618 MR. H. H. ARNOLD-BEMROSE ON THE MICROSCOPICAL [Nov. 1 894, 



It differs from iddingsite in having the plane of the optic axes 

 generally perpendicular to the c axis (001) instead of the b axis 

 (010) of the olivine. In one case it is perpendicular to the b axis 

 and in another parallel to a dome face. Another difference is that 

 sometimes various parts of a pseudomorph, though behaving as 

 crystallographic individuals, have a different optical orientation. 

 These variations, and the occurrence of fresh olivine in two of the 

 same outcrops, though not in the same specimens as this pseudo- 

 morph, point to the latter as being a replacement of the olivine, 

 and not an original separation from the magma. This view is 

 confirmed by the occurrence of fresh olivine-kernels with the cracks 

 altered to serpentine, and the olivine also partly replaced by the 

 Peak Forest kind of pseudomorph in the Pittle Mere outcrop, which 

 was probably continuous with the Potluck outcrop. 



Cleavage-flakes from the most altered specimens of the Potluck 

 rock were detached by the knife, and separated from fragments of 

 other minerals under a low power. They are not elastic like mica, 

 but are brittle, often, though not always, breaking along the cracks 

 of the original olivine. Treated with dilute hydrochloric acid on a 

 glass slide and warmed, they soon change from red to yellow and 

 become colourless. They lose their dichroism, but flakes lying on 

 the cleavage-plane still show an almost uniaxial figure. If the 

 action be continued a little longer, they become isotropic. When 

 heated in the closed tube, a little water is given off. 



Mr. L. Archbutt, F.C.S., F.I.C., kindly tested some of the flakes 

 for me and obtained the following results : — 



" The quantity of mineral received for analysis weighed about 

 11^ milligrammes. Six and a half milligrammes were finely 

 powdered, and digested with a little pure hydrofluoric acid and 

 a small drop of sulphuric acid in a platinum crucible, at a gentle 

 heat for some time, and then evaporated to dryness and heated to 

 faint redness. The residue was dissolved in hydrochloric acid, 

 which gave a clear solution ; this was diluted with a strong solution 

 of hydrogen sulphide, and a trace of brownish precipitate mixed 

 with sulphur was filtered off. The sulphuretted hydrogen was 

 expelled by evaporation, the iron oxidized by a few drops of bromine 

 water, and the excess of bromine evaporated off. A slight excess of 

 ammonia was then added, which produced a red precipitate : this 

 was tested for alumina. It was dissolved in hydrochloric acid and 

 boiled with pure potash in excess, which threw down a compara- 

 tively large precipitate having the colour of pure ferric hydroxide, 

 which was not further examined ; the filtrate was evaporated with 

 excess of pure solid ammonium chloride, and a comparatively fair- 

 sized precipitate of alumina was thus obtained, which was not 

 further examined. As potash purified by alcohol always contains 

 traces of alumina, the precaution was taken of testing rather more 

 of the potash than was used in the analysis, in the same way, 

 but the amount of alumina obtained was insignificant. The filtrate 

 from the ammonia precipitate was mixed with a few drops of a 

 strong solution of hydrogen sulphide, but no precipitate of manganese 



