Ch. xyii.] animal origin of white chalk. 319 



quite vague and visionary, until its probability was strengthened by 

 new evidence brought to light by modern geologists. 



We learn from Captain Nelson tbat in the Bermuda Islands, and in 

 the Bahamas, there are many basins or lagoons almost surrounded and 

 enclosed by reefs of coral. At the bottom of these lagoons a soft 

 white calcareous mud is formed, not merely from the comminution of 

 corallines (or calcareous plants) and corals, together with the exuvias 

 of foraminifera, niollusks, echinoderms, and crustaceans, but also, as 

 Mr. Darwin observed upon studying the coral islands of the Pacific, 

 from the fsecal matter ejected by echinoderms, conchs, and coral-eat- 

 ing fish. In the West Indian seas, the conch (Stromhus gigas) adds 

 largely to the chalky mud by means of its faecal pellets composed of 

 minute grains of soft calcareous matter, exhibiting some organic tissue. 

 Mr. Darwin describes gregarious fishes of the genus Scarus, seen 

 through the clear waters of the coral regions of the Pacific browsing 

 quietly in great numbers on living corals, like grazing herbs of gram- 

 inivorous quadrupeds. On opening their bod- 

 ies, their intestines were found to be filled with 

 impure chalk. This circumstance is the more in 

 point when we recollect how the fossilist was 

 formerly puzzled by meeting, in chalk, with cer- 

 tain bodies, called " larchcones," which were 

 afterwards recognized by Dr. Buckland to be 

 the excrement of fish. Such spiral coprolites 

 (fig. 280), like the scales and bones of fossil fish 

 in the chalk, are composed chiefly of phosphate Coprolites of fish, from 

 of lime. . 



In the Bahamas, the angel-fish, and the unicorn or trumpet-fish, and 

 many others, feed on shellfish, or on corals. 



The mud derived from the sources above mentioned may be actu- 

 ally seen in the Maldiva Atolls to be washed out of the lagoons 

 through narrow openings leading from the lagoon to the ocean, and 

 the waters of the sea are discolored by it for some distance. When 

 dried, this mud is very like common chalk, and might probably be 

 made by a modern pressure to resemble it still more closely.* 



Mr. Dana, when describing the elevated coral reef of Oahu, in the 

 Sandwich Islands, says that some varieties of the rock consist of 

 aggregated shells, imbedded in a compact calcareous base as firm in 

 texture as any secondary limestone ; while others are like chalk, hav- 

 ing its color, its earthy fracture, its soft homogeneous texture, and be- 

 ing an equally good writing material. The same author describes, in 

 several growing coral reefs, a similar formation of modern chalk, un- 

 distinguishable from the ancient. f The extension over a wide sub- 



* See Nelson, Geol. Trans., 1837, vol. v. p. 108 ; and Geol. Quart. Journ., 1853, 

 p. 200. 



f Geol. of U. S. Exploring Exped., p. 252. 1849. 



