546 F. Hardy — Mineral Gompositioji of Fenland Silt. 



(A) Mechanical Analysii 

 Coarse sand (TOO to 0'20 mm.) . 

 Fine sand (0-20 to 0-04 mm.) . 

 Coarse silt (004 to 001 mm.) . 

 Fine silt (001 to 0-002 mm.) 

 Clay (below 0-002 mm.) . 

 Loss on ignition . . r . 



Moisture . . . . , 



0-05 

 84-80 

 1-65 

 0-95 

 1-14 

 3 20 

 0-50 



92-29 



(B) Chemical Analysis. 



Insoluble residue ..... 8'4-750 



Total carbonate (as CaCOg) . . . 8-980 



Total phosphoric oxide (P20,5) . . . 0-112 



Total potash (K2O) , . ' . . . 1-224 



Total nitrogen 0034 



1. Examination op the Air-dried Silt without Preliminary 



Treatment. 



A slide of the silt was prepared and examined under the petro- 

 graphical microscope. The majority of the grains were found to be 

 subangular quartz fragments. Their shape indicates that the grains 

 had not been subjected to excessive rolling ; they resemble typical 

 boulder-clay quartz particles. 



A few grains of felspar were also noted. Certain of these showed 

 twin lamellae and are probably of soda felspar. 



The rest of the minerals (of density greater than 2' 70) are described 

 in the next section under the title of " heavy minerals ". They 

 constitute approximately 12 per cent of the bulk of the silt. 



2. Examination of the Silt with a view to Identifying the 



Heavy Minerals. 



In order to obtain grains of convenient size for petrographical 

 examination, a quantity of the silt was subjected to a 75 seconds 

 sedimentation process in pure water from the 7'5 cm. mark of a 

 beaker in the manner of a mechanical soil-analysis. 



The sand fraction thus obtained was collected and dried. The 

 heavy minerals were then separated by the use of a mixture of 

 bromoform and benzine of density 2*70. 



An examination of slides prepared from this heavy mineral portion 

 of the sand fraction of the silt yielded the following results : — 



The chief minerals identified were blue-green hornblende 

 (arfvedsonite), magnetite, glauconite and chlorite, aragonite (in the 

 form of shell fragments and broken oolitic grains), dolomite, 

 ferriferous chert, augite, and tourmaline. In this list the minerals 

 are arranged in their aj)proximate order of abimdance. 



Besides the above there occur also in lesser amount rutile, 

 muscovite, biotite, staurolite, ej)idote, zircon, kyanite, apatite, and 

 garnet. 



(1) Arfvedsonite is perhaj)s the commonest heavy mineral present 

 in the silt. It has undoubtedly been derived from the igneous 



