312 CRETACEOUS GROUP. [Ch. XVII. 



tion. Mr. Tuomey considers it as the lower portion of the series. 

 It may, perhaps, be a form of the Claiborne beds in places where 

 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 cellular 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 forma- 

 tions in Belgium and France are of intermediate age — Pisolitic limestone — Divis- 

 ions of the Cretaceous series in North-Western 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 — A similar rock now in 

 progress in the depths of the Atlantic made up of Globigerinse — Origin of Flint 

 in Chalk — Siliceous Diatomacese of the Atlantic — By what intermittent action 

 the alternate layers of white chalk and flint may have been caused — Pot- 

 stones of Horstead — Isolated pebbles of quartz and foreign rocks in chalk — 

 Fossils of the Upper Cretaceous rocks — Echinoderms, Mollusca, Bryozoa, 

 Sponges — Upper Greensand and Gault — Blackdown beds — Flora of the Upper 

 Cretaceous period — Fossil plants of Aix-la-Chapelle — Large proportion of Dico- 

 tyledonous Angiosperms — Their coexistence with large extinct genera of reptiles 

 — 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, com- 

 monly 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 those parts of Europe where it w T as 

 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 of 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 between the 

 Eocene and Recent periods, to the history of which the last seven 

 chapters have been devoted. Several fragmentary deposits have been 

 met with here and there, in the course of the last half century, of an 

 age intermediate between the white chalk and the plastic clays and 

 sands of the Paris and London districts, monuments which have the 

 same kind of interest to a geologist which certain mediaeval records 

 excite when we study the history of nations. For both of them 



