18 MONOGRAPH OF DURA DEN. 



the strata for quarries — in Glen Tummel, Glenlyon, near Loch 

 Earn head, at Aberfoyle, Auchmar, Loch Lomond — when it is 

 again traceable through all the western isles, from Lismore to 

 the more quartzj regions of Jura and Isla. 



The Clay-slate forms a very narrow strip, of about five or 

 six miles in breadth, and may be described as the outer envelope 

 of the primary series and upper crust of the Grampian range. 

 It extends from Stonehaven, in a continuous belt, to Roseneath, 

 and through Bute to Arran, where, at Loch Ranza, it is pene- 

 trated by the granite, and is seen in connexion with the 

 mica-slate. It consists of a fine-grained argillaceous basis, of 

 considerable hardness, of various colours, from a greenish-black 

 to a deep mottled purple, and, from its splintery fissile structure, 

 is admirably adapted for roofing-slate. Some of the smaller 

 islands in the vicinity of Oban, as Luing, Eisdill, and Seil, are 

 composed of a different slate, chiefly the chlorite, and of older 

 formation. Various other bands of slate occur, in groups of 

 different kinds and qualities, among the primary mountains. 

 They are confined to no particular mineralogical district, but 

 are distributed at long intervals, and appear as outliers indis- 

 criminately in the granite, gneiss, and mica-slate series. They 

 are termed talc, chlorite, actinolite, and hornblende schists, 

 according to the prevalence of any one of these mineral sub- 

 stances in the mass. They have less or more an unctuous feel, 

 a foliated or fibrous structure, an extremely flexible texture, 

 and a fine glossy lustre. The chlorite schist is very abundant 

 in the Cairnwell and Glenshee group ; the hornblende variety, 

 also strongly impregnated with cubical iron pyrites, occurs at 

 Ballahulish and Appin, and Ben Lair, in Ross-shire ; talc-slate 

 is not abundant, and is generally incorporated with the mica- 

 slate, by the substitution of the talc for the mica plates. Acti- 

 nolite schist is usually associated with, as it differs little in 

 character from gneiss and some specimens of granite, and is 

 found in considerable quantity in Glenelg, and the high and 

 beautifully sloping passes of Glen Shiel. 



