Dr. Charles RickeUs — On Subsidence and Accumulation. 119 



to this period, in a pause of wliicli we are now living. These silts 

 are again in places being denuded or ^^ fretted " away, and though 

 there is and has been since the Koman occupation a cessation of 

 vertical movement in the land, horizontal displacements by erosion 

 and re-formations by deposition are stUl taking place, due solely to 

 changes of tidal and river currents — agencies capable of affecting 

 much greater change than is usually supposed, and the effects of 

 which, even within the limits of the last century, I hope at some 

 future time to show are capable of measurement and exact calculation. 



YI. — On Subsidence as the Effect of Accumulation.^ 



By Charles Eicketts, M.D., F.G.S. 



THE majority of geological formations have certainly been de- 

 posited in what was at their respective periods the sea ; but 

 Sir Charles Lyell, Prof. Geikie and others have shown that the action 

 of the waves and currents upon sea-cliffs, and their power to remove 

 matter from above to below the sea-level, is very insignificant com- 

 pared with that effected by atmospheric agents, by rain and rivers, 

 in the interior, in consequence of the immensely more extended area 

 upon which these act ; so that, even supposing the height of ancient 

 cliffs to have been very much greater than those of the present time, 

 which have been considered to average not more than twenty-five 

 feet, it appears difficult or rather impossible to attribute the origin of 

 the materials of which they consist, sometimes amounting to several 

 miles in thickness, to the erosion of coast-lines, with the exception 

 of a comparatively thin stratum situated at their base. Those who 

 have ascribed their origin to marine denudation have given no satis- 

 factory explanation of the manner in which the debris from the dis- 

 integration of the cliffs has been redistributed to form these strata. 



In determining the source whence the deposits entering into the 

 composition of Paleeozoic rocks have been derived, attention must 

 be directed to the frequent recurrence, at or near the base of these 

 formations, of evidences of deposition in shallow water, such as 

 ripple-marks, sun-cracks, tracks of annelids, etc., current-bedded 

 strata, and conglomerates, whilst an accumulation of materials has 

 subsequently occurred amounting it may be to several miles in 

 thickness ; thus proving that, simultaneously with the deposit, sub- 

 sidence of the land has taken place to at least as great an extent as 

 the whole thickness of the superincumbent strata. It therefore 

 follows that if the then contour of the land was similar to the 

 present, though the beds at the base may at their formation have 

 been situated near the shore-margin, those overlying them, but 

 separated by this great thickness of strata, must in consequence of 

 the subsidence have been deposited at a distance of very many miles 

 from the coast, so far that by no possibility could the sediment have 

 been derived from marine denudation acting on cliffs. 



^ An Abstract of a Paper read before the Liverpool Geological Society, as the 

 President's Address for the Session 1871-72. 



