276 
THE CRETACEOUS ROCKS OF BRITAIN. 
(d) An interesting feature is the presence of a number of large 
grains of Rutile, showing crystal forms, the prism being in 
a few cases clearly capped by pyramidal faces of a different 
order. Some of the prismatic faces are strongly striated 
parallel to the vertical axis. Owing to the thickness of the 
fragments, it is difficult to determine the exact pleochroism, 
but changes from yellow to reddish-brown have been ob¬ 
served in the grains. 
3. Residue between * 25 to ' 5 mm. diameter. This differs from 
the above in the greater abundance of delicately silicified tests of 
Foraminifera, mostly broken, and often still filled with glauconite. 
Some of the glauconitic casts are well preserved, the biserial 
character of Textularia and the spiral arrangement of Buli- 
mina being well illustrated by the examples obtained. The smaller 
varieties of the former have the casts of the chambers standing out 
as rectangular projections, branching alternately from opposite sides 
of a common stem of extreme thinness ; in the latter the stem is flat¬ 
tened, the projecting portions arising from it after the manner of 
young leaves near the apex of a growing plant. 
4. The lightest material differs but little from the above, but a 
number of heavy minerals were obtained by means of borotung- 
state of cadmium. These included numerous irregular deep red- 
brown and red fragments of Rutile, Tourmaline in good crystals, 
the prisms being perfect, and capped by rhombohedral termin¬ 
ations. In the latter, as usual, the absorptic n for the ordinary 
ray is very great; cleavage is well setn, especially in the bluish- 
grey varieties. Zircon is rare, two or three crystals having doubly 
pyramidal terminations were seen. In these the zoning is well 
shown, and the pyramidal terminaticns are very acute. In one 
case the simple tetragonal pyramid, without prism faces, was 
observed. Annular microlites occur as inclusions. 
The noticeable features in connection with this bed are therefore: 
The predoihinance of the coarser residue over the more clayey con¬ 
stituents, in this-‘respect differing markedly from corresponding 
strata further to tlie-Oast; the extreme tenuity of the siliceous tests 
of the Foraminifera ; the delicate preser vatic n of the glauconitic 
casts ; arid the abundance, variety, and size of the heavier minerals. 
The results of these two analyses are tabulated on p. 293, with 
the chalks of this zone. 
• 
0. t6e Marls and Chalks of the Am. varians Zone. 
• ' . . . ■ • 
. Macroscopic Aspect of the Book, 
t 
The colour and aspect of these beds vary according to the 
locality and the horizon from which they come, these variations 
being due partly to the difference in the nature of the deposit 
and partly to circumstances not connected with its depositicn. 
