1825.J of Alluvial and Diluvial Formations, 245 



It is not, however, the external contour so much as the inter- 

 nal structure of the district, which bears on the subject of this 

 paper. The whole of the alluvial delta exhibits, as might be 

 expected, a great uniformity in the arrangement of its consti- 

 tuent beds. VVhen the vegetable coating is removed from any 

 part of it, we may generally find below a brownish black earth 

 which is formed of a variable mixture of common vegetable soil, 

 of peat, and of alluvial silt. The different qualities of fen land 

 arise out of the variable proportions of these constituents. In 

 those tracts which are pent up between high artificial banks 

 and upon which water frequently stagnates, the soil is almost 

 exclusively composed of decayed vegetable matter converted 

 more or less perfectly into the state of peat. In other more 

 favoured tracts, more especially on the sloping skirts of the 

 diluvial hills, the soil is of great fertility, and is composed prin- 

 cipally of the accumulated silt of successive inundations. 

 Materials possessing some of these characters are in many 

 places accumulated upon the regular strata of the country to the 

 thickness of nearly twenty feet. When they are laid bare by 

 any artificial section, we may often see various modifications 

 which are so far interesting as they throw Hghtupon the ancient 

 history of these deposits. In one part of such a section we 

 may find the prevailing black earth interrupted by thin beds of 

 peat, each of which indicates the temporary residence of stag- 

 nant water. In another part of the same section, the prevailing 

 soil is seen to alternate with layers of sand and silt which mark 

 the effects produced by extraordinary land floods. Alternations 

 like these are so common as hardly to deserve any notice. If 

 the section descend still further, we not unfrequently find the 

 whole series of alluvial deposits separated from the true substra- 

 tum (which in many places is composed of a stiff blue clay)* by 

 a very thin bed of light coloured, unctuous, marly silt. This 

 marly silt is, if I mistake not, of great antiquity, and must have 

 been deposited by the waters prior to the existence of any por- 

 tion of the alluvial covering. 



If all the soil and accumulated detritus were removed from the 

 district I am considering, it is certain that the surface of the 

 ground would present many considerable irregularities. It is 

 further evident that such a surface must in ancient times have 



inches ; Peterborough great fen thirteen feet two inches above the level which ought to 

 form the base of the drainage near the sea. — (See Bower's Report of the New Drainage 

 near Boston.) 



» In all the central parts of tlie fens, tlie blue substratum contains innumerable 

 specimens of the cliaracteristic grijphwa dilatala of the Oxford clay ; but near Ely, 

 under the alluvial and diluvial detritus, there is a bed which contains the ostrea dellui- 

 lif.a of the Kinmicridge clay. If I mistake not, the coral-rag formation thins off before 

 it reaches the fens, where the two clays are probably brought into immediate contact, 

 Below Cambridge, the tr»ct* of fen land rest on the gait or Folkstone clay. 



