F. Hardy — Mineral Composition of Fenland Silt. 547 



rocks of Scandinavia, which are especially characterized by the 

 presence of this blue-green strongly pleochroic soda-amphibole. 



(2) Magnetite occurs as numerous well-rounded opaque grains. 



(3) The glauconitic material present closely resembles that occurring 

 in the Lower G-reensand deposits of certain districts of East Anglia.^ 

 The glauconitic grains vary in colour from pale green to deep brown. 

 Possibly much of the pale-green material is composed of chlorite, 

 which is closely related to glauconite. (4) Aragonite occurs in the 

 silt chiefly as abundant fragments of shells. Prisms of the mineral 

 (possibly derived from shells such as Inoceramus and from 

 Echinoids) also occur. In addition, oohtic grains are common. These 

 show the characteristic zonary structure, and give dark crosses when 

 viewed under two nicols. Many of the oolitic grains are highly 

 ferriferous, and the aragonite of which they were originally composed 

 may have been altered by impregnation with iron and magnesium 

 into some such mineral as ankerite. (5) The dolomite present occurs 

 as typical " fresh "-looking crystals of rather large size and with 

 well-marked rhombohedral cleavages. Dr. Harker, who examined 

 certain of the slides, expressed the opinion that the appearance 

 of the dolomite grains suggests a secondary and comparatively 

 recent origin of the mineral. Possibly the mineral was deposited 

 from sea-water, and the crystals gradually built up during 

 the successive wettings and dryings to which the modern 

 Fenland salt-marsh silts are subjected. (6) The mottled 

 grey-brown grains of chert owe their high density to the 

 presence of abundant iron oxide in them. The grains are of com- 

 paratively large size, and possess a cryptocrystalline structure. 



(7) Augite occurs as colourless or pale-green angular fragments, 

 which owe their shape to the imperfect cleavage of this mineral. 



(8) The presence of tourmaline is noteworthy. The commonest 

 colour exhibited by the well-rounded drop-like grains of this mineral 

 is olive-green, although a very few pink grains were seen. The 

 tourmaline grains resemble on the whole those which characterize 

 the Lower Greensand strata as described by Rastall. (9) Of the less 

 common heavy minerals, rutile is conspicuous. It occurs chiefly 

 as rounded brownish-red grains of typically high refractive index. 

 (10) Muscovite is fairly abundant as thin flakes of various sizes. Its 

 presence was rendered conspicuous by illuminating the slides under 

 examination by light of grazing incidence. (A further comment on 

 the occurrence of muscovite in the silt is to be found in the next 

 section.) (11) Biotite is not very abundant, but it is undoubtedly 

 present in appreciable amount. The mineral is of exceptional 

 occurrence in sediments generally. (12) Staurohte occurs as rounded 

 yellowish fragments of strong pleochroism and high refractive index, 

 (13) Epidote was seen in a few angular fragments, of which a certain 

 number possessed a thin coating of iron oxide. (14) Zircon is rare. 



^ R. H. Rastall, "Mineral Composition of Lower Greensand Strata of East 

 England " : Geol. Mag., Vol. LVI, 1919, p. 209. 



