s--_creta
marga_ of Linnaeus--known locally as malm, marl, clunch, &c.; and
harder, more stony kinds, called rag, freestone, rock, hurlock or
harrock in different districts. In certain parts of the formation layers
of nodular flints (q.v.) abound; in parts, it is inclined to be sandy,
or to contain grains of glauconite which was originally confounded with
another green mineral, chlorite, hence the name "chloritic marl" applied
to one of the subdivisions of the chalk. In its purest form chalk
consists of from 95 to 99% of calcium carbonate (carbonate of lime); in
this condition it is composed of a mass of fine granular particles held
together by a somewhat feeble calcareous cement. The particles are
mostly the broken tests of foraminifera, along with the debris of
echinoderm and molluscan shells, and many minute bodies, like
coccoliths, of somewhat obscure nature.

The earliest attempts at subdivision of the Chalk formation initiated
by Wm. Phillips were based upon lithological characters, and such a
classification as "Upper Chalk with Flints," "Lower Chalk without
Flints," "Chalk marl or Grey chalk," was generally in use in England
until W. Whitaker established the following order in 1865:--

Upper Chalk, with flints

/ chalk rock
Lower Chalk < chalk with few flints
\ chalk without flints

Chalk Marl / Totternhoe stone
\ " marl

In France, a similar system of classification was in vogue, the
subdivisions being _craie blanche_, _craie tufan_, _craie chloritee_,
until 1843 when d'Orbigny proposed the term _Senonien_ for the Upper
Chalk and _Turonien_ for the Lower; later he divided the _Turonien_,
giving the name _Cenomanien_ to the lower portion. The subdivisions of
d'Orbigny were based upon the fossil contents and not upon the
lithological characters of the rocks. In 1876 Prof. Ch. Barrois showed
how d'Orbigny's classification might be applied to the British chalk
rocks; and this scheme has been generally adopted by geologists,
although there is some divergence of opinion as to the exact position
of the base line of the Cenomanian.

The accompanying table shows the classification now adopted in
England, with the zonal fossils and the continental names of the substages:--

+-----------------------------------------------+---------------------+----------+-----------+
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