THE MIDDLE LIAS OF NORTHAMPTONSHIRE. 
121 
It is important to notice that the lower beds of the Upper 
Lias were formed whilst the area was sinking, whereas the 
land was pretty certainly rising during the latter portion of 
the Middle Lias period. In the former case, therefore, fossils 
from remote districts would successively be introduced ; in the 
latter the area of deposit would become more and more 
restricted, and so the introduction of new forms more 
difficult. This seems at first entirely opposed to the fact 
that there are a good many characteristic fossils in the Rock- 
bed and Transition-bed. Two suggestions may be made to 
account for them. One is that these fossils were present in the 
Middle Lias before, though scarce, and that their large 
numerical increase was a result of restricted migration ; 
the relative abundance of species of Bracliiopods in the 
Rock-bed and Gasteropods in the Transition-bed being a 
result of altered though not necessarily unsuitable conditions. 
The second explanation I would suggest is that the land was 
sinking during the period marked by the deposition of the 
sandy micaceous beds of the “ Margaritatus" Zone, reaching 
its greatest, though never very great depth, when the 
impervious clays just below the Rock-bed were deposited, 
and that this was the time when the Bracliiopods and other 
fossils were introduced, after which the land commenced and 
continued to rise to near the end of the Middle Lias period. 
The similarity of the “ Margaritatus ” Zone to the lower part 
of the Upper Lias in the frequent alternation of hard and 
soft beds, and in the distribution of fossils in them, seems 
to favour this latter view. 
Of course if the theory of palaeontological zones being 
bounded by hard stone beds, which I have sketched out here, 
has any truth in it, the bed which I have placed at the base 
of the Middle Lias could scarcely be the commencement of a 
zone, though it might be the termination of one. I must 
confess the belief is growing with me that the “ Margaritatus ” 
Zone must include the clay bed immediately below “ L.” This 
would alter the boundary between the Middle and Lower 
Lias, as drawn by the Geological Survey, so I hesitate to 
make this change without some better reason than that of 
a theory only imperfectly worked out. 
There are certain well-marked palaeontological differences 
between the three zones of the Middle Lias we are concerned 
with. The “ Margaritatus ” Zone is characterised by Lamel- 
libranchs ; the “ Sjrinatus ” Zone by Bracliiopods ; and the 
“ Transition-bed ” by Gasteropods. These differences are 
much more evident to those practically acquainted with the 
