Si ratification of France and England, 137 



<»oiTie of that within the basin of Paris ; but we cannot I 

 think infer, that tliis same plastic clay underlays the Lon- 

 don clay, in the neighbourhood of London, as there is ee- 

 iieraliy a sand here imrnediaiely upon the chalk, but which 

 varies greatly in thickness, from nmny yards (as in the 

 sand-pits by the Thames between Greenwich and Wool- 

 wich) to a few feet only or perhaps inches, in the borings oF 

 some of the modern wells in and near the metropolis : still 

 however this sand has always I believe be?'n found in this 

 district, and it has generally been forced up wiih q;reat vio- 

 lence into the well, by the water issuing from the joints in 

 the chalk beneath, on first pricking this spring with the au- 

 gef, or on any after occasion when the column of v/ater in 

 the well is much and suddenly difninishctl, as often happens 

 by the pumping at Mcux's brew house and Oiher places. 



Mr. Smith and myself have been nsed to consider the 

 London clays as regular strata, and the layers of small re- 

 gular flattened chert-stones which it contains, as nodules 

 peculiar thereto, and not As rovnded pebbles and indicating 

 an alluvial origin to these clays: I thought these coiiclu- 

 gions well warranted by the regular beds of which this clav 

 in many places consists; its regidar layers of Indus helmonti ; 

 the uniformity in the shape of its chert pebbles, many of 

 which appear concentric in their structure, and by their 

 having an external coat or covering, which is nearly simi- 

 lar in all of them: but an able student undcr>Mr. Williaiu 

 Smith, of the same standing with myself, Mr. Benjamiii 

 Bevan of Leigh ton Buzard, Beds., whose situation as engineer 

 to the Grand- Junctio!! Caa*il Company, has for some years 

 past given liim better opportunities of examining the north- 

 ern edcic of the London clays than I have possessed, has for 

 some time held the clays and sands above the chalk to be 

 alluvia, and produces instances of chert pebbles taken from 

 their bed on Rislip-common, Middlesex, in a pit uherb 

 sand containing layers of ihese pebbles has chalk in its bot- 

 tom, to piove these pebbles to have been rounded, and 

 even some oF them to have been broken and since partially 

 rounded (which is perhaps the most unerring test of round- 

 ing, that we nieet with). It has also been suggested hy 

 this gentleman, triat the peculiar dark-coloured and uneven 

 coat of these pebbles, is occasioned by a partial decomposi- 

 rion which they liavc sufi'ered, since their rounding, the 

 cfieets of which are also visible in the concentric stains 

 within many of them, which give them so very nearly llie 

 appearance of original nodules. In candour 1 ought also 

 to state, that the accounts which I have collected of thi; 



sinkings 



