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 Globigerina and other Foraminifera, and of the triturated tests of marine Mollusca. The most abundant fossil in this district is Ostrea vesicularis var. bay lei; other less common forms are Rbynchonella cuvieri, Plicatula injiata, and Actinocamax plenus (Belemnitella plena). The central division of the Middle Chalk, comprising the zones of Terebratulina gracilis (or lata, as it is now called) and Holaster 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 in depth. 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 is a conspicuous fault about 10 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 Globigerina-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 or 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 20
