The Geologist/Volume 5/Notes and Queries, Subdivisions of the Chalk Formation
SECTION OF THE CHALK CLIFFS UNDER DOVER CASTLE, KENT.
NOTES AND QUERIES.
Subdivisions of the Chalk Formation.—The generally accepted subdivisions of the chalk are,—1, Upper White Chalk with bands of Flint nodules; 2, Middle or Lower White Chalk; 3, Grey Chalk or Chalk Marl.
These have been in undisputed use for very many years, not because they do not require any modification to render the accordance more definite and more rigidly corresponding to the accumulation of information which has been going on since their introduction, but chiefly because chalk,—at least English chalk,—is white or of a pale grey, which when the beds are in a dry state is so nearly white, that ordinary eyes do not see the difference, and ordinary collectors do not care about it so long as they can get hold of a fine fossil.
Still, however, it is very necessary, and high time that some one should take in hand to define accurately the lines of division, especially that between the upper and lower white chalks.
I doubt very much that the cessation of the bands of flints denotes the demarcation between the upper and lower white chalk (middle chalk of some authors): they should be properly, and must be ultimately, separated by a characteristic difference in the distinguishing organic remains.
With the lowermost bands of flints (Plate II. a) very numerous beds of ventriculites and sponges set in, and are continued far below the termination of the layers of flints, down to a very thick bed of pure white chalk (b), that rests upon a very marked and peculiar stratum about two feet thick (c), which, from the weathering out of its upper and under surfaces, forms a marked line as far as the eye can see the distinctions of bedding all along the coast.
This bed, in my own note-books and in conversation, I have familiarly termed the "two-foot stratum."
Below this we have again a thick bed of white chalk, free from flints. At least, such is the order in the section to which these remarks more particularly refer, namely, that presented by the East or Castle Cliff at Dover, of which we give a view in Plate II.
This "two-foot stratum" is persistent throughout Kent, and I have met with it both in Surrey and Sussex, and it will therefore probably form one of the best and most unmistakable guides in inland quarries to those particular beds of white chalk to which we wish to draw attention, for the purpose of getting all the information we can as to their geographical area, order of succession, and organic contents in other chalk districts, so that the true horizon of division, as formed by distinctiveness of organic remains, may be properly made out.
We shall be obliged, by communications, and stratigraphical lists of fossils from our readers and correspondents, to assist us in our labours in determining this interesting point of Whether the ordinary division into "white chalk with flints," and "white chalk without flints," is not merely a mineralogical division, and not a proper geological subdivision characterized by distinctive organic remains, and marking out a positive zone in the succession of geological events and of life-forms; or Whether a distinguishing alteration in the organic remains of the white chalk does not happen so near the horizon of cessation of flint layers, that by including or excluding some few beds of chalk, those valuable and characteristic petrological features (of chalk with, or without flints) may not be made more precisely valuable and definite than at present. S. J. Mackie.