A HISTORY OF BEDFORDSHIRE 
with silt. The upper portion has a yellowish tinge, it is less fossili- 
ferous than the lower, and its nodules are not so easily distinguished from 
the surrounding matrix. 
The coarse gritty texture of the Melbourn Rock, to which the term 
‘rag’ is applied by the workmen, is mainly due to the presence of the 
remains of G/obigerina and other Foraminifera, and of the triturated tests 
of marine Mollusca. The most abundant fossil in this district is Ostrea 
vesicularis var. baylei; other less common forms are Rhynchonella cuviert, 
Plicatula inflata, and Actinocamax plenus (Belemnitella plena). 
The central division of the Middle Chalk, comprising the zones of 
Terebratulina gracilis (or Jata, as it is now called) and Ho/aster planus, 
consists of a soft white limestone about 200 feet in thickness, which is 
much more homogeneous in texture than the Melbourn Rock. The 
lines of deposition are occasionally indicated by thin seams of grey marl 
which persist over a considerable area. When exposed on the face of a 
section these seams are seen to be broken at intervals by slight faults 
varying from a few inches to 2 or 3 feet indepth. Examples are present 
in the cuttings for the Midland Railway south-east of Luton. In the 
cutting at the twenty-ninth mile from London there are at least half a 
dozen faults in 200 yards, appearing like a series of steps, and there are 
others which are obscured by vegetation. In the next cutting south- 
eastwardly there isa conspicuous fault about 1o yards in length and with 
a downthrow of 2 or 3 feet. These faults appear to have been pro- 
duced by the upheaval of the Chalk and its shrinkage from desiccation. 
The sea in which this portion of the Chalk series was formed 
seems to have been deeper than that in which the Lower Chalk was laid 
down, though not so profound as the ocean in which the G/obigerina-ooze 
of the present day is being deposited. 
The fossils in this division are both numerous and _ interesting, 
including the typical species Terebratulina lata and Holaster planus, well- 
preserved examples of Terebratula semiglobosa, Spondylus spinosus, and Gal- 
erites albo-galerus, and occasionally the teeth of Lamna, Corax, and 
Ptychodus. Broken fragments of Inocerami are abundant. Other less 
common forms are the curious Hippurites mortoni, and aptychi, the man- 
dibles of belemnites, three specimens of which have been found in the 
neighbourhood of Luton and are now in the Blackmore Museum, 
Salisbury. 
This portion ot the Middle Chalk occupies a considerable surface 
over the south of the county, and is largely devoted to arable agriculture. 
The hard seams of chalk above and below it (the Chalk Rock and 
Melbourn Rock) have been important factors in the production of the 
existing outlines of the Chalk escarpment, and by their resistance to 
erosion they have greatly assisted in the formation of the ‘lynchets’ 
which are not uncommon over this area. 
The uniformity in texture of the great mass of the Middle Chalk, 
and the slight changes in the general features of its fauna, suggest that 
subsidence and deposition nearly balanced each other, so that an almost 
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