374 



THE WONDERS OF GEOLOGY. Lect. JV. 



interesting example of the perfect similarity of a natural 

 process, in periods separated from each other by immense 

 intervals of time.* 



9. Subdivisions and extent of the wealden. — The 

 Wealden is subdivided into several groups, which are 

 characterized by the lithological nature of the strata, and the 

 prevalence of certain kinds of fossils ; but throughout the 

 whole, the fluviatile character of the formation is main- 

 tained : in the lowermost part of the series only are there 

 any intrusions of a marine or estuary nature. In fact, it 

 is not possible to conceive an accumulation of sedimentary 

 detritus more purely fluviatile — a delta more free from 

 marine exuviae. Yet foreigners, and even some English 

 geologists, describe it as a fluvio-marine formation ; " un 

 melange de fossiles marines et d'eau douce." f 



Although it is not within the scope of these lectures to 

 enter upon details of stratification, it is necessary to point 

 out the principal subdivisions of this extensive system of 

 fresh-water deposits.^ 



1. Wealden upper clays and sands. — Stiff blue clays, with 



septaria, argillaceous ironstone, and beds of shelly limestone, 

 called Sussex or Petworth marble. 



2. Tilgate-grit and hastinqs-sands. — Fawn-coloured sand, and 



sandstones, with beds of calcareous grit or Tilgate-stone, alter- 

 nating with blue clays and limestones, marls, and lignite. 



3. Ashburnham beds. — Alternations of clays, shales, and bluish-grey 



shelly limestones and shales. 



4. Pdrbeck beds. — Clays, sandstones, and shelly limestone called 



Purbeck marble. Limestone, with layers of vegetable mould, 

 and trunks of trees in a vertical position — the petrified Forest 

 of Portland. 



* For a particular account of the Wealden strata in the south-east of 

 England, see my Geology of the South-East of England ; Fossils 

 of Tilgate Forest ; and Geological Excursions round the Isle of 

 Wight, p. 269. 



f M. Pictet— Traite Elementaire dc Palaeontologie. 



X Geology of the South-East of England, p. 182. 



