20 ME. W. WHITAKEB, OX THE [vol. lxXV, 



geologic scale, of local occurrence, of little thickness, of somewhat 

 monotonous composition, merely deposits of pebbles and sand. 



They are alike in composition, being almost wholly composed of 

 well-rolled, smooth-surfaced pebbles of Chalk-flints. The differences 

 in them consist in the variation in the few pebbles of other rocks. 

 Thus the Blackheath Beds yield a peculiar quartzite which has not 

 been found in other deposits, but no quartz; the Bagshot Beds, 

 on the other hand, yield small pebbles of quartz, but so far none 

 of quartzite ; the Barton Beds, besides quartz, yield chert from 

 the Lower Greensand. Thus it is the highest beds only that give 

 evidence of overlap on to Lower Cretaceous strata, the quartz and 

 quartzite being indefinite ; and this overlap is of great extent, 

 the inferential loss of beds at the surface southward (whether 

 by thinning or by erosion) being comparable to the loss of beds 

 underground northward, from the Wealden area to the London 

 Basin, though some hundreds of feet less. 



Again, confining our view to the Lower London Tertiaries. 

 looking beyond the pebble-beds, and taking the Series as a whole, 

 we find that though it is not of mere local occurrence and varies a 

 great deal in composition, character, and origin, yet it may be looked 

 on as insignificant, being in many parts less than 50 feet thick 

 and in few reaching to about thrice that thickness. 



Yet the study of these small things bears on questions of great 

 import, on erosion and overlap of considerable extent; and it may 

 throw light on events of appreciable magnitude and wide interest, 

 even on that highly debatable question the erosion of the Weald. 

 For, if the general conclusions that have been here brought forward 

 hold good, that great erosion must have started in Lower Eocene 

 times, as, indeed, I have occasionally suggested, and the process may 

 have been continued in Middle Eocene times. I will not enter into 

 the question of what happened later ; but it seems to me that the 

 planing-oif of the Chalk began in Lower Eocene times, though 

 to what extent it went over the central part of the Wealden area 

 one can hardly say. 



Although much work has been done on the Lower London Ter- 

 tiaries as well as on the beds above them, up to the Barton Sand, 

 in the London Basin, there is still much to be done in various ways 

 in the field and in the laboratory, and it has been well started with 

 means that were not in the hands of the older workers. This work 

 I leave to my younger brethren, content if I have dropped them a 

 bint here and there. 



VI. Remarks ox the Clay-with-Fltxts. 



I forbear from alluding to the literature of this subject and to 

 the various theories that have been brought forward to explain the 

 occurrence of the material : enough now to consider that locally. 



The sections that have been seen at Worms Heath, with some in 

 other outliers of the Blackheath Beds in Surrey, are of interest 

 in regard to the Clay-with-Flints, that irregular material (which 



