FORMATION OF CHALK. . 15 



remains. Indeed, it is held that some of the forms of life found 

 in the Chalk must have drifted from shallower waters. Under 

 the microscope foraminifera (Globigerina, etc.) ostracods, frag- 

 ments of corals, crinoids, echinoids, bryozoa, brachiopods and 

 molluscs, of a calcareous nature, may sometimes be detected, 

 together with other organisms having siliceous structures such 

 as certain sponges, diatoms and radiolaria (Polycystina). There 

 are also minute oval bodies called Coccoliths, some of which 

 appear to be organic, and there is much amorphous material 

 that may be in part detrital, while portions may have been 

 voided by fishes and other animals. The calcareous constituents 

 formed a kind of ooze of fairly uniform composition over wide 

 areas, akin to accumulations now taking place under the deep 

 water of the Atlantic. In reference to this it is interesting to 

 notice that in 1863 S. P. Woodward described a new and 

 anomalous genus of echinoid from the Chalk of Kent under the 

 name Echinothuria floris 1 one example was obtained from 

 Charlton. He then remarked that ' it seems to indicate the 

 former existence of a family or tribe of creatures whose full 

 history must ever remain unknown.' Ten years later C. Wyville 

 Thomson in his ' Depths of the Sea ' founded the family Echino- 

 thuridae to contain the original genus, together with two recent 

 forms brought to light by the deep-sea dredgings in the Atlantic. 



The nodules which occur at several horizons in the Chalk 

 may have been formed and partially rolled in tracts where the 

 deposit had to some extent been consolidated, and they appear 

 to indicate shallower conditions than the mass of the Chalk. 2 



In 1857 Godwin -Austen obtained from the Chalk at Haling 

 pit, near Croydon, a waterworn boulder of granite and pebbles 

 of other rocks, which in his opinion had been transported from 

 a beach by a berg derived from coast-ice. Other erratic stones 

 have since been found in the Chalk, and the problem of their 

 origin may be variously explained. Drifted tree -trunks with 

 gravel adhering to the roots may have been one cause, and it 

 was suggested by H. G. Seeley 3 that pebbles may have been 

 transported by some of the larger fishes and saurians, whose 

 habits of swallowing stones are now well known. 



FLINT. 



The origin of the nodules and bands of flint has ever been a 

 source of wonder and curiosity. The nodules and some of the 

 more regular bands are due to the concentration or segregation 

 of siliceous matter from the soft calcareous ooze from which the 

 bulk of the Chalk was formed. The siliceous matter in the 



1 Named after J. Wickham Flower, of Croydon ; Geologist, vol. vi, 1863, 

 p. 327. 



2 Jukes -Browne, A. J., 'Cretaceous Rocks of Britain' (Mem. Geol. Surv.), 

 vol. iii, 1904, pp. 361, etc. 



3 * Handbook of the London Geological Field Class,' 1891, p. 91. See also 

 Stebbing, W. P. D., Quart. Journ. Geol. Soc., vol. liii, 1897, p. 213. 



