186 LIAS OF ENGLAND AND WALES : 
but a thin band. Some of the layers at the eastern end of the 
Mendip range, are conglomeratic, and contain pebbles of Car- 
boniferous Limestone. Thence to Bath the Rock Bed is probably 
represented by soft beds. In Gloucestershire it is well developed 
near Wotton-under-Edge and Dursley, but near Stroud it is 
insignificant as a Rock Bed. Eastwards at Bloxham, Adderbury, 
and King's Sutton near Banbury, it becomes a well-marked 
formation, and has been extensively worked as an iron-ore ; while 
further north at Edge Hill it furnishes an important building- 
stone. Near Market Harborough the stone-beds are again feebly 
represented, but near Melton Mowbray and onwards to Grantham 
there are further valuable beds of ironstone. Northwards in 
Lincolnshire the stone-beds are but occasionally developed to 
any prominent degree. 
The Middle Lias has been divided into two zones, as 
follows : 
"Marlstone of variable thick- 
ness. 
Ammonites spinatus (Fig. 54) I ,,. . , . 
I Micaceous sands with in- 
durated bands. (Local.) 
f Laminated sands and clays 
/- ee\\ with indurated bands. 
Ammonites margaritatus (rig. 55 )< rn -,i 
v 6 ' | Blue micaceous clays with 
[_ nodules of limestone. 
Ammonites margaritatus is not confined to the lower beds, but 
some of the finest examples occur in the same beds with A. spinatus 
in many localities. North of Cheltenham, as remarked by Dr. 
Wright, it is difficult to separate the beds belonging to the zones of 
Ammonites spinatus and A. margaritatus.* 
Fossils are usually to be met with in abundance in the 
Marlstone. The sands as a rule are not fossili ferous. The 
laminated beds are rarely exposed inland, but the blue clays are 
used for brick-making, and usually yield small specimens or 
fragments of A. margaritatus. 
The thin bed of Marlstone exhibited on the Dorset coast 
includes a layer which has yielded, among other fossils, a great 
many Gasteropoda, and this has been termed the Pleurotomaria- 
bed ; but the Rock Bed there represents only a portion of the 
Marlstone of other localities. 
North-east of Banbury there is a remarkably rich bed on top 
of the Marlstone ; it yields in abundance Ammonites acutus, and 
has been termed the zone of that fossil. The term " Transition- 
bed " has also been applied, for in some respects it presents 
evidences in its fauna of a passage between the Middle and Upper 
Lias. No doubt it is approximately on the same horizon as the 
Pleurotomaria-bed of Dorsetshire, as pointed out by Mr. E. A. 
Walford and Mr. Beeby Thompson ; and they have suggested, 
with good reason, that it may represent the zone of Ammonites 
annulatus, which forms a distinct horizon at the base of the Upper 
Lias of Yorkshire. 
* Wright, Lias Ammonites (Palseontograph. Soc.), p. 94. 
