314 CLASSIFICATION OF CRETACEOUS ROCKS. [Ch. XVII. 



dation previous to the Eocene period ; but many able palaeontologists, 

 and among others MM. C. d'Orbigny, Deshayes, and d'Archiac, dis- 

 puted this conclusion, and, after enumerating 54 species of fossils, 

 declared that their appearance was more tertiary than cretaceous. 

 More recently, M. Hebert, having found the Pecten quadricostatus, a 

 cretaceous species, in this same pisolitic rock, at Montereau near 

 Paris, and some few other fossils common to the Maestricht chalk, 

 and to the Baculite limestone of the Cotentin, in Normandy, classed 

 it as an upper member of the cretaceous group, an opinion since 

 adopted by M. Alcide d'Orbigny, who has carefully examined the 

 fossils. The Nautilus Danicus, fig. 278, and two or three other spe- 

 cies found in this rock, are frequent in that of Faxoe in Denmark, but 

 as yet no Ammonites, Hamites, Scaphites, Turrulites, Baculites^ or 

 Hippurites have been met with. The proportion of peculiar species, 

 many of them of tertiary aspect, is confessedly large ; and great 

 aqueous erosion suffered by the white chalk, before the pisolitic lime- 

 stone was formed, affords an additional indication of the two deposits 

 being widely separated in time. The pisolitic formation, therefore, 

 may eventually prove to be somewhat more intermediate in date be- 

 tween the secondary and tertiary epochs than the Maestricht rock. 



It should, however, be observed, that all the above-mentioned 

 strata, from the Thanet Sands to the Pisolitic limestone inclusive, 

 and even the Maestricht rock, next to be described, exhibit marks 

 of denudation experienced at various dates, subsequently to the con- 

 solidation of the white chalk. This fact helps us in some degree to 

 explain the remarkable break in the sequence of European rocks, 

 between the secondary and tertiary eras, for many strata which once 

 existed have doubtless been swept away. 



CLASSIFICATION OF THE CRETACEOUS ROCKS. 



The cretaceous group has generally been divided into an Upper 

 and a Lower series, each of them comprising several subdivisions, 

 distinguished by peculiar fossils, and sometimes retaining a uniform 

 mineral character throughout wide areas. The Upper series is often 

 called familiarly the chalk, and the -Lower the greensand, the last- 

 mentioned name being derived from the green color imparted to cer- 

 tain strata by grains of chloritic matter. The following table com- 

 prises the names of the subdivisions most commonly adopted : — - 



UPPER CRETACEOUS. 



A. 1. Maestricht beds and Faxoe limestones. 



2. White chalk with flints. 



3. Chalk marl, or gray chalk slightly argillaceous. 



4. Upper Greensand, occasionally with beds of chert, and with chloritic marl 



(craie chloritee of French authors) in the upper portion. 



5. Gault, including the Blackdown beds. 



