734 PROF. J. W. JUDD ON THE NATURE AND RELATIONS OF 
A similar rock was met with in the shafts of the Marden Park 
Tunnel on the Croydon and Oxtead Railway, the sections in which 
have been so well described by Mr. Caleb Evans*. ; 
The Melbourn Rock of Mr. Jukes-Brownet appears to be very 
similar to that found in the Richmond boring; and there can be no 
doubt that in this latter we have a representative of the Zone of 
Belemmtes plenus in the remanié condition which has been so 
admirably described by M. C. Barrois. In the cliff-section at 
Folkestone, Mr. F. G. H. Price found the same bed with a thick- 
ness of 4 ft.£ The Melbourn Rock contains 0-92 per cent. of 
insoluble matter, the rock from the Richmond well 1-3 per cent. 
The detection of these two important horizons (the Chalk Rock 
and the Zone of Belemnites plenus) in the Richmond boring, furnishes 
us with the most valuable aid in classifying the Chalk strata of the 
London area. 
The whole of the strata, 300 ft. in thickness, which lie above 
the Chalk Rock, belong to the Upper Chalk, or Senonian of Conti- 
nental authors, and to the lower part of that division, the Chalk 
with Micrasters; the Upper Senonian, or Chalk with Belemnitellas, 
not being represented in the London area. 
Through the upper, 200 ft. of the Chalk with Micrasters in the 
Richmond well, layers of chalk flints occur at pretty regular intervals. 
In the lower 100 ft. the bands of flint are less regular and constant, 
and before the Chalk Rock is reached they cease altogether. Forty 
feet above the Chalk Rock a band of dark grey marl about 3 in. 
thick occurs in the midst of the Chalk. 
The Chalk Rock itself is probably a diminutive representative of 
the Zone ot Holaster planus. 
The next 137 ft. of strata, consisting of chalk of a less purely 
white colour and with only few and scattered flints, but with oc- 
casional thin seams of marl, represents the Zones of Terebratula gracilis 
and of Inoceramus labiatus, but we have no means of estimating 
the thickness of the beds belonging to each of these divisions in the 
section under consideration. 
The Zone of Belemnites plenus is, as shown by M. C. Barrois, 
the fragmentary representative of a great series of strata, which in 
some parts of France contains a remarkable and distinctive fauna. 
The three zones, having a united thickness at Richmond of less than 
150 it., are the diminished representative of the Middle Chalk, or 
Turonian of Continental authors. 
The Chalk strata which lie below the Zone of Belemnites plenus, 
220 ft. in thickness at Richmond, consist of grey marly chalk 
without flints. In passing downwards these strata become more 
and more argillaceous, and of a darker colour, the bottom beds being 
‘‘ chalk-marl;” but so gradually does the transition from grey chalk 
to chalk marl take place, that in the absence of any representative 
* Proc. Geol. Assoc. 1870. 
+ Mem. Geol. Sury., ‘Geology of the Neighbourhood of Cambridge,” p. 55 
881). 
{ Quart. Journ. Geol. Soc. vol. xxxili. (1877), pp. 489, 440, 445. 
