GNEISS, MICA SCHIST, AND SLATE SYSTEMS. 659 



western point of Scotland. This is a lofty headland of gneiss, interstratified with 

 hornblende, and intersected with granite veins. It projects far out into the rough 

 Atlantic. The waves driven by the northern storms have cut deep fissures and caverns 

 in the promontory ; the pinnacles around it, and the rocks at its base starting up from the 

 bottom of the ocean, striking the imagination as so many guards against the further 

 invasion of the Cape. 



Mica-schist System. The mica-schist is a compound essentially of quartz and mica, 



and thus differs from granite and gneiss by the absence of felspar as a constituent. 



Sometimes the mica predominates, or the quartz, and occasionally felspar occurs with 



other substances, as hornblende, chlorite, and talc, producing hornblende, chlorite, and 



talcose schists. Common garnet, also, and staulorite, are often so abundant as to become 



constituents, and originate the varieties of garnetif erous and staurolitiferous mica-schist. 



In some varieties, it exhibits a fine and regular laminar structure, which gives to the 



rock a fibrous aspect, as on the preceding page. In talcose and chlorite schist, the distinctive 



elements take the place of mica, quartz forming the other ingredient. They are usually 



found associated together, but the chlorite is more abundant than the talo, and easily 



distinguishable by the green colour, flexible structure, and soapy feel of its characteristic 



mineral. Hornblende-schist has a dark-green colour approaching to black, but it is of 



rare occurrence in an independent form, Ben Lair in Ross-shire being the principal 



mass in Great Britain. Mica-schist, on the contrary, is often silvery or pearly white 



with intermingling shades of grey. The same able Scotch geologist, whose description 



of the gneiss districts of his native country has been given, thus speaks of the associate 



system : " Mica slate has but little economical value, though in exposed situations 



forming a useful substitute for roofing slate. Either alone or mixed with other beds, it 



forms extensive districts, whose scenery assumes various features, according to the power 



of the rock to resist decomposition. Where soft and easily destroyed, the mountain 



districts to which it is mostly confined are low, tame, undulating, and seldom broken by 



precipices or projecting cliffs ; where it is hard and quartzose, they have a more 



picturesque character, the hills rising to 4000 feet or upwards, and running out in long 



serrated edges, with peaked or dome-shaped summits, abrupt precipices, and deep craggy 



ravines, overhung with birch, or partially covered with vegetation. As illustrations of 



these, it is enough to mention Killicrankie, Loch Katterin, and the Trosachs, the fantastic 



ridges of Glencroe, or the bold, rugged, and sterile mountains on the west coast." 



The annexed sketch exhibits the two great formations in their order of succession 



above the granite. Classifying both 

 groups together, the careful geologist 

 just quoted proposes the following 

 arrangement, introducing the sub 

 ordinate strata, which form component 



parts of them in the order of super- 

 Granite. Gneiss. Mica Schist. r 



position. 



UPPER SERIES. Chlorite slates, apparently uniting the clay-slate and mica-slate deposits. 

 No organic remains. It ranges on the whole south-east border of the Grampians, from 

 near Aberdeen to Argyleshire. 



MIDDLE SERIES. Mica-schists, primary limestone, quartz-rock, in various combinations, 

 the former by far the most predominant, the others only locally important ; the limestone 

 occurs in different parts of the series. That of Loch Earn, Inverary, and Balahulish, is 

 in the upper part, approaching to chlorite-slate ; that of Glen Tilt, and of the vale of 

 Loch Tay, is in the lower part. No organic remains. This occupies a great part of the 

 eastern and southern Highlands, the north-west of Ireland. 



