_a._ The _Chalk_ is too well known to need description, though
technically it may be described as a soft white limestone chiefly built
up of the microscopic shells of _Foraminifera_, and characterized in its
upper part by nodules and bands of flint. These flints frequently
inclose casts of fossils (sponges, sea-urchins, etc.), and sometimes
shells themselves. Fossils, too, are fairly abundant, scattered
throughout the mass. Amongst the commoner may be noticed the
sea-urchins, such as the "sugar loaf" (_Ananchytes_) and the
heart-shaped _Micraster_, the Brachiopods or Lamp-shells (_Terebratula_,
_Rhynchonella_), a "Thorny Oyster" (_Spondylus spinosus_), besides
Ammonites, Belemnites (part of the internal shell of a kind of
cuttle-fish), and the teeth of several species of sharks. Altogether the
chalk is about 1,000 feet thick.
[Illustration: _Ammonites various_ (from the chalk).]
_b._ _Upper Greensand_ is a series of greenish-grey sands and
sandstones. The green colour, on close inspection, is seen to be due to
the presence of innumerable small green grains of a mineral called
glauconite. These are frequently casts of the chambers of the very same
foraminifera that the chalk is so largely composed of.
[Illustration: _Rhynchonella depressa_ (a Brachiopod, from the Upper
Greensand).]
Nodules and layers of "chert" (an impure kind of flint) occur in it,
whilst in places it forms a hard rock called "firestone." The commonest
fossils are Brachiopods, very similar to those in the chalk, a
scallop-shell with four strongly marked ribs on it (_Pecten
quodricostatus_), an oyster with a curved beak (_Exogyra columba_), and
a pear-shaped sponge (_Siphonia pyriformis_). The Upper Greensand is
better seen at places in the southern part of the Isle of Wight, in
cliffs on the Dorsetshire coast, in Wiltshire, at Sidmouth, and in some
parts of Surrey.
[Illustration: _Ammonites auritus_ (from the Gault).]
_c._ _Gault_, a stiff blue clay abounding in fossils: Ammonites often
retaining their pearly shell; Belemnites, a bivalve with very deep
furrows on it (_Inoccramus sulcatus_), and its first cousin (_I.
concentricus_, p. 21), in which the ridge-like markings correspond with
the lines of growth, besides many others, may be obtained in abundance
from it. Layers of phosphatic nodules occur at irregular intervals. The
gault is best studied at East Wear Bay, near Folkstone; it may also be
seen in Dorsetshire, Wiltshire, and Cambridgeshire; lately it has been
found as far west as Exeter.