Ch. XVII.] ANIMAL OEIGIN" OF WHITE CHALK. 241 



mollusks, echinoderms, and crustaceans, but also, as Mr. Darwin observed 

 upon studying the coral islands of the Pacific, from the faecal matter 

 ejected by echinoderms, conchs, and coral-eating fish. In the West 

 Indian seas, the conch (Strombus gigas) adds largely to the chalky mud 

 by means of its faecal pellets, composed of minute grains of soft calca- 

 reous 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 Hying corals, like grazing herds of graminivor- 

 ous quadrupeds. On opening their bodies, 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 certain bodies, 

 called " larch-cones," which were afterwards rec- 

 ognized by Dr. Buckland to be the excrement of 

 fish. Such spiral coprolites (fig. 251), like the 



^ ^ ^ n « ., n i . .1 in Coprolites offish, called Iulo 



scales and bones ot fossil nsh m the chalk, are eido-copri, from the chalk. 

 composed chiefly of phosphate of lime. 



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

 many others, feed on shell-fish, or on corals. 



The mud derived from the sources above mentioned may be actually 

 seen in the Maldiva Atolls to be washed out of the lagoons through nar- 

 row 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 moderate pressure 

 to resemble it 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 aggre- 

 gated shells, imbedded in a compact calcareous base as firm in texture as 

 any secondary limestone ; while others are like chalk, having its color, 

 its earthy fracture, its soft homogeneous texture, and being an equally good 

 writing material. The same author describes, in many growing coral 

 reefs, a similar formation of modern chalk, undistinguishable from the 

 ancient.f The extension, over a wide submarine area, of the calcareous 

 matrix of the chalk, as well as of the imbedded fossils, would take place 

 more readily in consequence of the low specific gravity of the shells of 

 mollusca and zoophytes, when compared with ordinary sand and mineral 

 matter. The mud also derived from their decomposition would be much 

 lighter than argillaceous and inorganic mud, and very easily transported 

 by currents, especially in salt water. 



Single pebbles in chalk. — The general absence of sand and pebbles in 

 the white chalk has been already mentioned ; but the occurrence here 

 and there, in the southeast of England, of a few isolated pebbles of 



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

 p. 200. 



•J- GeoL of U. B. Exploring Exped. p. 252, 1849. 



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