360 
THE CRETACEOUS ROCKS OF BRITAIN. 
In tlie publication above mentioned the Middle Chalk included 
three zones, the succession being as follows : — 
r3. Zone of Holaster planus, including the Chalk Rock. 
Middle j 2. Zone of Terebratulina gracilis. 
Chalk 1 1. Zone of Rhynchonella Cuvieri or Inoceramus mytiloides, 
( with the Melbourn Rock at its base. 
The question of what ought to be regarded as the zone of 
Holaster planus, and whether the Chalk Rock should be placed 
at the top of the Middle Chalk or the base of the Upper Chalk, 
are points upon which we have spent much time and consideration, 
for even where the Chalk Rock is well developed there is not always 
a clearly marked plane of division either at its base or at its top, 
such as there is generally between the Lower and the Middle Chalk. 
The history of the zone of Holaster planus, so far as England is 
concerned, may be summarised as follows. Hebert, writing in 1874, 
indicated the zone as comprising the central part of the “ nodular 
chalk ” of Dover.* * * § Professor Barrois in 1876f traced this zone 
through various parts of England, and regarded the Chalk Rock as 
its equivalent. The result of collecting in Cambridgeshire between 
1876 and 1877 showed that Hoi. planus was fairly common below the 
Chalk Rock, and in 1880 it was proposed to group 50 or 60 feet 
of this soft chalk with the Chalk Rock in a zone of Holaster planus.X 
The Dover section was again described in 1886,§ and as the 
Holaster was found to be commonest in the lower beds, while 
Micro ster, especially the species breviporus (= M. Leskei), was com¬ 
moner in the upper part, it was proposed to make two zones, one 
of Holaster planus and another of Micraster breviporus. This, how¬ 
ever, has not proved satisfactory, the faunas are not sufficiently 
distinct, and the fauna of the Chalk Rock is found both in and 
above the zone of M. breviporus. 
It is, no doubt, quite possible to indicate at Dover a set of beds 
characterised by a special fauna, of which H. planus might be taken 
as the index, but there is no well marked summit to such a zone, 
which graduates up into that usually known as the zone of Micraster 
cortestudinarium. Moreover, the zone so defined may be regarded 
as the expansion of the Chalk Rock, but for that very reason is 
not the equivalent of the zone of Holaster planus as defined in 1880 
for Cambridgeshire and in 1889 for the counties of Oxford and 
Buckingham.il 
The fact is that Holaster planus ranges through a considerable 
thickness of chalk, commencing high up in the Terebratulina zone, 
and dying out in that of Micraster cortestudinarium. It is not, 
of course, necessary that the range of the species which is chosen 
as the index of a zone should be restricted to that zone, but only 
* Bull. Soc. Geol. de France, Ser. 3, Tom. II. p. 423. 
t Recherches sur le Terr. Cret. Sup., passim, 
t See Geol. Mag., Dec. II., Vol. vii. p. 248. 
§ By William Hill. See Quart. Journ. Geol. Soc., Yol. xlii. p. 232. * 
il See Memoirs of the Geological Survey, Geology of the Neighbourhood 
of Cambridge (1881), and Geology of London (1889). 
