MIDDLE CHALK — GEN3BHAL ACCOUNT. 
359 
CHAPTER XXVI. 
GENERAL ACCOUNT OF THE MIDDLE CHALK OR 
TURONIAN STAGE. 
Definition and Limitation of the Stage. 
There have been several attempts to establish a Middle or Medial 
division of the English Chalk as mentioned in a previous chapter 
(see p. 4); but the lirst endeavour to found such a division on 
a strictly palaeontological basis was made by Caleb Evans in 1870.* 
His “ Middle Chalk,” however, consisted of but one zone, that which 
is now known as the zone of Terebratulina gracilis , and he did 
not attempt to trace it beyond the immediate neighbourhood of 
the line of section which he described. 
It was reserved for Professor Barrois, in 1876, to show that we 
possessed in England a full and complete equivalent of the Turonian 
of the North of France ; that its lowest zone with Inoceramus 
mytiloides could be traced without difficulty from Dorset to Norfolk, 
and at each extremity of the Wealden area ; and that its upper limit 
was defined by the “ Chalk Rock ” of Mr. Whitaker. 
The survey of the Cambridgeshire Chalk by Mr. Penning and 
myself convinced us that in this county the formation fell naturally 
into three great divisions, and the general results of our work were 
expressed in the following terms! : ~ 
“ (1) That the zones of life [in the Cambridgeshire Chalk] 
are similar to those established elsewhere, and have been actually 
traced over a considerable area. 
“ (2) That there are two breaks of more or less importance 
marked by the occurrence of the hard rocky beds which are 
termed the Melbourn Rock and the Chalk Rock. 
“ (3) That a middle division of the Chalk is thus marked 
off, and that its establishment is confirmed by the palaeonto¬ 
logical evidence.” 
The facts on which this “ Middle Chalk ” was founded were fully 
set forth in the Geological Survey Memoir on the Neighbour¬ 
hood of Cambridge published in 1881. Since that time there has 
never been any doubt about the lower limit of the Middle Chalk, 
the Melbourn Rock being an horizon which is easily recognised 
and easily followed across the country, except in the western part 
of Dorset. Its upper limit, again, is conveniently marked by the 
Chalk Rock wherever that rock is developed as a single set of hard 
rocky beds, but where this is not the case upper limit of the Middle 
Chalk is less definite. 
* On some Sections of Chalk between Croydon and Oxsted, Geol. Assoc. 
Separate Paper, (1870). 
t Geol. Mag., Dec. If., Yol. vii., p. 254 (1880). 1 
