| Middle Chalk. |  | Terebratulina lata. Inoceramus labiatus. |
| Lower Chalk. | | Holaster subglobosus | | Actinocamax plenus (at top). | |
| Schloenbachia varians | | Stauronema carteri (at base). |
The method of study according to zoological zones is of great interest. The period of the White Chalk was of long duration, and the physical conditions remained very uniform. So that by studying the succession of life during this period we may learn much about the gradual change of life on the earth, and the evolution of living things.
We have seen that the whole mass of the chalk is made up mainly of the remains of living things,—mostly of the microscopic foraminifera. We have seen that sponges were very plentiful in that ancient sea. Of other fossils we find brachiopods—different species of Terebratula and Rhynchonella—a large bivalve Inoceramus sometimes very common; the very beautiful bivalve, Spondylus spinosus, belemnites, serpulæ; and different species of sea-urchin are very common. A pretty heart-shaped one, Micraster cor-anguinum, marks a zone of the higher chalk, which runs along the top of our northern downs. Other common sea urchins are various species of Cidaris, of a form like a turban (Gk. cidaris, a Persian head-dress); Cyphosoma, another circular form; the oval Echinocorys scutatus, which, with varieties of the same and allied species, abounds in the Upper Chalk, and the more conical Conulus conicus. The topmost zone, that of B. Macronata, would yield a record of exuberant life, were the chalk soft and horizontal. There was a rich development of echinoderms (sea urchins and star fishes), but nothing is perfect, owing to the hardness of the rock (Dr. Rowe). The general difference in the life of the Chalk period is the great development of Ammonites and other Cephalopods in the Lower Chalk, and of sea urchins and other echinoderms in the Upper, while the Middle Chalk is wanting in the one and the other. Shark's teeth tell of the larger inhabitants of the ocean that flowed above the chalky bottom.
Many quarries have been opened on the flanks of the Chalk Downs, of which a large number are now disused. They occur just where they are needed for chalk to lay on the land, the pure chalk on the north of the Downs to break up the heavy Tertiary clays, which largely cover the north of the Island; the more clayey beds of the Grey Chalk on the south of the downs to stiffen the light loams of the Greensand.[10]
[8] See Common Stones, by Grenville A. J. Cole, F.R.S. 1921.
[9] 1,472 ft. at the western end of the Island, 1,213 ft. at the eastern.—Dr. Rowe's measurements.
[10] Dr. A. W. Rowe.
Chapter VIII