452 EOCENE PERIOD. CHAP. 



and the laminated shell beds are more frequent in the east. The 

 pebble beds of Blackheath, the thickest known example, include 

 generally small masses, but occasionally pieces occur six or eight 

 inches in diameter, and suggest sea-shore action, somewhat like 

 that on the Chesil beach, which, arriving at a maximum there, 

 is yet part of a system represented by feebler shingle beds as 

 far west as the extremity of Dorsetshire. 



The London clay requires a totally different set of conditions 

 to account for its comparative uniformity and freedom from sandy 

 or gravelly admixture. Deep and tranquil water, instead of shallow 

 and disturbed currents ; fine sediment transported from afar, instead 

 of pebbly aggregates left near the shores ; quiet residence of 

 mollusca in several zones, through long spaces of time these 

 are the main elements. There is, however, this to be added. The 

 truly argillaceous character of the lower part changes to a fine- 

 grained sandy clay in the upper part; a change quite natural 

 if the sea depth were gradually reduced by mere accumulation 

 through time ; for thus the transporting power of watery movement 

 would gradually increase, and larger and larger particles become 

 capable of transport. Finally, this power of drifting sands and 

 pebbles returned, and the London clay became covered by the 

 Hampstead sands, but received, or at least has preserved, no later 

 deposits of the eocene ages; while elsewhere, in another basin, 

 that of Hampshire and the Isle of Wight, they were continued 

 in a long succession; and these also came to an end, and were 

 followed, after a long interval and in another branch of the caino- 

 zoic sea, by the fossiliferous zones of the crag. 



The following Synopsis of the genera of fossils discovered in these 

 eocene strata, within the drainage of the Thames, is offered as 

 a convenient, though incomplete, list with which to terminate 

 the long series of the forms of life, which, always varying with 

 the lapse of time, and continually readjusted to new physical 

 conditions, but always contained within the same organic formula, 

 constitute one great body of evidence, one full and compre- 

 hensive scale by which to measure the succession and determine 

 the periods of all the great revolutions 



'Of Nature, constant in her ceaseless change.' 



