48 On the Stratification of England, 



comprehended in the London clay. I have mentioned the 

 above two points, one sufficiently near in direction to the 

 south and the other to the north of the proposed archway, as 

 a guide to the inquiries which I am going to suggest J but in 

 all probability some other line, nearly north and south, (par- 

 ticularly on the north side of London,) may be found more 

 eligible, in which I beg to recommend that it should be mi- 

 nutely inquired by means of the many different Wells which 

 have been sunk, by the recent cuttings of the Croydon Canal 

 and other excavations, as well as by the croppingof the several 

 strata which compose the London clay, both at its southern 

 and northern edge, what arc the nature^ the actual succession, 

 and thicknesses of the several strata, which may be expected 

 to exist under the vale of the Thames at London, after the 

 alluvial matters therein have all been sunk through, and the 

 strata reached. During the last autumn, I, and others, had an 

 opportunity of making part of the above inquiries on the line 

 of the Croydon canal, and am enabled to state, that, pro- 

 ceeding northward from Sydenham town to near Brockley- 

 Green, the canal will be found all the way cut in a thick 

 stratum of very strong red clay, whose characteristic mark 

 seems to be, two or more remarkable layers of ludus Helmo7itii, 

 or clay-balls ; which clay-ball stratum forms the surface in 

 most parts near London that I have examined, except where 

 cither gravel or alluvial matters exist thereon, or where an 

 excavation or abrasion of the surface has taken place, and 

 exposed strata which are to be found lower in the series. 

 In going southward from Brockley-Green, and descending 

 from the surface of the stratum above described (for the 

 course of one mile and a half, or rather less,) to the foot of 

 Plough-garlick Hill, near the London and Greenwich road, 

 a great number of the upper strata of the London clay may 

 be traced in succession, and ample specimens of each may 

 be obtained from the new banks of the canal. It may be 

 necessary here to mention, that the exposure of the edges of 

 the strata, in the cutting of the canal between Brockley and 

 the Greenwich road, and in numerous other places on the 

 southern side of the Thames vale, seems owing to an enor- 

 mous dislocation or lift of the strata in those places, and 

 to an amazing abrasion or wearing away of the edges of the 



uppermost 



