Scalbert and the other 

 hills north of Campbeltown, where it is quarried in several places. 

 Dr. Macculloch represents the whole of this tract as limestone, but, 

 as the sketch-map will show, it is far less abundant than the igneous 

 rocks, and even than the associated mica-slates*. Some of the beds 

 on the west side of Loch Ruag are more compact and slaty, but still 

 show the same dark colour. Other beds again on the declivity of the 

 hill are a kind of cipollino, or rather calcareous mica-slate, consisting 

 of alternate layers of white granular limestone and mica. This lime- 

 stone series is again seen in the low ridge to the south of Campbeltown 



* Perhaps the omission has arisen from the blue used for the trap-rocks being 

 mistaken for that employed for the limestone, — a not uncommon cause of error in 

 other parts of the country. 



