THE CRETACEOUS PERIOD. 269 



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 thick- 

 ness attained by the Cretaceous rocks of North America is 

 about 9000 feet, as in Wyoming, Utah, and Colorado. Ac- 

 cording 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 por- 

 tion 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, 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 (Alga} have been de- 

 tected in the White Chalk; and the flints of the same forma- 

 tion 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, Cy- 

 cads, and Conifers. The Upper Cretaceous rocks, however, 

 both in Europe and in North America, have yielded an abun- 

 dant flora which resembles the existing vegetation of the globe 

 in consisting mainly of Angiospermous Exogens and of Mono- 



