THE CHALK.
The chalk formations or “cretaceous group of beds” include strata of various mineral substances; but the white chalk which forms the cliffs of Dover and the adjoining coasts, and the downs and chalk quarries of the South of England, is the chief and most characteristic formation. Chalk, immense as are the masses in which it has been deposited, owes its origin to living actions; every particle of it once circulated in the blood or vital juices of certain species of animals, or of a few plants, that lived in the seas of the secondary period of geological time. White chalk consists of carbonate of lime, and is the result of the decomposition chiefly of coral-animals (Madrepores, Millepores, Flustra, Cellepora, &c.), of sea-urchins (Echini), and of shell-fishes (Testacea), and of the mechanical reduction, pounding, and grinding of their shells. Such chalk-forming beings still exist, and continue their operations in various parts of the ocean, especially in the construction of coral reefs and islands.
Every river that traverses a limestone district carries into the sea a certain proportion of caustic lime in solution: the ill effects of the accumulation of this mineral are neutralised by the power allotted to the above-cited sea-animals to absorb the lime, combine it with carbonic-acid, and precipitate or deposit it in the condition of insoluble chalk, or carbonate of lime.
The entire cretaceous series includes from above downwards:
Maestricht beds of yellowish chalk.
Upper white chalk with flints.
Lower white chalk without flints.
Upper green-sand.
Gault.
Lower green-sand and Kentish rag.