132 BRITISH EOCENE FLORA. 



the ever-increasing depth led to a change in the sediment. Thus, though beds of Green- 

 sand or Chalk may be perfectly continuous, with precisely the same lithological characters, 

 it is absurd to assert that portions of either when separated widely apart by degrees of 

 latitude and longitude must be synchronous. So far from this, the Chalk-with-flints of 

 one locality must most certainly have been deposited synchronously with the Chalk- 

 without-flints of another, and this in turn with the Chloritic marl of another, and the 

 Greensand of another. The shallower-water zones, such as the Greensand, would travel 

 forward so long as the sea continued to encroach, and along the farthest confines of the gulf 

 would recede again when elevation set in, without any Chalk having been deposited over 

 them, so that some " Upper Greensands " might be newer than any Chalk. It is probable 

 that each minor zone was a zone of depth, characterised by the same quality of sediment, 

 and a Fauna to some extent peculiar to it, which kept up with it as it travelled farther 

 and farther landwards. There might thus be great similarity (homotaxis) in the Fauna of 

 each zone at long intervals of distance, and its distinct characteristics maintained over the 

 most extensive areas, without, for all that, its contents having lived synchronously over 

 the whole area. 



We have noticed that the Neocomian and Gault of England and Western France 

 contain a varied and considerable Flora, represented mainly by foliage and fruits of 

 Coni/era, without affording the slightest trace of the presence of angiospermous 

 Dicotyledons. Even the Grey Chalk and the Blackdown Beds have only yielded Conifers 

 and a Williamsonia of Jurassic type. We cannot account for their absence by supposing 

 our area to have been isolated, for in the preceding Wealden period neither its Fauna nor 

 Flora differed from that of Europe. But when we reach Aix-la-Chapelle, we find the 

 Chalk and Greensand resting upon beds containing a Flora largely made up of 

 Dicotyledons, and still farther off, in the Cenomanian 1 of Bohemia, living genera such as 

 Magnolia, and farther on still, equally developed Dicotyledons in the slightly newer 

 Turonian. Such facts were hitherto completely inexplicable, but it now appears probable 

 that the interval required for the Chalk to progress even only 300 or 400 miles, 

 may have endured long enough to permit an enormous progress in the evolution of 

 phanerogams. Nor does the 1200 or 1400 feet of vertical Chalk remaining in our area, 

 at all represent the completed formation ; for, as the prolonged subsidence finally ceased 

 and gave place to an equally slow elevation, all the lessening zones of depth must have 

 travelled back with the receding ocean, and left a series of beds arranged inversely to that 

 preserved to us. The planing action of the sea has removed all this newer series, just 

 as it has planed away an older mass of the width of the English Channel ; and it is still 

 slowly but inexorably cutting down to its own level all the cliffs that form its 

 shore-lines. The Eocene seas, from beginning to end, were ceaselessly engaged in 

 this work, and their enormous deposits of flint shingle mark how much had fallen a 

 prey to them. Nor have the Cretaceous rocks enjoyed any respite from the work of 



1 Equivalent of Grey Chalk and Chalk-marl. 



