Holding a similar position to the Maestricht beds, and showing a similar intermixture of Cretaceous forms with later types, are certain beds which occur in the island of Seeland, in Denmark, and which are known as the Faxöe Limestone.
Of a somewhat later date than the Maestricht beds is the Pisolitic Limestone of France, which rests unconformably on the White Chalk, and contains a large number of Tertiary fossils along with some characteristic Cretaceous types.
The subjoined sketch-section exhibits the general succession of the Cretaceous deposits in Britain:—
GENERALIZED SECTION OF THE CRETACEOUS SERIES OF BRITAIN.
Fig. 185.
In North America, strata of Lower Cretaceous age are well represented in Missouri, Wyoming, Utah, and in some other areas; but the greater portion of the American deposits of this period are referable to the Upper Cretaceous. The rocks of this series are mostly sands, clays, and limestones—Chalk itself being unknown except in Western Arkansas. Amongst the sandy accumulations, one of the most important is the so-called "marl" of New Jersey, which is truly a "Greensand," and contains a large proportion of glauconite (silicate 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 palæontological 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 (Algœ) 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 Proteaceœ, some of which are referred to existing genera (Dryandra, Banksia, Grevillea, &c.)
[Footnote 23: The "Flowering plants" are divided into the two great groups of the Endogens and Exogens. The Endogens (such as Grasses, Palms, Lilies, &c.) have no true bark, nor rings of growth, and the stem is said to be "endogenous;" the young plant also possesses but a single seed-leaf or "cotyledon." Hence these plants are often simply called "Monocotyledons." The Exogens, on the other hand, have a true bark; and the stem increases by annual additions to the outside, so that rings of growth are produced. The young plant has two seed-leaves or "cotyledons," and these plants are therefore called "Dicotyledons." Amongst the Exogens, the Pines (Conifers) and the Cycads have seeds which are unprotected by a seed-vessel, and they are therefore called "Gymnosperms." All the other Exogens, including the ordinary trees, shrubs, and flowering plants, have the seeds enclosed in a seed-vessel, and are therefore called "Angiosperms." The derivation of these terms will be found in the Glossary at the end of the volume.]
In North America, the Cretaceous strata of New Jersey, Alabama, Nebraska, Kansas, &c., have yielded the remains of numerous plants, many of which belong to existing genera. Amongst these may be mentioned Tulip-trees (Liriodendron), Sassafras (fig. 186), Oaks (Quercus), Beeches (Fagus), Plane-trees (Platanus), Alders (Alnus), Dog-wood (Cornus), Willows (Salix), Poplars (Populus), Cypresses (Cupressus), Bald Cypresses (Taxodium), Magnolias, &c. Besides these, however, there occur other forms which have now entirely disappeared from North America—as, for example, species of Cinnamomum and Araucaria.