Owing to the fact that so large a portion of the Cretaceous formation
has been deposited in the sea, much of it in deep water, the _plants_
of the period have for the most part been found special members
of the series, such as the Wealden beds, the Aix-la-Chapelle
sands, and the Lignitic beds of North America. Even the purely
marine strata, however, have yielded plant-remains, and some of
these are peculiar and proper to the deep-sea deposits of the
series. Thus the little calcareous discs termed "coccoliths," which
are known to be of the nature of calcareous sea-weeds (_Algoe_)
have been detected in the White Chalk; and the flints of the same
formation commonly contain the spore-cases of the microscopic
_Desmids_ (the so-called Xanthidia), along with the siliceous
cases of the equally diminutive _Diatoms_.
The plant-remains of the Lower Cretaceous greatly resemble those
of the Jurassic period, consisting mainly of Ferns, Cycads, and
Conifers. The Upper Cretaceous rocks, however, both in Europe and
in North America, have yielded an abundant flora which resembles
the existing vegetation of the globe in consisting mainly of
Angiospermous Exogens and of Monocotyledons.[23] In Europe the
plant-remains in question have been found chiefly in certain
sands in the neighbourhood of Aix-la-Chapelle, and they consist
of numerous Ferns, Conifers (such as _Cycadopteris_), Screw Pines
(_Pandanus_), Oaks (_Quercus_), Walnut (_Juglans_), Fig (_Ficus_),
and many _Proteaceoe_, some of which are referred to existing
genera (_Dryandra, Banksia, Grevillea_, &c.
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