LOWER CHALK—ISLE OF WIGHT. 
95 
Melbourn Rock, we have had some doubt whether this inter¬ 
mediate band should be regarded as belonging to the Lower or 
to the Middle Chalk. It contains very few fossils, and such as 
we could find were not of characteristic Lower Chalk species ; 
more recently I). A. Rowe has obtained Middle Chalk species 
from these beds, and has consequently placed them in the zone 
of Rhynch. Cuvieri. 
STR ATIGR APHIC AL . DETAILS. 
In describing the coast sections in this district it will be convenient 
to give the complete vertical succession at each locality first, and 
then to indicate the zonal divisions so far as it is possible to do so. 
The first section to be noticed is that of Ballard cliff, on the north 
side of Punfield Cove, near Swanage. Part of the Lower Chalk 
here was described by Mr. H. G. Pordham in 1876* and very 
briefly by Dr. Barrois in the same year.f It was measured more 
carefully by Mr. A. Strahan in 1888.J The following account 
is based on that of Mr. Strahan, with the addition of some details 
given by Mr. Pordham : — 
ft. in. 
B 
Smooth grey marl (Belemnite Marl) - - - - 5 0 
Alternations of greyisli-white chalk and bluish-grey 
marl— Am. [Acantk. ] Mantelli and Am. [Acanthi] 
navicularis- - - - - - - - -84 0 
Similar beds, with two or three layers of greenish plios- 
phatic nodules —Am. [. Acantli ]. navicularis about 20 0 
/Alternations of bluish shaly marl and harder grey 
chalk — Am. [Schloenb.] varians , etc.- - - - 22 O 
Hard chalk marl, weathering into rhomboid pieces - 6 0 
( Very hard splintery chalk ----- 13 
Sandy and glauconitic marl - - - - - - 010 
Glauconitic sandy chalk, with phosphatic nodules and 
scattered fossils - - - - - 4 0 
About 143 0 
Mr. Rhodes was instructed to make a careful search for Am. 
(, Schloenb .) varians in these beds, but he could not find a fragment of 
one higher than 30 feet above the top of the basement bed, nor was 
it common anywhere except in that bed. It would appear, there¬ 
fore, that the zone of Am- varians is barely more than 34 feet thick, 
but when we recollect that its thickness at Compton Bay (omitting 
the subzone of Stauronema Garteri) is only 52 feet the difference 
is not greater than might be expected. As however we are un¬ 
certain whether this is really the upper limit of the zone, we have 
grouped the beds into bracket A and bracket B. 
If the overlying beds with phosphatic nodules do not belong to 
the Am. varians zone they must be grouped with those that represent 
the zone of Holaster subglobosus, which will thus have a thickness 
of 104 feet, and be thicker than in the Isle of Wight; but, instead 
* Proc. Geol. Assoc., Vol. iv. p. 506. 
t Recherches sur le Terr. Cret. Sup. p. 101. 
I See Geology of the Isle of Wight, 1889, and Geology of the Isle of Pur- 
beck, p. 167 (1898). 
