98 NATURAL SCIENCE. August. 



Above the Chalk is a series of sands, clays, and pebble-beds 



forming the Lower London Tertiaries and London Clay : the latter is 



covered in places by sands and pebble-beds forming the Bagshot 



series. After this there is a great blank in the Thames Basin : when 



the geological record again commences, it does so only with a series of 



gravels, clays, and brickearths, which rarely contain any fossils 



except some derived from older deposits. In consequence, their 



correlation and methods of formation present problems of unusual 



difficulty. The following table summarises these beds in descending 



order : — ' 



L — Pleistocene. 



{a) Post-Bouldcv Clay. — Brickearths and gravels at low levels in 

 valleys of existing rivers. Most of the molluscs and 

 mammals of existing species. 

 High-level terrace gravels and brickearths with remains of 

 mammoth, arctic plants, etc. 



(b) Boulder Clay. — Chalky Boulder Clay with fragments of rocks 

 and fossils from the north of England. 

 IL — Pre-Boulder Clay, of Uncertain Age. 



Newer Plateau Gravels : Gravels containing many northern frag- 

 ments and quartzite boulders. 



Older Plateau Gvavels : Gravels with many small quartz pebbles. 

 The " Westleton Shingle" of Professor Prestwich and the 

 Pre-Glacial Pebble Gravels of the Geological Survey. 



" Southern Drift." — Partly synchronous with above. 



in. — Pliocene. 

 Crags of Suffolk and Sand on Chalk at Lenham in Kent. 



IV. — Eocene. 

 Pebble Beds of Bagshot series. 



The most important recent attempt to solve the problems of the 

 origin of the Thames Valley is that of Professor Prestwich in a series 

 of three papers entitled "On the Relation of the Westleton Beds 

 . . . . and their Extension Inland," published in the Quarterly 

 Journal of the Geological Society for 1890. The papers are the result of 

 over 50 years of work, and describe some sections that were closed 

 from examination before the younger generation of geologists saw the 

 light. They are a summary of a life's work by the Nestor of British 

 geologists, and from this, as well as from their intrinsic importance, 

 demand respectful notice. 



Professor Prestwich's theory of the sequence of events in the south- 

 east of England is as follows. In Lower Pliocene times a sea spread 



1 In reference to the above table it should be noted that the only bed positively 

 assigned to the Glacial is the Chalky Boulder Clay. The gravels that underlie this 

 are no doubt closely connected with it, but were obviously deposited or re-arranged 

 by water. To avoid ambiguity in the article the term " Glacial" is not used as a 

 time name ; and beds are simply referred to as being earlier or later than the Boulder 

 Clay instead of as Pre- or Post-Glacial. 



