232 LIAS OF ENGLAND AND WALES: 
The local attenuation of the Middle Lias in this area is borne 
out by the observations of Prof. Judd, who in describing the beds 
in Rutlandshire and bordering tracts, remarks that while 150 feet 
thick in the northern part of that area, they are reduced to less 
than half that amount in the southern part.* 
The divisions observed by Prof. Judd in Rutlandshire and 
adjoining tracts, may be summarized as followsf : 
5. Marlstone Rock-Bed. Limestone, more or le&s ferruginous, and 
passing in places into a good ironstone. The rock is often crowded with 
fossils, its mass being made up of fragments of Crinoids, spines of Eehino- 
derms, Serpulao, and fragments of shells, while certain beds in it (locally 
known to quarrymen as "jacks") consist of an agglomeration of shells of 
Rhynchonella tetrahedra and Terebratula punctata, usually filled with finely 
crystallized calcspar. Bdemnites paxillosus and B. elongatus are extremely 
abundant in the Rock-bed, and serve to distinguish it from the Northampton 
Sand, which often resembles it in mineralogical characters, but in which 
Belemnites are exceedingly rare. Ammonites are not abundant in the Rock- 
bed in this district, but at some points, as Edmondthorpe, Loddington and 
Horninghold, Ammonites communis, and A. annulatus occur in considerable 
numbers; A. spinatus, and some varieties of A. maryaritatus, are also found in 
it, but much more rarely, in this district. Large specimens of Pecten 
asquwalvis, with the highly-characteristic P. dentatus, also Hinnites abjectus, 
and Avicula inaquivalvis, are among the most abundant forms in the 
Rock-bed. 
The Rock-bed is very variable both in thickness and mineralogical character ; 
it is finely developed in the neighbourhoods of Tilton-on-the-Hill and 
Somerby, near the former of which places, it is seen to measure 18 feet 6 inches 
in thickness ; towards the east and south, however, it attenuates very rapidly, 
being only 8 or 9 feet thick about Oakham, 2| feet at Alexton, 2 feet at 
Godeby and at Horninghold, and less than 1 foot between Keythorpe and 
Hallaton. Besides being greatly diminished in thickness, the bed sometimes 
loses its calcareous character and becomes sandy, in these cases often resemb- 
ling the other hard beds which occur lower in the Middle Lias. When the 
junction of the Upper Lias clay and the Rock-bed is seen, the latter often 
presents the appearance of having suffered erosion before the deposition of the 
Jormer. Insignificant, however, as the Rock -bed often becomes, there is 
no certain evidence of its actual disappearance within the area, but in places, 
its presence being doubtful, it is indicated on the map by broken lines only. 
4. Light blue clays, with bands of ironstone-balls of concentric structure, 
and usually very unfossiliferous. These beds are exposed in some brickyards 
about Oakham, at Langham, and at Market Harborough. At some places 
they contain beds of green and brown sand, as near Horninghold. 
3. Beds of blue clay with septaria, the latter not unfrequently containing 
Specular Iron, and weathering to a red colour. They contain many of the 
fossils recorded from the preceding beds, but less abundantly. They are 
exposed in Belton, Hallaton, and Cranhoe brickyards. 
2. Beds of blue, highly micaceous clay, with large septaria crowded with 
fossils. There were two brickyards in these beds at Ouston, and between 
Whissendine and Pick well. The most abundant species in these beds are 
Ammonites margaritat us (the large typical form), Belemnites elongatus, Cryptania 
(Helicina) expansa, Avicula incequivalvis, Hippopodium ponderosum, Modiola 
scalprum (very abundant), Cardium truncatum, Pleuromya costata, and 
Pentacrinus subangularis, 
1. Soft, yellowish-brown, sandy and micaceous ironstone, crowded with 
casts of shells, and alternating with light blue clays. These ferruginous bands 
vary very greatly in number and thickness, and are sometimes nodular. They 
* Geol. Ktttland, p. 64. 
f Geol. Rutland, pp. 64-66. 
