The Mollusca of the Chalk Rock: Part I.

Abstract

The Chalk Rock is a bed of hard, usually cream–coloured limestone, occurring at the top of the Middle Chalk; it succeeds the zone of Holaster planus and is followed by the zone of Micraster cor–testudinarium . Noticeable features in the bed are the presence of grains of glauconite and numerous green–coated nodules of phosphatic chalk. Under the microscope it is characterized by the abundance of foraminifera, especially Globigerina , and by numerous fragments of the tests of mollusks and echinoids; the bright green grains of glauconite are also conspicuous. The insoluble residue, obtained by treating with hydrochloric acid, was found by Dr. W. F. Hume to consist mainly of quartz and glauconite; augite, hornblende, and tourmaline occur very rarely. Frequently the Chalk Rock is not hard throughout, but contains irregular cavities filled with soft mealy chalk or with red sandy clay. Analyses of the rock have been made by M. Duvillier, Dr. Frankland, Mr. C. W. Meanwell and Mr. P. G. Sandford, and they show from ·20 to ·713 per cent. of phosphoric acid—a large amount compared with that usually found in chalk. The best development of the Chalk Rock occurs in Berkshire, Oxfordshire, Buckinghamshire, and Bedfordshire; its average thickness is about 3 feet. In the Isle of Wight it varies from 8 inches to 2 feet 7 inches. In Dorset and Devon it consists of a layer of green–coated nodules only.Near Winchester (at Twyford) the zone can be traced by means of the characteristics fossils, but the rock is not hard