THE CRETACEOUS PERIOD. 261 
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 gooo 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 paleontological 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 A/ants 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 (A/g@) 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 Dzatoms. 
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- 
cotyledons.* In Europe the plant-remains in question have 
* The ‘‘ Flowering plants” are divided into the two great groups of the 
Endogens and Exogens. The Zxdogens (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 
