xi.] CHANGES OF CLIMATE. 277 



You must know that certain sorts of shells live in 

 deep water, and certain in shallow. You may prove 

 this to yourselves, on a small scale, whenever you go 

 to the seaside. You will find that the shell which crawl 

 on the rocks about high-water mark are different from 

 those which you find at low-tide mark ; and those again 

 different from the shells which are brought up by the 

 oyster- dredgers from the sea outside. Now, the lower 

 part of the clay, near here, contains shallow-water 

 shells : but if you went forty miles to the eastward, 

 you would find in the corresponding lower beds of the 

 clay, deep-water shells, and far above them, shallow- 

 water shells such as you find here : a fact which shows 

 plainly that this end of the clay sea was shallowest, 

 and therefore first filled up. 



But again and this is a very curious fact between 

 the time of the Plastic clays and sands, with their 

 oyster-beds and black pebbles, and that of the London 

 clay, great changes had taken place. The Plastic clay 

 and sands were deposited during a period of earth- 

 quake, of upheaval and subsidence of ancient lands ; 

 and therefore of violent currents and flood waves, 

 seemingly rushing down from, or round the shores of 

 that Wealden island to the south of us, on the shore of 

 which island Odiham once stood. We know this from 

 the great irregularity of the beds : while the absence 

 of that irregularity proves to us that the London clay 

 was deposited in a quiet sea. 



But more. A great change in the climate of this 

 country had taken place meanwhile ; slowly perhaps : 

 but still it had taken place. 



In the lowest clay above the chalk are found at 

 Beading many leaves, and buds, and seeds of trees, 



