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 GEOLOGY uniform depth of water was maintained during its formation. Flint nodules become numerous in its higher part, indicating an increase in the num- ber of silex-secreting organisms living in the seas of the period. The Chalk Rock consists in this district of two bands of indurated limestone, each about 2 feet thick, separated by about 10 feet of soft white chalk known as the Micraster-bed. The ' rock ' itself, so termed from its hardness, is cream-coloured, sometimes with a greenish tinge owing to the many glauconitic grains which it contains. It is often sub-crystalline, sometimes shows a conchoidal fracture, and in places is so hard that when struck with a hammer it has a metallic ring. The hardest seams are nodular, the nodules being slightly phosphatic, very irregular in shape, and cream-coloured with a green coating. The fossils of the Chalk Rock are numerous and varied, consisting chiefly of Foraminifera, sponges, echinoderms, and Mollusca. The delicate microscopic tests of the Foraminifera are often in fragments, and the former existence of branched sponges is sometimes indicated by tubular cavities occasionally containing a powdery substance in which sponge-spicules are present. The state of preservation of the remains of the Mollusca varies considerably. As a rule the shells of the gastro- pods and cephalopods have perished, casts of both the interior and ex- terior being present, the latter exhibiting the ornamentation, which can be reproduced by means of a wax impression. The lamellibranchs usually have their shells well preserved, as is also the case with the tests of the echinoderms and brachiopods, but they are difficult to extract owing to the hardness of the matrix. The general features of the organic remains indicate a marked difference in the conditions under which they lived from those which existed when the softer beds of the Middle Chalk were deposited. The prevailing species, especially of the gastropods, prove that at this period the sea-floor was not deeper than from 150 to 200 fathoms, this being the extreme depth at which their modern representatives are found. Taking all the observed facts into consideration, it is probable that the Chalk Marl and Chalk Rock, which are separated by 400 feet of Creta- ceous beds, were deposited in seas the depths of which were very similar. A few species are common to both horizons, 1 indicating that, although absent from this area, they maintained their existence during the interval in some other locality. The Chalk Rock appears to form the summit of the escarpment of the Chalk along the Dunstable Downs as seen from the plain below, but the higher ground behind is on the Upper Chalk. It may be traced westwards along the ridge from the Five Knolls, and sections of it are exposed in a pit near the top of the downs. Sections may also be seen on both sides of the Luton valley near the 500-feet contour line. Owing to its extreme hardness it is affected much more slowly by denudation than the associated softer beds, so that its presence has largely determined 1 See H. Woods, £>uart. Journ. Geo/. Soc. liii. 396.