GEOLOGY cent, of carbonate of lime, from 8 to lo per cent, of peroxide of iron, and a little sand and clay. Whether the iron ore was original or derived from the underlying ferruginous carstone is a question difficult to answer. Tiny pebbles or grains of quartz are met with in the bed, which is but 4 feet thick. In its upper part it contains a branching structure, formerly supposed to be a fossil, and named Spongia paradoxica. This occurs also in the lowest part (' Sponge bed ') of the White Chalk. It has been shown by Prof. T. McK. Hughes to be simply a concretionary structure.* Fossils occur abundantly in the Red Chalk, as is the case elsewhere with formations that are represented in an attenuated condition. It was evidently formed slowly, and the zonal distinctions elsewhere met with in equivalent strata cannot be defined. Belemnites minimus and Terebratula biplicata are abundant. In a well-boring at Holkham a bed resembling Red Chalk was found beneath the White Chalk, and above a bed of Gault clay, and it is probable that in one form or another the Gault extends from its outcrop in West Norfolk eastwards beneath the greater portion of the county. CHALK The White Chalk undoubtedly forms the foundation of the main part of Norfolk. Its base, exposed in the cliff at Hunstanton, extends southwards along the borders of the Gault in the west of Norfolk, while the whole formation is inclined very gently (at an angle of less than i°) towards Norwich and Yarmouth, where on the coast its upper limit occurs about 500 feet below sea-level. Probably the Chalk attains its full local thickness where it is covered by Eocene strata at Yarmouth, and it has been estimated to be about 1,400 feet. At Norwich the Chalk was proved to a depth of 1,152 feet, and some further thickness is exposed in the adjacent hills. The Chalk is divided into Lower, Middle, and Upper Chalk, but the scenery produced by the formation in Norfolk differs markedly from that in the southern and southern-midland counties. We find no conspicuous escarpments nor downs, because the Chalk has been abraded and covered by Glacial Drifts, which greatly modify its features. The Lower Chalk in Norfolk is for the most part a hard white lime- stone with the ' Sponge-bed ' before noticed at its base, and it exhibits in places the curved jointing so conspicuous in the Cambridge clunch. It is a comparatively pure chalk with 95 per cent, of carbonate of lime, but it contains occasional seams of marl. It is characterized by Ammonites varians. Overlying the Lower Chalk is a representative of the Totternhoe Stone — a sandy and glauconitic chalk with green-coated phosphatic nodules, from 2 to 4 feet thick, and yielding Holaster subglobosus. This bed was first observed near Stoke Ferry and Roydon by Mr. William Whitaker. Above it is a hard grey chalk 35 to 40 feet thick, char-
- Quart. Journ. Geol. Soc, vol. xl. p. 273.
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