200 



CHAPTER XIV. 



ON CHALK, AND THE SUBJACENT BEDS OF GREEN SAND. 



Extent of the Chalk Formation. — Green Sand divided into lower and upper Green 

 Sand by a Bed of Clay called Gait.— Chalk Marl. — Chalk, its Mineral Charac- 

 ters. — Change of Character in the Alps. — Flints in the upper Chalk. — On the 

 formation of Flints. — Remarkable Organic Remains in Chalk.— Recent Discov- 

 ery of Beds belonging to the Chalk Formation, in the United States of America. 

 —On the Scaglia of the Alps supposed to represent Chalk. 



The well-known mineral, chalk, with its subjacent beds of green 

 sand, comprises a formation or series of strata of great depth, which 

 are spread over a large portion of the south-eastern and eastern coun- 

 ties of England, and are found covering a large extent of surface in 

 the northern parts of France, preserving nearly the same characters 

 as the English chalk. Similar beds are found in Germany and in the 

 north of Europe ; but on approaching the mountain ranges of the 

 northern chain of Alps, the mineral characters of chalk undergo a 

 considerable change. Scarcely a trace of chalk is found in any part 

 of Scotland ; but it occurs on the north coast of Ireland. 



The animal remains in chalk and its subjacent green sand, are 

 exclusively marine, proving that this great calcareous and arenaceous 

 deposition, a thousand feet or more in thickness, was formed under 

 the ocean. 



Chalk is regarded as the last, or uppermost, of the secondary stra- 

 ta ; and there is a marked difference between the organic remains 

 in chalk, and those in the tertiary strata that in many situations cover 

 it. The geological position of chalk is over the oolite formation; but 

 we have seen, in the last chapter, that in the counties of Sussex and 

 Kent, chalk and green sand rest immediately upon the freshwater 

 beds of the Wealden ; and in the western counties of England, where 

 the oolite is wanting, chalk covers lias and red marl. The thick beds 

 of green sand under chalk are regarded as constituting, with the 

 chalk, one marine formation, as they contain many of the same ge- 

 nera of fossil remains, both in England and on the continent of Eu- 

 rope ; and the lower beds of chalk or chalk marl, pass gradually into 

 the green sand, by a close intermixture with it, and have, on account 

 of their greenish or yellowish colour, been denominated Glanconie 

 crayeuse and Craie chloritee, by the French. 



Green sand has received its English name from its intermixture 

 with particles of green earth ; it is very variable in its mineral char- 

 acters, being sometimes found composed of loose siliceous sand ; in 

 other situations, it forms sandstone, cemented by calcareous earth; 

 it abounds in siliceous concretions, which vary from an opaque bluish 

 white chert or hornstone, to flint and chalcedony. The geodes 



