12 F. F. Tnckett — Erosion of Rocks in Corsica. 



III. — Eemarkable Examples of Atmospheric Erosion of Eocks 



IN Corsica. 



By F. F. TicKETT, F.R.G.S, 

 (PLATE II.) 



IN the course of repeated visits to Corsica I have been much 

 struck by the extraordinary erosion, not only of cliifs, but even 

 more so of detached masses or bouklers, from near sea-level to 

 heights of 5, COO to 6,000 feet; and, having taken some photographs 

 last January of specimens of the kind last referred to, I sent them 

 to my friend Professor Bonney, who informed me that he had never 

 met with any instances of erosion of such a peculiar and unusual 

 character, and asked whether I could furnish him with a fragment 

 of the rock. 



Unfortunately, owing to the inaccessibility of two of the objects 

 photographed, and my hesitation to break away any of the third 

 ("Tete de Chien"), I was only able to send him a piece obtained 

 from a cliff by the roadside at some distance, which had been 

 scooped out by erosion into overhanging eaves and other curious 

 forms ; and his report on this, after having a microscopic section 

 prepared from it, was as follows : — 



" The specimen is about 4^ in. long, 2 in. wide, and 1 in. in 

 maximum thickness, weathered on both sides and on the blunter 

 edge, apparently' having been broken from a thin, flake-like pro- 

 jection such as would form the edge of one of the peculiar cavities 

 in the photograph. The weathered surface is irregular, lumpy, and 

 inclining to be flaky, of a dull dark -brown colour in the less 

 prominent parts, elsewhere a pale brownish-green. The rock itself, 

 quite close to the exterior, is a rather pale greenish-grey colour, 

 somewhat mottled witli small whiter and one or two darker patches, 

 showing elsewhere a fibrous structure. 



"Microscopic examination of a slice cut from one end, transverse 

 to the length of the flake, proves the rock to consist largely of 

 microlithic minerals, and to have been greatly affected by pressure 

 — probably almost crushed. It exhibits two or three small grains 

 ■of rhombic pyroxene, probably bastite ; a number of small grains of 

 augite, probably residual ; a large quantity of rather minute actinolite, 

 and perhaps a few flakes of a greenish to white mica. Some small 

 grains, however, of a colourless, slightly flaky mineral, like bastite, 

 but with oblique extinction, are certainly secondary, occurring 

 somewhat after the manner of albite in certain crushed Alpine 

 schists. Brown iron oxide is present only as an occasional staining 

 or in granules, and sphene (possibly) in the latter condition. The 

 rock has undergone so much secondary change that its original 

 condition has been obliterated. I think it most probably has been 

 a pyroxenite, with a little enstatite and possibly a few grains of 

 olivine, allied to, but hardly to be classified with, the peridotites. 

 It reminds me a little of some of the augite-serpentines of the Valais 

 Alps, but in it actinolite practically takes the place of the mineral 

 serpentine." 



