OLDER ROCKS OF BRITTANT. 



303 



that the group, as a whole, not seldom reminded me in its mineral 

 composition and structure of some of the schists, rich in silvery 

 mica and garnet, which in the Alps, so far as my knowledge goes, 

 appear to occupy a rather definite horizon, some little distance 

 below the base of the great and widespread uppermost group of 

 schists (the schistes lustrees in part of some authors ; bilndner Scliiefer 

 in part of others, the graue Schiefer and grune ScJiiefer of the Swiss 

 survey map). 



Whatever may be the earlier history of these chloritoid-schists, 

 they bear the impress of some later and very potent earth-move- 

 ments. These have produced the dominant foliation, which sometimes 

 renders the rock almost fissile, and in the planes of this cleavage- 

 foliation, so far as I could ascertain, the larger scales of chloritoid 

 (sometimes j inch diameter) have been developed. In the bay, 

 south of Locmaria, about the middle of the south side of the island, 

 the dip (about 20°) of the cleavage-foliation is slightly to the east of 

 south, but afterwards it becomes roughly south-west ; and this 

 appears (allowing for local disturbances) to be the prevalent direc- 

 tion in the eastern part of the island, the angle of inclination being 

 commonly from 15° to 20°. 



The amphibolites are chiefly developed to the south of Locmaria, 

 and near the Fort de la Croix at the eastern end of the island. They 

 occur in bauds, apparently interstratined with the other schists, at 

 most about 50 yards thick, but often less. 



Besides glaucophane, which is usually present, they contain 

 garnets (commonly abundantly), together with epidote, green 

 hornblende, white mica, quartz, sphene, rutile, and hasmatite. They 

 are com monly schistose in structure, and occasionally exhibit mineral 

 banding, both structures coinciding with the " cleavage-foliation " 

 in the chloritoid-schists. The banding is often produced by a pre- 

 dominance of epidote, sometimes of glaucophane. Occasionally, as in 

 a case south of Locmaria, the amphibolites are rather massive. 

 Sometimes, as to the north of the Fort de la Croix (where the rock is 

 coarser, the garnets being as large as a pea), they are conspicuously 

 schistose. Not seldom the amphibolites are rotten, and I believe 

 that any one who wished to collect the rock simply for microscopic 

 sections would obtain the most suitable specimens from the rolled 

 pebbles on the beach. The schistose structure is not always clearly 

 marked, and some of the rock in its mode of decomposition reminded 

 me of the more massive varieties of the " chloritic schists" of South 

 Devon *. The constituent minerals have been so admirably described 

 by Dr. Barrois in the memoir already quoted, that I will merely say 

 that some of the varieties of the rock are extraordinarily rich in 

 glaucophane, though the individual crystals of the mineral are not 

 generally large f . I quite agree with Dr. Barrois that the crystalli- 

 zation of the glaucophane is posterior to the pressure which has in 



* Quart. Journ. Geol. Soc. vol. xl. p. 1. 



t Some of the less schistose forms curiously resemble, both macroscopically 

 and microscopically, the glaucophane eclogite which I have described from the 

 Val d'Aoste (Mineral. Mag. vol. vii. p. 1). 



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