122 J. 8. GARDNER ON BRITISH CRETACEOUS NUCULIDZ. 
beds hitherto regarded as equivalent in age become actually and 
relatively newer. ‘The Cretaceous series of Devonshire, Ireland, 
Limburg, and Denmark, though almost identical in their sequence 
and lithological characters with those of Kent and Sussex, never- 
theless contain assemblages of fossils of much younger aspect. The 
approach to Tertiary types is least marked in the Blackdown Beds 
at Blackdown, but they contain types, and even genera, not met with 
elsewhere in the Cretaceous series except at Aix-la-Chapelle, where 
at least 50 per cent. of the Mollusca incline to Tertiary forms. In 
the Cretaceous rocks of Denmark, which are still more remote from 
our centre, the approach to Tertiary forms, though naturally less 
obvious in the White Chalk with flints, is very intimate in the oyer- 
lying coral band and limestone, the only Cretaceous form among a 
large number of Cypraa, Tritons, Volutes, Mytili, Rostellarie, 
Turbinelle, &¢.—ali of Eocene aspect—being a Plewrotomaria. In 
the Irish Chalk, again, we have Turritelle and Scalarie, as well as 
casts of Mitra-like shells, which approach Tertiary types. In an 
area of prolonged and gradual subsidence, as Europe undoubtedly 
was throughout the Upper Cretaceous period, the deep sea, or ancient 
gulf of the Atlantic, as represented by the Chalk, would gradually 
encroach further and further in every direction as the levels were 
lowered. At a certain depth the physical conditions rendered the 
deposit of Upper Greensand alone possible, and the deposition of 
this quality of sea-bed seems to have been accomplished in deeper 
water than that in which the Gault mud was formed, but less deep 
than that required for the Grey Chalk. It precedes the Chalk in 
what I designate its central area, and was continued in ever widen- 
ing contours as long as the depression was maintained, or, in other 
words, for as long as the Cretaceous period lasted. Its ultimate 
margins must be beyond and younger than all but the newest true 
Chalk, if, as appears certain, the deposition of Greensand or Chlo- 
ritic Marl was a necessary phase leading up to Chalk. During the 
Cretaceous subsidence, wherever the Atlantic Ocean penetrated, a 
Greensand bottom was nearly invariably followed by one of Chloritic 
Marl, and this by Chalk without flints, and this again by Chalk with 
flints. What we take for horizons or zones of age are no more 
than travelling zones of depth, which radiated from a centre through- 
out a long period of depression, commencing in our area at the base 
of the Middle Cretaceous series and enduring elsewhere without 
interruption to a period immeasurably more recent than the newest 
of our English Cretaceous rocks, which have since suffered vast de- 
nudation. It is thus misleading to speak of the Upper Greensand, 
as a formation, being older than the Chalk, though in any given 
area it is older than the Chalk of the same area, because its depo- 
sition preceded that of Chalk on that spot. Purely local causes 
seem to have led to the chert formations of Blackdown and Aix, 
the Firestone, Marne rock, the Red and Grey Chalks, &c.; but these 
causes may have recurred at any interval and anywhere along the 
margin of the Chalk ocean, and similarity of lithological character 
does not in this case imply contemporaneous deposition. The old 
