XI. On the Strata of the Northern Division of Cambridgeshire. 
By FRANCIS LUNN, Esq. 
In a Letter addressed to G. B. GREENOUGH, Esq. Pres, Geol. Soc, 
[Read May 1, 1818.] 
T? H E tract of country examined is of an extent so limited, that I 
should be disposed to think the facts scarcely worth your notice, if 
I were not aware that it is only from the united labours of many 
that a perfect knowledge of the features of a country can be 
acquired. 
The lowest stratum found in the county is the ferruginous sand, 
which on quitting Potton in Bedfordshire, is observed on the borders 
of Cambridgeshire at Gamlingay ; in this parish may be seen the 
sand, and resting upon the sand the next stratum in the scale, the 
blue clay, which in this county is universally termed gait. 
In some of the pits the two are seen having the line of their 
junction as well defined as that of two immiscible fluids, which 
proves that in this county there is no intermediate stratum between 
the sand and the gait. It may however be invariably observed, that 
a layer of the sand contiguous to the clay is much more highly im- 
pregnated with iron than the lower part of the same stratum, the 
oxide of this metal cementing together the particles of sand so as to 
form a hard rock. Leaving to the south-east the two villages of 
Hatley, which are both on the gait, we are led by the boundary of 
the strata between Little and Great Gransdon, and passing to the 
westward of Caxton, which is also on the clay, we proceed through 
