PART II. CHAPTER XV. 



181 



Cretaceous Group. 



CHAPTER XV. 



CRETACEOUS GROUP. 



White chalk — Its marine origin shown by fossil shells — Extinct genera of 

 cephalopoda — Sponges and corals in the chalk — No terrestrial or fluviatile shells, 

 no land plants — Supposed origin of white chalk from decomposed corals — 

 Single pebbles, whence derived — Cretaceous coral-reef in Denmark — Maes- 

 tricht beds and fossils — Origin of flint in chalk — Wide area covered by chalk 

 — Green-sand formation and fossils — Origin of — External configuration of 

 chalk — Outstanding columns or needles — Period of emergence from the sea — 

 Difference of the chalk of the north and south of Europe — Hippurites — Num- 

 mulites — Altered lithological character of cretaceous formation in Spain and 

 Greece — Terminology, 



The next group which succeeds to the tertiary strata in the 

 descending order has been called Cretaceous or chalky, because 

 it consists in part of that remarkable white earthy limestone called 

 chalk (creta). With this limestone however are usually asso- 

 ciated other deposits of sand, marl, and clay, called the Green- 

 sand formation, because some of its sands are remarkable for 

 their bright green colour. 



The following are the subdivisions into which the Cretaceous 

 Strata have been divided in the south of England : — 



Cretaceous 

 group. 



1. Chalk 

 formation. 



2. Green-eand 

 formation. 



a. soft white chalk, with 

 flints 



b. hard white chalk, with 

 few or no flints 



c. chalk marl 



a. upper green-sand - 



Gault, or blue marl, 10 to 

 lower green-sand and 

 iron-sand, with occa^ 

 sional limestone 



md ^ 

 ca- > 



250.t 



The accompanying section (Fig. 143.) will show the manner 

 in which the tertiary strata of the London and Paris basins, as 

 they are called, rest upon the chalk, and how the white chalk in 

 its turn reposes throughout this region upon the green-sand form- 

 ation. 



* Conybeare, Outline, &c., p. 85. 



t Fitton, Geol. Trans., Second Series, vol. iv. p. 319. 



