12 THE CRETACEOUS ROCKS OF BRITAIN. 
Chalk or Turonian stage. It includes, therefore, the beds which 
have been called Chloritic Marl, Chalk Marl, and Grey Chalk, to¬ 
gether with a certain thickness of white or whitish chalk which has 
a band of grey marl at the top. 
The question of the line of demarcation between the Upper Green¬ 
sand (Selbornian) and Lower Chalk has given rise to much con¬ 
troversy, for the junction of the two formations presents very diffe¬ 
rent aspects in different parts of the country. In some localities, 
especially where the highest part of the Selbornian consists of soft 
greensand, there is a gradual passage upward from this sand into 
the Chalk Marl, a part of the passage being generally occupied by 
sandy glauconitic marl containing small phosphatic nodules and casts 
of fossils. In other places, however, the plane of division is clearly 
marked, and a certain amount of current-erosion seems to have 
taken place, so that the base of the Chalk Marl rests on a waterworn 
surface, and contains a large number of phosphatic nodules and 
fossils, which in some places have clearly been derived from the 
erosion of pre-existent deposits. 
In the southern counties any layer of glauconitic and noduliferous 
marl occurring at the junction of Greensand and Chalk has generally 
been called “ Chloritic Marl ; ” but in the counties of Bedford, Herts, 
Cambridge, and Suffolk the junction bed is known as the “ Cam¬ 
bridge Greensand .” 
The name Chloritic Marl appears to have been a translation of 
the French marne or craie chloritee, and the first suggestion of it 
which we can find is a passage in a paper by Mr. Godwin-Austen 
(1843), in which he describes the base of the Chalk in Surrey as 
“ becoming first a craie chloritee, till by further diminution of the 
calcareous matter we reach the bright green beds of the Upper Green¬ 
sand with Plicatula inflataC* Professor E. Forbes and Captain 
L. L. B. Ibbetson seem to have been the first to employ it as a designa¬ 
tion for a particular chloritic (i.e., glauconitic) marl, the mineral 
now known as glauconite being then confused with chlorite. Their 
“ Chloritic Mail” was that at the junction of Greensand and Chalk 
in the Isle of Wight,f and Captain Ibbetson gave a more particular 
account of this stratum in a later paper,J the substance of which 
was incorporated in a little book published in 1849, entitled “ Notes 
on the Geology and Chemical Composition of the Isle of Wight.” 
From that date this particular bed has always been known as 
“ The Chloritic Marl,” notwithstanding the misnomer involved in 
the term. Captain Ibbetson’s description of the bed is, however, 
not free from ambiguity, and difficulties arose in ascertaining how 
much he meant to include in his Chloritic Marl, and which bed 
ought to be regarded as the base of the Chalk. Thus Mr. M. Nor¬ 
man, describing the Ventnor section in 1858,* distinguished what 
* Proc. Geol. Soc., VoL iv. p. 169. 
t Brit. Assoc. Rep. for 1844. Trans. Sec. p. 144. 
1 Brit,. Assoc. Rep. for 1848. Trans. Sec. p. 69. 
;§ Geologist, Vol. i. p. 480. 
