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 (_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_.