Yol. 68.] COMPOSITION" OF THE ARCTIC "BED. 247 



Table I brings out the fact that a low proportion of sand (as in' 

 Samples 1 and 3) is associated with high proportions of vegetable 

 and calcareous matter and of iron sulphides, and with a low pro- 

 portion of detrital heav}^ minerals. This is what one might 

 anticipate, seeing that both platy shell-fragments and plant-debris 

 tend to be deposited chiefly with the finer sediment, and heavy 

 detritus with the coarser, and that decaying vegetation seems to 

 have been the cause of the precipitation of the sulphides. 



IV. Description of Minerals, 



Pew of the minerals enumerated in Table II require special 

 description. The grains described as quartzite are compound quartz- 

 grains, showing a mosaic structure in varying degrees of minuteness ; 

 they were probably derived in the first case from schists, but some 

 may be silicified sandstones of Mesozoic or Tertiary age. The felspars 

 include orthoclase, microcline, and plagioclase. The tourmaline is 

 usually brown, but blue grains also occur ; perfectly rounded grains 

 are common, reminding one of the well-rounded tourmalines of the 

 Bagshot Sands. The hornblende is green and strongly pleochroic. 

 Many of the andalusites are colourless or nearly so, but several 

 grains show the characteristic pink tint for light vibrating parallel 

 to the axis of elongation, which is also the negative or fast direction 

 (c = a = red). 



The iron ]3yrites occurs as spherical, cylindrical, and irregular 

 aggregates, showing crystal facets and sometimes encrusting plant- 

 remains. The surface is often fresh and brass}', but sometimes 

 oxidized. The crystalline form is not sufficiently developed to 

 determine whether the mineral is pyrite or marcasite, and the 

 grains are too minute for a satisfactory determination of their 

 density. The magnetic sulphide of iron, pyrrhotite, commonly 

 occurs in spherical aggregates intermixed with large amounts of 

 clayey and organic matter and iron-oxide. The pyrrhotite itself 

 appears to be in a very finely divided condition, and its presence is 

 only indicated by the magnetic character of the aggregates and the 

 evolution of hydrogen sulphide in hydrochloric acid. 



V. Origin of the Sulphides. 



It is well known that decaying organic matter, under anaerobic 

 conditions, can precipitate the sulphides of iron from iron-bearing 

 solutions. The black mud deposited in insufficiently aerated 

 sewage-beds owes its colour to the formation of ferrous sulphide. 

 Prof. IST. Andrussov ^ has described a black mud from the bottom of 

 the Black Sea in which the pigment consists of minute globules of 

 ferrous sulphide (FeS). These globules have been observed in the 

 interior of diatoms. In other parts a blue mud occurs which con- 

 tains nail-shaped concretions of ferric disulphide (FeS^). These 



^ 'La Mer Noire': in Gruide des Excursions, Ylleme Cougres Geol. 

 Internat. (St. Petersburg) 1897, Fascicule xxix. 



