Page:VCH Norfolk 1.djvu/39

 GEOLOGY they were formed around large sponges whose subsequent decay gave rise to the central cavity. Paramoudras or Pot-stones were first noticed by Dr. Buckland in the Chalk of Antrim, and the name is probably derived from an Erse word signifying ' sea pear.' In form many are smooth pear-shaped bodies from 8 inches to 2 feet and more in length, and from 8 inches to i foot or more in diameter. Others are of very irregular shape, with numerous flinty protuberances. All have a central cavity, and occasionally two cavities have been observed. They were noticed by Lyell in 1825 at Horstead, and they occur in many of the pits near Norwich. Noteworthy examples are also to be seen on the foreshore at Sheringham and Runton, where sometimes they are surrounded by rings of nodular flints.^ Tabular flints occur in the Chalk near SwafFham and Wells. At Swaffham some layers are eight or more feet in length, and from 9 to 12 inches thick. At Thetford the Chalk contains cylindrical flints which are very sonorous when struck. The lowest portion of the Upper Chalk at Swaffham, North and South Creake, and Burnham Overy, is characterized by Micraster coranguinum ; Echinoconus conicus is also found in it. The middle portion of the Upper Chalk, as at Wells, Walsingham, and perhaps at Diss, is characterized by Marsupites; while the upper portion, so well exposed near Norwich, is known as the Belemnitella Chalk, containing Belemnitella mucronata and B. lanceolata, also Pecten nitUus, P. concentricus, Terebratula carnea, Echinocorys vulgaris^ Cardiaster granulosus, and remains of a large reptile allied to Mosasaurus. On the Norfolk coast the Chalk which appears at Hunstanton does not again form cliffs until we reach Weybourne, where it rises to about 20 feet and is covered by the Crag and Glacial Drifts. It disappears below the sea-level east of Cromer, to reappear only for a short distance in the cliffs of Trimingham. The very highest Chalk observable in Norfolk is that at Trimingham, which has been disturbed by Glacial agency (see p. 19). It contains many porous spongiform flints, and one sandy seam a foot thick. Among the fossils are Belemnitella mucronata, which serves to link it with the Norwich Chalk ; also a large Gryphcea, Magas pumilus, and the true form of Terebratulina gracilis. The Chalk has been largely worked for lime-burning, for whiting, and for its flints near Norwich ; and often the pits have been extended underground in galleries of great length in order to avoid moving the superficial strata or ' uncallow.' Some of these workings, as on Heigham Hill, date back more than three hundred years. Chalk from Whitling- ham has been sent to Burgh Castle, near Yarmouth, where mixed with river-mud it has been manufactured into Portland cement. The black flints have been extensively used for building purposes, and notably for the inlaid flint-work so characteristic of church towers {Geol. Survey), 1882, p. 4. 9
 * See C. B. Rose, Proc. Geol. Assoc, vol. v. p. 514, and C. Reid, ' Geology of Cromer '