548 F. Hardy — Mineral GomiJosition of Fenland Silt. 



When it occurs it resembles the grains of that mineral in the Lower 

 Greensand. (15) Kyanite is present as large blade-like fragments, 

 several of which exhibit the characteristic re-entrant angles, giving 

 a waist-like appearance to the grains. (16) Apatite occurs in small 

 amount as tiny rounded, colourless, highly refractive grains. (A 

 further note on its significance in the silt is given below.) (17) Garnet 

 is fairly abundant as colourless, more or less angular chips of high 

 refractive index. Such grains of garnet are characteristic of many 

 geologically recent sands of East Anglia,^ and also of certain of the 

 Lower Greensand strata. 



3. Note on the Composition of the Finer Fractions of the 



Silt. 

 An attempt was made to identify the minerals present in the silt 

 and clay fraction of the sample, but it was only found possible to 

 recognize definitely the presence of quartz as the main constituent 

 of the material ; of muscovite in abundant small flakes ; of tiny 

 angular fragments of zircon, and of much calcareous matter. 



IV. THE ORIGIN AND GEOLOGICAL RELATIONSHIPS 



OF THE MODERN FENLAND SILT. 



Assuming that the sample of Fenland silt described above is 

 fairly typical of the uniform deposit which is slowly being 

 accumulated along the north-west border of the Wash, certain 

 conclusions as to the relationship of the silt may be drawn from its 

 mineralogical composition. 



The general assemblage of minerals is very similar to that of certain 

 of the geologically recent deposits of Cambridgeshire, as described 

 by Rastall. It is almost identical with that of the finer parts of the 

 plateau-gravels, which are most probably the residue of the boulder- 

 clay of the North Sea glacier. Hence it may be assumed that such 

 boulder-clay has furnished the bulk of the material of the modern 

 Fenland silt. 



The presence in the silt of calcareous oolitic grains, which have 

 been generally somewhat altered by secondary processes, may be 

 due to the admixture of detritus from weathered Jurassic rocks over 

 which the Fenland rivers flow in their courses to the Wash. These 

 conclusions are in full accord with the geological evidence which has 

 already been summarized in Section II. 



V. THE BEARING OF THE MINERALOGICAL COMPOSITION 



OF THE MODERN FENLAND SILT ON CERTAIN 

 CHEMICAL CHARACTERISTICS OF THE SILT. 



(A) Carbonate. 

 The establishment of the presence of two distinct kinds of 

 carbonates in the silt sample examined is perhaps the chief point 

 of chemical interest in the investigation. The writer has elaborated 



1 R. H. Rastall, " The Mineral Composition of some Cambridgeshire Sands 

 and Gravels " : Proc. Camb. Phil. Soc., vol. xvii, 1912, p. 136, etc. 



