470 Reviews — Cornet & JBriarfs Divismis of the White Chalk. 



abounds in large greyish-brown flints, disseminated and in con- 

 tinuous bands. The average thickness of this deposit is about 500 

 feet. 



The Cra'e de Spiennes rests on the Crate de Noiwelles, the next 

 division, the upper surface of which is much eroded, hardened, and 

 perforated by lithophagous molluscs. Phosphatic nodules, and frag- 

 ments of shells, echinoderms, sponges, etc., with Chalk debris, 

 together make up a thick conglomerate, which forms the base of the 

 former stratum. 



The Craie de Nouvelles is the purest White Chalk of the entire 

 mass, it is soft to the touch, and can be written with. It is irregu- 

 larly stratified in thick and much-jointed beds. Very black flints 

 are numerous throughout. Its thickness is about 66 feet. 



The third member of the series, the Craie d' Ohourg, underlies the 

 last, but no exact point of juncture between them is apparently to 

 be discovered. It is a greyish white soft Chalk, irregularly stratified 

 in thin and much-fissured beds. Black flints are disseminated 

 through it in places ; but in some localities they are entirely absent. 

 A conglomerate similar to that at the bottom of the Craie de Spiennes 

 divides this mass into two unequal portions, each of which, the 

 authors think, it may be necessary in time to raise to the rank of a 

 division, in which case the upper one would preserve the name of 

 €raie d' Ohourg, and the lower would be known as the Craie de 

 Strepy. The thickness of the whole reaches in the north of the 

 district to 500 feet, in the south it is only 100 feet. 



The fourth and last of these White Chalk divisions is the Craie de 

 St. Vaast, which separates itself, it seems to us, more naturally than 

 the foregoing Craie d' Ohourg into two subdivisions. Of these the 

 upper consists of soft White Chalk, not marly, irregularly bedded in 

 thin layers, with numerous nodules of iron pyrites, and without 

 flints. The lower one is less white than the last, is somewhat 

 marly, is irregularly bedded in thick beds, and contains very nu- 

 merous black and white flints. In the west the Craie de St. Vaast 

 is 166 feet thick, but it thins considerably to the south where it is 

 scarcely 60 feet. 



The palseontological evidence on which the necessity of the new 

 divisions must naturally chiefly depend will be seen by glancing 

 at the accompanying table, which we have drawn up for the purpose, 

 from MM. Cornet and Briart's lists of fossils. 



Most of the fossils which are shown in the table to belong to the 

 Craie de Spiennes only, are rare, and all of them, Iwith the ex- 

 ception of Ananchytes ovata, and possibly Nodosaria Zippei and 

 Bulimina variahilis, are to be found in the Brownish Chalk of Ciply, 

 which, as we said before, is the lowest member of the Maestricht 

 series. The stratigraphical evidence on the other hand leaves no 

 doubt as to the propriety of separating the deposits in question. 



Magas pumila, which is absent in the other members of the 

 White Chalk, occurs in great abundance in the Craie de Nouvelles, 

 •of which it is characteristic. 



