LOWER CHALK—MICROGRAPHIC STRUCTURE. 
281 
matter is also present. The higher beds, as usual, are the most 
calcareous; the mineral grains become smaller and the proportion 
of inorganic matter less as one progresses from the base to the 
summit of the zone. 
Mineral grains which, judging from thin sections, reach their maxi¬ 
mum in size and quantity between Wantage and Tring are more 
abundant in the lower than in the higher part. They are never 
very large, and do not approach the size of the mineral grains of 
South Dorset. So far as can be measured in the sections the dimen¬ 
sions do not exceed '25 mm. in the longest diameter, though they 
probably exceed this. 
In Buckinghamshire and extending westward also into Oxford¬ 
shire is a bed of hard compact limestone in the middle of this division. 
It does not differ much from the beds above and below it in general 
structure, but the usual ingredients are now inclosed in a matrix 
of granular crystalline calcite which binds the whole into a com¬ 
pact limestone (see p. 292). This change has doubtless been 
accomplished by the aid of percolating water saturated with 
calcic carbonate, for the “ Marl Bock,” as it may be termed, is 
the source of a continuous line of springs, the water being thrown 
out by a bed of argillaceous marl beneath. 
Bedfordshire and Cambridgeshire.—We have no information of 
the composition of the lower beds of this zone in the western part 
of Bedfordshire. The upper 35 feet is well exposed in a cutting of 
the Midland Kailway at Charlton, three miles east of Dunstable. 
The portion seen here is a dark grey marl containing a consider¬ 
able quantity of fine sand, the grains of which are smaller than 
those of Buckinghamshire; so fine, indeed, are they that it is 
almost impossible to separate them from the amorphous matter by 
levigation. It passes up into a paler grey calcareous marl, in which 
there is a decided decrease in the amount of sand-grains and fine 
inorganic matter. Shell-fragments are fairly numerous ; many are 
too small for identification, but the larger pieces are nearly all prisms 
of Inoceramus, though here and there plates or spines of 
echinoderms may be recognised. Foraminifera are sparingly 
distributed through the deposit. (See Plate IV., Fig. 2.) 
At Arlsey, in the north-east corner of the county, the lowest beds 
are exposed ; the aspect of the marl in a specimen 10 feet above the 
Cambridge Greensand (17) reminds one of that of North Dorset. 
“ Spheres ” regain their premier place, and with shell-fragments 
(Inoceramus-'pnsms) of irregular size form nearly half the rock. 
A few Foraminifera ani here ani there a mineral or glauconitic 
grain of small size constitute the recognisable elements embedded 
in the amorphous matrix, which in this locality ani at this horizon 
is largely calcareous. 
But nearer the summit of the zone, 30 feet below the Totternhoe 
Stone (18), the material is nearly identical with that seen in Charlton 
