234 ♦ CRETACEOUS GROUPS. [Ch. JXil 



like our chalk downs, or is covered exclusively by the Juniperus Virgini' 

 ana, as certain chalk districts in England by the yew-tree and juniper. 



Some of the shells of this limestone are common to the Claiborne beds, 

 but many of them are peculiar. 



It will be seen in the section (fig. 244, p. 232) that the strata of Nos. 

 1, 2, 3 are, for the most part, overlaid by a dense formation of sand or clay 

 without fossils. In some points of the bluff or cliff of the Alabama river, 

 at Claiborne, the beds Nos. 1, 2 are exposed nearly from top to bottom, 

 whereas at other points the newer formation, JSTo. 4, occupies the face of 

 nearly the whole cliff. The age of this overlying mass has not yet been 

 determined, as it has hitherto proved destitute of organic remains. 



The burr-stone strata of the Southern States contain so many fossils 

 agreeing with those of Claiborne, that it doubtless belongs to the same part 

 of the Eocene group, though I was not fortunate enough to see the rela- 

 tions of the two deposits in a continuous section. Mr. Tuomey considers 

 it as the lower portion of the series. It may, perhaps, be a form of the 

 Claiborne beds in places w^here lime was wanting, and where silex, derived 

 from the decomposition of felspar, predominated. It consists chiefly of 

 slaty clays, quartzose sands, and loam, of a brick-red color, with layers of 

 chert or burr-stone, used in some places for mill-stones. 



CHAPTER XVII. 



CRETACEOUS GROUP. 



Lapse of time between the Cretaceous and Eocene periods — "Whether certain 

 formations in Belgium and France are of intermediate age — Pisolitic limestone 

 — Divisions of the Cretaceous series in N^orthwestern Europe — Maestricht beds 

 — Chalk of Faxoe — White chalk — Its geographical extent and origin — Formed 

 in an open and deep sea — How far derived from shells and corals — Single 

 pebbles in chalk — Chalk flints — Potstones of Horstead — Fossils of the Upper 

 Cretaceous rocks — Echinoderms, Mollusca, Bryozoa, Sponges — Upper Green- 

 sand and Gault — Chalk of South of Europe — Hippurite limestone — Cretaceous 

 rocks of the United States. 



Having treated in the preceding chapters of the tertiary strata, we have 

 next to speak of the uppermost of the secondary groups, commonly called 

 the chalk, or the cretaceous strata, from creta, the Latin name for that 

 remarkable white earthy limestone, which constitutes an upper member of 

 the group in these parts of Europe, where it was first studied. The marked 

 discordance in the fossils of the tertiary, as compared with the cretaceous 

 formations, has long induced many geologists to suspect that an indefinite 

 series of ages elapsed between the respective periods of their origin. 

 Measured, indeed, by such a standard, that is to say, by the amount ot 

 change in the Fauna and Flora of the earth effected in the interval, the 

 time between the cretaceous and Eocene may have been as great as that 



