336 Mr. Parkinson on the Strata, 



broken and rounded by rolling, but its characters were still capable 

 of being ascertained. It possessed, in the softer parts, the colour 

 and appearance of the Essex mineralised bones so distinctl)^, as to 

 leave not a doubt of its having been imbedded in this stratum ; 

 whilst in the enamel it manifested decided, characters of the tooth of 

 some species of the mammoth, ox mastodon of Cuvier. 



The actual limit of this stratum has not been ascertained ; it Is 

 however known to -extend through Essex, Middlesex, part of Kent, 

 -and Surry, and through Hertfordshire, Buckinghamshire, and indeed 

 much further both to the northward and westward. In many parts 

 its continuity has been interrupted, apparently by partial abruptions 

 of it, together even with a portion of the stratum on which it rests. 

 The shells of this stratum have hitherto been discovered only in the 

 •parts already noticed. 



Blue CLAY stratum. This, the next subjacent bed, is formed 

 of a ferruginous clay exceeding two hundred feet in thickness. Its 

 colour for a few feet in the upper part is a yellowish-brown, but 

 through the whole of its remaining depth is of a dark bluish grey, 

 verging on black. It is not only characterised by these circum- 

 stances, but by the numerous scptaria which are dispersed through it>, 

 and by the peculiar fossils which it contains. 



The difference of colour observed between its superior and 

 inferior part, and which has generally been supposed to be owing 

 to a difference in the degree of oxidation of the iron present in it, 

 appears to be the result of a difference in the quantity of it, oc- 

 casioned by the washing away of this metal in the upper part by 

 tlie water which percolates through it, and which runs off laterally 

 by the numerous drains made near the surface. The dark red 

 colour of tiles made from the blue clay, the reddish-yellow co- 

 lour of the place bricks made of the yellowish-brown clay, and the 



