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 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 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.)