Palwo-Geological and Geographical Maps of the British Islands. 283 
producing a complete unconformity of stratification between the formations 
themselves, along with which there is a complete change in the fauna ; so much so, 
that with the exception, perhaps, of some Foraminifera, no species passes from the 
Jurassic into the Cretaceous rocks, and of 300 Lower Greensand species, only about 
20 per cent. survive into the Upper Cretaceous series.* 
Nature of the Cretaceous Strata—Speaking generally, the Lower Cretaceous 
strata consist of gravel, sands, and clays, of sedimentary origin, indicating a process 
of formation not far remote from the land of the period, and in a sea of no great 
depth. They are altogether absent on the borders of Devon and Dorset, where the 
Upper Greensand rests directly on the New Red Marl and Lias. The Upper 
Cretaceous strata indicate the prevalence of oceanic conditions (during the later 
stages) in the formation of the chalk, which is a white limestone composed in the 
main of shells of Foraminifere, and containing molluses, crinoids, and echinoderms 
in great numbers. Spicules and casts of sponges are common, and are often found 
enclosed in flints. 
The beds and nodules of flint of the Upper Chalk, are due to a process of pseudo- 
morphism, whereby the free silica in the waters of the ocean has, from time to time, 
been consolidated around some body, such as an echinus, a sponge, a shell, cr other 
foreion body, and has replaced the original carbonate of lime of which the body 
itself was formed, or by which it was enveloped.t The Upper Greensand formation, 
at the base of the Chalk, has been shown by Ehrenberg to be formed of the casts 
of Foraminifera preserved in silicate of iron.{ Similar casts were brought up from 
deep waters in the Indian Ocean by the officers of the “ Challenger” expedition. 
With the Upper Cretaceous beds commences the appearance of Dicotyledenous 
plants, both in Europe and America,§ giving a perfectly new aspect to the flora of 
the world, or as it has been expressed by Dr. Oswald Heer, “introducing a new 
fundamental conception of the vegetable kingdom.” 
Distribution of Land and Sea.—At the commencement of the Cretaceous period, 
the British area was probably almost entirely in the condition of dry land, and was 
but slightly submerged during the formation of the Lower Greensand. Professor 
Ramsay considers that this submergence was so slight that the Oolitic strata, which 
then extended far to the west into the borders of Wales, were not entirely 
submerged.|| After its deposition, the land was tranquilly raised out of the sea, and 
subjected (along with the older strata) to atmospheric waste. 
The deposition of the Gault in our area, first took place on the surface of a 
* Ramsay. Supra cit., p. 217. 
t The process has been fully explained by the late Dr. Bowerbank, and more recently by Professor 
Rupert Jones, F.R.s. 
t “Ueber den Griinsand.” Abhand. der K. Acad. der Wissenschaft, zu Berlin, 1855, p. 85. 
§ Viz. :—At Aix-la-Chapelle, and in America, from the Rocky Mountains to the Arctic Regions at 
Noursoak. 
|| Supra cit., p. 230. 
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