250 A. J. Jukes-Browne—The Subdivisions of the Chalk. 
by Professor Hébert and himself, and that these zones are arranged 
under the three great sections into which D’Orbigny had originally 
divided the French Chalk: viz. Cenomanien, Turonien, and Senonien. 
It is also clear that these divisions are to some extent correlative 
with the Lower, Middle, and Upper Chalk of Woodward and Rose. 
The result, therefore, of the more careful and detailed observations 
that have recently been made is to set aside the old divisions depend- 
ing on the proportions of flint nodules in the Chalk, and to introduce 
the new element of the zonal distribution of life. It consequently 
becomes necessary to re-consider the questions of primary subdivision 
and nomenclature ; the latter is involved in the former, and before 
that can be determined, we must ascertain where the greater paleeon- 
tological breaks in the series occur, whether they coincide with any 
beds of marked lithological character, and whether the natural 
division of the whole formation thus suggested is into two, or three 
parts. 
As the survey of the Cambridgeshire Chalk by Mr. Penning and 
myself has thrown some light on the points above mentioned, I 
propose to give a short account of the zonal divisions which we 
have established in that county; for a full description of these beds 
and their fossil contents I must refer to the Memoir on the neigh- 
bourhood of Cambridge, which will shortly be published. 
During the progress of this survey in 1875-77 we recognized the 
representatives of the two hard bands which had been described by 
Mr. Whitaker in more southern counties under the name of Chalk 
Rock and Totternhoe Stone ; we furthermore discovered the existence 
of a third intermediate band of rock, to which we gave the name of 
the Melbourn Rock; and which proved to be of great stratigraphical 
importance. These three beds we succeeded in tracing across 
Cambridgeshire for a distance of about twenty-five miles, and have 
laid down lines on the Ordnance Map which give a sufficiently 
accurate indication of the course taken by each outcrop. The result 
of this is to divide the Chalk into four stages, which are as clearly 
defined as if they were subdivisions founded on difference of litho- 
logical character, like those of the Lower Greensand in the Wealden 
area. 
Between these boundary-lines certain paleontological zones are 
recognizable, and the distribution of the fossil remains in these 
confirms the stratigraphical evidence as to the relative importance of 
the rock beds for the purposes of classification. The following is a 
brief account of these zonal divisions in Cambridgeshire (see Table 
on p. 257) :— 
1. Cambridge Greensand.—This is the homotaxial equivalent of 
what has elsewhere been called the Chloritic Marl, and should 
without doubt be regarded as the basement bed of the Chalk Marl. 
This thin noduliferous layer has been so often described that it is 
hardly needful to give much description of it here; I will only 
remind you of the following facts,—that the nodules are enclosed in 
* See Quart. Journ, Geol. Soc. vol. xxxi. p. 272; and Gzox. Mac. Decade II. 
Vol. IV. p. 350. 
