icate of iron and potash). It also contains a little phosphate
of lime, and is largely worked for agricultural purposes. The
greatest thickness attained by the Cretaceous rocks of North
America is about 9000 feet, as in Wyoming, Utah, and Colorado.
According to Dana, the Cretaceous rocks of the Rocky Mountain
territories pass upwards "without interruption into a coal-bearing
formation, several thousand feet thick, on which the following
Tertiary strata lie unconformably." The lower portion of this
"Lignitic formation" appears to be Cretaceous, and contains one
or more beds of Coal; but the upper part of it perhaps belongs
to the Lower Tertiary. In America, therefore, the lowest Tertiary
strata appear to rest conformably upon the highest Cretaceous;
whereas in Europe, the succession at this point is invariably an
unconformable one. Owing, however, to the fact that the American
"Lignitic formation" is a shallow-water formation, it can hardly
be expected to yield much material whereby to bridge over the
great palaeontological gap between the White Chalk and Eocene
in the Old World.

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