236 CLASSIFICATION OF CEETACEOUS EOCKS. [Ch. XVIL 



already known is about 100 feet. Its geographical range, according tc 

 M. Hebert, is not less tban 45 leagues from east to west, and 35 from 

 north to south. "Within these limits it occurs in small patches only, rest- 

 ing unconformably on the white chalk. It was originally regarded as 

 cretaceous by M. E. de Beaumont, on the ground of its having undergone, 

 like the white chalk, extensive denudation previous to the Eocene period ; 

 but many able paleontologists, and among others MM. C. D'Orbigny, 

 Deshayes, and D'Arcniac, disputed 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 quadri- 

 costatus, a cretaceous species, in this same pisolitic rock, at Montereau 

 near Paris, and some kw 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. 249), and two or three other species found in this 

 rock, are frequent in that of Faxoe in Denmark, but as yet no Ammonites, 

 Hamites, Scaphites, Turrilites, 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 limestone was found, affords an additional indi- 

 cation of the two deposits being widely separated in time. The pisolitic 

 formation, therefore, may eventually prove to be somewhat more inter- 

 mediate in date between the secondary and tertiaiy 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 consolidation of the 

 white chalk. This fact helps us in some degree to explain the remark- 

 able break in the sequence of European rocks, between the secondary 

 and tertiaiy eras, for many strata which once existed have doubtless been 

 swept away. 



CLASSIFICATION OF THE CRETACEOUS KOCKS. 



. The cretaceous group has generally been divided into an Upper and 

 a Lower series, each of them comprising several subdivisions, distin- 

 guished by peculiar fossils, and sometimes retaining a imiform mineral 

 character throughout wide areas. Tbe Upper series is often called famil- 

 iarly the chalk, and the Lower the greensand, the last-mentioned name 

 being derived from the green color imparted to certain strata by grains 

 of chloritic matter. The following table comprises the names of tbe sub- 

 divisions most commonly adopted : 



UPPEE CRETACEOUS. 



A. 1. Maestricht beds and Faxoe limestones. 



2. White chalk -with flints. 



3. Chalk marl, or gray chalk slightly argillaceous. 



