ple a Ne ina. 
TRANSACTIONS OF THE SECTIONS. 41 
spurs of lower chalk have, by faults and denudations, been cut off from the general 
mass and thus form a series of obscure outliers, the exact limits of which it is ex- 
tremely difficult to define, in consequence of the featureless nature of the country 
and the great beds of drifted clay and gravel which cover many parts of the more 
regular formations. A small patch of chalk near Castle hill, and the chalk quarries 
near Madingley, worked under a thick covering of drifted brown clay, are given as 
examples. To the west of this irregular escarpment or boundary of the chalk, the 
following formations break out in a regular descending order:—1. Upper green- 
sand; 2. Galt; 3. Lower greensand; 4. Kimmeridge clay; 5. Coral rag; 6. Ox- 
ford clay, which forms the subsoil of the great Bedford Level. 
1. Upper Greensand.—This appears, as far as regards its thickness, in a very de- 
generate form. The portion to which the term greensand is strictly applicable is 
only a few inches thick, and above it there is sometimes an ambiguous deposit of a 
few feet which forms a passage into the lower chalk. It is not true, as has formerly 
been stated, that the chalk marl forms a passage into the galt; for the upper green- 
sand, without a single exception, makes a natural break between them. As the 
elialk formation is generally pervious to water and the galt always impervious, it 
necessarily follows that copious springs are thrown out by the greensand bed. The 
springs at Cherry Hinton, the Nine-springs at the foot of the Gogmagog hills, and 
those of Coton and Madingley, are given as examples; and if a curved line be drawn 
through the principal springs of the neighbourhood, it will give a good approxima- 
tion to the true line of outcrop of the lower greensand. Thus it appears, by reference 
to the map, that Cambridge is situate on the galt in a bay formed by two irregular 
spurs of the lower chalk. Though the greensand is thus degenerate, its fossils are 
numerous and extremely characteristic. Well known species of Hxogyra and Tere- 
bratula are in great abundance; characteristic greensand Ammonites are not rare ; 
Hippurites occur occasionally ; sharks’ teeth and fish palates of several families are 
abundant. Many fragments of the head-bones of several species of Chimera occur 
here and there, and paddle-bones and teeth of a Plesiosaurus have been found abun- 
dantly near Barnwell, though mutilated and ill-preserved. But the most abundant 
fossils are the well-known black nodules which resemble coprolites, but have none 
of the structure of those bodies, being generally almost amorphous. They contain 
however a large per-centage of phosphate of lime. The author does not consider 
that their origin is yet well made out. The greensand, though partly incoherent and of 
such inconsiderable thickness, yet appears to have protected the upper surface of the 
galt from denudation ; for it spreads out over a considerable area to the west of the 
chalk, forming the upper surface of the galt where that formation rises into the dry _ 
lands that skirt the neighbouring marshes, 
2. Galt.—This formation, as has been proved by repeated borings for Artesian 
wells, is more than 150 feet thick. None of the borings, though apparently com- 
mencing at the top of the formation, reach 200 feet. As this formation occupies 
a country almost at a dead level, and partially covered by marsh lands and gravel, 
its superficial extent cannot be accurately represented. Its western limits are how- 
ever, here and there, well-defined by the outcrop of the lower greensand. The fos- 
' sils from this formation are derived from the upper part of it, as seen in the brick- 
pits near Cambridge, and they agree very closely, so far as they go, with the fossils 
of the Folkstone clay. Hamites, Crioceratites, and shells of the genus Inoceramus, 
&c. abound. A few shells of the genus Plicatula appear to be of the same species 
with those found in the lower beds of the neighbouring chalk marl. 
3. Lower Greensand and Sandstone.—This formation is well seen at Denny Abbey, 
on the road to Ely. To the east of that place it is, for many miles, concealed under 
the marshes and makes no escarpment; but it is probably continuous, as it breaks 
out in greater force, forming an under terrace to the chalk escarpment of Norfolk, 
and, as is well known, may be traced to the sea at Hunstanton Cliff. It is seen 
near Cottenham, Rampton and Willingham (as represented on a map). It then 
crosses the Huntingdon road about the seventh milestone, buried however under a 
great thickness of the brown clay (Till). Again, it breaks out at Elsworth, and ina 
denudation between Caxton and Bourne, and at Great Gransden. Beyond the last- 
named place it increases in thickness and blends itself with the sand-hills of Bed- 
fordshire. In the range above given it is perhaps, on the average, not more than 
