2O2 Pur beck and Wealden. 



I have now to describe a series of deposits that were 

 formed at the mouth of a river in a large delta, com- 

 parable in size to the largest deltas of the living world, 

 and consisting of the following subdivisions, the oldest 

 being placed at the bottom : 



Purbeck r Weald Clay. 



and < Hastings Sands and Clays. 



Wealden Series. I Purbeck Limestone Marls and Clays. 



The events that brought about the formation of 

 these strata seem to have been as follows : 



By the deposition of that series of beds of limestone 

 and shales that constitute the Oolitic strata, a great 

 marine area was more or less filled with sediments, the 

 last of which is the Portland Limestone. Probably 

 aided by partial upheaval of the flat-lying strata, a por- 

 tion of this area was invaded by the waters of a large 

 continental river, the rise of land having been sufficient 

 to unite Britain with the Continent of what is now 

 Europe, which, however, at that time presented very 

 different contours from those of the present day. We 

 must now conceive the old islands, which I described in 

 the last chapter, as forming groups of hills and moun- 

 tains, rising out of vast plains, the surface of which con- 

 sisted of horizontal or nearly horizontal Upper Oolitic 

 strata, through which, from some far-off unknown sources, 

 a long and broad river ran. The earliest strata of the 

 Purbeck Beds must have been formed in open, clear, fresh 

 water, in the broad mouth of this river, for near Tisbury in 

 Wiltshire, they pass gently into each other, the marine 

 strata of the Portland and Purbeck Limestones being 

 firmly united in the same quarries. The lowest beds of 

 the Purbeck strata are of fresh-water origin, and on the 

 whole the transition from the uppermost marine beds 

 of the Portland, to the lowest fresh-water strata of the 



