350 Wood — Structure of Yalleys in Essex. 



been accumulated subsequent to the deposit of tbe Upper Brickearth 

 {x^'Tf^^ and that it has itself, as well as the gravel and brickearth, 

 been overspread by a newer gravel (a; 5'), which has again been 

 removed by the aqueous action that gave rise to the warp (?/). A 

 list is given by my father, in the monograph of the Crag Mollusca, 

 of a numerous fauna collected by himself there many years ago. 

 Since when, the Rev. 0. Fisher has obtained from it Cyrena fluminalis 

 and Pcduclina lenta. The occurrence of the former shell concurs with 

 the view to which the position of the fluviatile and iluvio-marine 

 deposits would lead us, viz., one nearly analogous to those at G-rays, 

 which are newer than the gravel and Upper Brickearth of the 

 Thames YaUey (see Section 4 in Gteological Magazine, Vol. Ill, 

 p. 62), as these are newer than the East Essex gravel and capping 

 brickearth. 



The valley of the Blackwater consists of two parts — the upper or 

 river portion, and the lower or estuary portion. The latter has been 

 subjected to disturbances subsequent to the creation of both parts, 

 from which the upper has escaped. 



In a paper " on the Structure of the River and other valleys of the 

 East of England," ^ I j)ointed out, by means of a map, that the 

 valleys of the eastern and south-eastern parts of England, which 

 are occupied by strata newer than the Trias, resolved themselves 

 into regular systems of concentric arcs, diverging from several 

 centres, of which the most important were three — one in the 

 North Sea off Flamborough Head, another near the Isle of Wight, 

 and a third near Canterbury ; and that these arcs, inosculated more 

 or less with each other, cutting through the Glacial series (at least 

 through the upper and middle portions of it) with such regularity, 

 that a rod laid over the Ordnance map from the Thames to the Nen. 

 and Welland would intersect a similar angle in all the chief valleys 

 that it crossed ; and I pointed out the evidence there was that these 

 arcs originated by the flexures produced from lateral pressure exerted 

 from the several centres which were foci of earthquake disturbance 

 under the Boulder-clay sea, and which flexures, by imparting direc- 

 tion to the denudation, had become deepened and strongly marked 

 by the action of denudation proportionately to the extent to which 

 they had undergone that process. I also endeavoured to show that 

 upon this original valley origin there had, at a later stage of the great 

 denudation, supervened a series of rectilinear movements, accom- 

 panied by denudation ; so that the valleys which having earliest 

 emerged, and thus escaped the later part of the denudation, were due 

 solely to the first part of this process, and those later emerged to the 

 joint operation of the two. 



The formation of the valley of the Blackwater — both the u.pper 

 and the lower portions in their original condition — has been due to 

 the first of these processes, the arcs by which they originated being 

 those emanating from the Canterbury centre. Section No. 9 (see 

 margin, of Map), drawn so as to cross both the upper and lower por- 

 tions of the valley, will show the relation borne by the East Essex 

 1 Phil. Magazine for March, 1864. 



