+3 
UMM. Wilson and Crick—The Lias Marlstone of Tilton. 297 
The Marlstone Rock in the Tilton railway-cutting, lying beneath 
a thick capping of Upper Lias Shales, is a hard massively-bedded 
grey ferruginous limestone, in appearance something like the Marl- 
stone Ironstone of the Cleveland district of Yorkshire, but more finely- 
grained and duller looking, and containing also a much less per- 
centage of iron. The rock is traversed by numerous joints, and 
along these and also along the bedding-planes to the depth of a few 
inches on either side the carbonate of iron has been changed, under 
the influence of percolating waters, into the hydrated ferric oxide. 
Coincidently with this chemical change the greenish-grey stone has 
assumed a rusty-brown colour, so that, when first opened out, the 
face of the rock presented a very prettily checked appearance. A 
few years’ exposure to the atmosphere, however, has gone far 
towards toning down this variegation and covering the whole face 
of the Marlstone Rock with a uniform dull brown tint. 
So far as the Leicestershire area is concerned, the ‘Transition 
Bed,” at the top of the Marlstone Rock, appears to be confined to 
this single locality. This bed is remarkable for the numerous and 
varied organisms which it contains. Of these Harpoceras acutum is 
especially abundant, and characteristic of this horizon. Several 
Ammonites usually confined to the Upper Lias are here found asso- 
ciated with other forms characteristic of the Middle Lias, so that 
palzontologically the “ Transition Bed” must be considered as really 
transitional between those two series, notwithstanding that it pos- 
sesses the mineral characters of, and is welded to the Marlstone 
Rock. We have not therefore hesitated to apply to this bed the 
name given by Mr. H. A. Walford! and Mr. B. Thompson’ to a 
similar bed which lies at the top of the Marlstone Rock in Oxford 
and Northamptonshire. It is mainly, if not solely, from this thin 
stratum that the Ammonites, as well as the Gasteropods, which are 
so numerous at Tilton, have been obtained. The “Transition Bed ” 
may be examined in situ on a narrow ledge which projects a little 
from beneath the Upper Lias shales on the west side of the railway- 
cutting, and the best way to work it is to turn over and break up the 
slabs with a pick in wet weather, when the stone is softer and works 
much more readily. A small dip—about 1° §.E.—carries this bed 
and the Marlstone Rock beneath the line towards the south end of 
the cutting. 
The material taken from the Tilton cutting has been used to 
construct the embankment at East Norton, about three miles to the 
south. Here blocks of the Tilton Marlstone lie about in great num- 
bers. Under the action of the weather during the ten or twelve 
years which have elapsed since the line was made, the hard grey 
Marlstone Rock has been changed superficially into a comparatively 
soft brownish arenaceous ironstone, a change of the same kind, if 
not carried to the same degree, as that which, in the course of ages, 
1 “On some Middle and Upper Lias Beds in the Neighbourhood of Banbury,” by 
Edwin A. Walford, F.G.S., Proc. Warwick Nat. and Arch. Field Club, 1878. 
2 “Notes on Local Geology,”’ by B. Thompson, F.G.S., part x. ‘* The Junction 
Beds of the Middle and Upper Lias,” Journal Northants. Nat. Hist. Soc. vol. ui, 
p. 239, 1883. 
