120 
THE MIDDLE LIAS OF NORTHAMPTONSHIRE. 
In studying the lower beds of the Upper Lias in North¬ 
amptonshire and some other districts, I have been led to the 
conclusion that the hard beds have a peculiar significance— 
that in fact they form the boundaries of distinct palaeonto¬ 
logical zones. Thus, at the base of the Upper Lias we have 
a paper shale in which fish remains are common, and at or 
near the top of this deposit occurs a limestone band in which 
they are exceedingly abundant. The hard bed is called the 
Fish and Insect-bed, or with us, insect remains being rare, 
simply the Fish-bed, and although for a few inches above fish 
fragments may still be met with, the limestone band practi¬ 
cally forms the upward limit of the Fish and Insect Zone. 
Next we have in Northamptonshire a clay, with very few 
fossils, capped by another limestone band containing tl falcifer’ 
ammonites in enormous numbers. This bed, locally named 
the Lower Cephalopoda-bed, forms the upper boundary of 
the “ Serpentinus-beds.” Above this again is a clay contain¬ 
ing great numbers of small ammonites, chiefly A. com¬ 
munis and A. Holandrei, with another limestone band at the 
top, in which they are, in places, still more abundant. This 
latter bed, which we call the Upper Cephalopoda-bed, forms 
the upper boundary of the “ Communis-beds above it there 
is scarcely a fossil to be found for a considerable thickness. 
There is no progressive increase noticeable in the number 
of ammonites and other fossils as we approach the limestone 
bands from below; hence it seems necessary to assume a 
scarcity of sedimentary matter during their formation, the 
shells of marine animals, chiefly ammonites, themselves con¬ 
stituting, in many cases, the greater portion of the bed. 
The question as to whether the change in the fauna which 
took place at the end of each period was brought about during 
the cessation of deposit, or when the fresh sediment was 
introduced, can be fairly well decided in favour of the latter 
view, because there is a noticeable lingering of certain forms 
of fossils just above two of the hard beds, though only for a 
few inches. 
It seemed to me probable that the hard beds of the Middle 
Lias might have a similar significance, and I think they have, 
though to a less noticeable extent. There certainly is a large 
increase in the number of fossils in the hard beds as compared 
with the soft ones below them respectively, and each one 
forms the upper boundary of one or more fossils, and the 
place of maximum development of perhaps several others for 
this neighbourhood, though of course out of this district they 
may be found higher or lower according to the direction in 
which migration took place. 
