MIDFORD SAND. 39 
MIDFORD SAND. (PASSAGE BEDS.) 
Zones of Ammonites jurensis and A. opalinus. 
The zones of A. opalinus (" Opalinnsthone ") and A. jurensis (" Jurensis- 
mergel ") were notified by Quenstedt in 1843 ; and when Oppel instituted 
his comparisons between the Jurassic rocks of this country and the 
Continent, he subdivided the beds as follows* : 
("Zone of Trigonia na-vis^i with A, opalinus 
Inferior I and 
Oolite. | Zone of Ammonites f Rliynclwnella 1 Gloucestershire 
L torulosus. J cynocephala. > Cephalopoda 
Upper Lias. Zone of A. jurensis. J Bed. 
The names applied to the upper two zones are not applicable to this 
country, as Trigonia navis is not present, nor is it a good zonal species, 
and A. torulosus is not sufficiently abundant : but the beds are conveniently 
grouped under the general name, zone of A. opalinus. 
The lowest zone of the Inferior Oolite is usually regarded as 
that of A, opalinus, while the uppermost zone of the Upper Lias 
is taken to be that of A. jurensis. 
Over large areas of the Midland counties the zone of A jurensis 
does not appear to be represented, although here and there, as at 
Northampton, it is partially developed. There, however, we find 
no difficulty in determining the respective stratigraphical limits 
of the Upper Lias clay and Inferior Oolite Series. 
In Gloucestershire, Somersetshire, and Dorsetshire the case is 
different : over considerable portions of that area wfi have a 
complete passage from Lias to Oolite, and the only possible 
stratigraphical divisions that can be made, consist in grouping 
together go far as possible, the main ma?s of sandy stratn, and the 
limestones locally associated with them, under the term Midford 
Sand, or Midford Beds, and separating them from the main 
mass of Upper Lias clay on the one hand, and from the Inferior 
Oolite on. the other. 
It is of course considered desirable by palaeontologists that the 
fossils of the zone of A. opalinus be separated from those of the 
zone of A. jurensis, because the one zone is considered Inferior 
Oolite and the other Lias. Where to make the desired separation 
between these zones in the absence of lithoiogical and strati- 
graphical guides is difficult enough: in the absence of fossils of 
course it is hopeless. Even at best the two zones, in the west 
of England, are so intimately connected by forms common to 
both, that a hard-and-fast line is out of the question. The field- 
geologist moreover has to deal with strata that in many places 
yield no fossils. 
On local palseontological grounds the Gloucestershire Cepha- 
lopoda Bed was split in two portions (see p. 104), and the name 
Cephalopoda Bed is by some authorities restricted to the lower 
part, so as to place it in the zone of A. jurensis. On strati- 
* Die Juraformation (1856-58), pp. 305, 321, and Table at end of volume. 
