SEDIMENTATION, ENVIRONMENT, AND EVOLUTION. 15 



outcrops the relative rapidity of the subsidence. It is likely, 

 however, that the eastern area of Palaeozoic rocks subsided 

 even more rapidly, so that it was speedily overstepped by a 

 covering of Gault, Greensand, and Chalk. 



The purity of the Upper and Middle Chalk and its freedom 

 from terrigenous matter is doubtless related to the low altitude 

 of the Cretaceous land-surface and the distance from large 

 rivers or from those with other than a gentle gradient. These 

 conditions followed naturally from the rapid and widespread 

 depression in Cenomanian times. 



In the earlier portion of this paper (p. 9 and Fig. 2), an 

 " interformational conglomerate " was demonstrated as being- 

 produced during elevation and subsequent subsidence of the 

 land. Such oscillatory action seems to have been connected 

 with the formation of the Blackheath Pebble-beds in Eocene 

 times. The extraordinarily wide area covered by this deposit 

 of rounded flint pebbles (derived obviously from the flints of 

 the Chalk) and its even thickness preclude the possibility of the 

 whole deposit being contemporaneous. It is doubtless a litho- 

 logical phase traversed by time-planes, but the absence of 

 fossils of determinative character makes proof impossible. 

 The overlap of the Thanet Beds by the Woolwich and Reading 

 Beds, and of the latter by Blackheath Beds, which rest directly 

 on the Chalk near its escarpment in Kent and Surrey, indicates 

 an initial subsidence of the area ; but the pebble-beds are 

 essentially shallow- water deposits, and their wide extension 

 from the Chalk, back over the Woolwich and Reading Beds 

 in turn, indicates a gradual subsequent elevation, accompanied 

 in all probability by penecontemporaneous erosion. 



The existence of similar spreads of pebbly deposits is in itself 

 evidence of instability. The Bunter pebble-beds of the Trias and 

 the Old Red Sandstone conglomerates may be cited, although 

 these deposits may be of the nature of intermontane detrital 

 accumulations rather than interformational conglomerates. 



