Ch. XYIL] chalk flixts, how formed. 321 



structure, as Mr. Dana has well observed, helps us to understand the 

 imperfect aggregation of that remarkable rock. At the same time 

 the continued growth of these Rhizopods over a wide exteut of deep 

 ocean enables us to conceive how formerly in European areas a vast 

 thickness of cretaceous limestone, very uniform in composition, and 

 devoid of sand, pebbles, terrestrial and freshwater plants and shells, 

 and all other signs of a neighboring continent, may have been formed, 

 That white chalk is now forming in the depths of the ocean, may now 

 be regarded as an ascertained fact, because the Globigerina bulloides 

 is specifically undistinguishable from a fossil which constitutes a large 

 portion of the chalk of Europe. It is not figured (p. 26) among the 

 cretaceous foraminifera discovered by Mr. Lonsdale in 1835, because it 

 occurs for the most part in fragments in the white chalk, and the 

 perfect shell was not well understood before it was obtained living from 

 the bed of the Atlantic. The Rosalina figured in the same page some- 

 what resembles externally a Globigerina, but it differs in the arrange- 

 ment of its cells. 



Chalk Flints. — The origin of the layers of flint, whether in con- 

 tinuous sheets or in the form of nodules, has always been found more 

 difficult to account for than that of the white chalk. In modern 

 coral reefs no such siliceous masses are known to be forming. But 

 here again the late deep-sea soundings have suggested a very proba- 

 ble source of such mineral matter. During the cruise of the " Bull- 

 dog," already alluded to, it was ascertained that while the calcareous 

 Globigerinm had almost exclusive possession of certain tracts of the 

 sea-bottom, they were wholly wanting in others, as between Green- 

 land and Labrador. Dr. "Wallich supposes that they flourished in 

 those spaces where they may derive nutriment from organic and 

 other matter, brought from the south by the warm waters of the 

 Gulf Stream, and that they may be absent where the effects of that 

 great current are not felt. In several of the spaces where the calcare- 

 ous Rhizopods are wanting, the microscopic plants called Diatomacece, 

 before mentioned (figs. 282, 284), the solid parts of which are siliceous, 

 monopolize the ground at a depth of nearly 400 fathoms, or 2400 feet. 



Mr. Dana also has reminded us that in the soundings made in the 

 Sea of Kamtschatka Professor Bailey found the same microscopic 

 vegetable organisms in as great profusion as are the Globigerinse in 

 the Atlantic, and he adds that when such Diatomacese decompose, 

 the alkaline waters of the ocean can take up and hold in solution 

 only a minute portion of the silica set free, so that an opportunity 

 would be given for the remainder to form concretionary nodules, or 

 to aggregate round any foreign body as a nucleus, especially when 

 such a body is undergoing chemical change or decomposition. This 

 would explain the frequent occurrence of fossils within nodules of flint, 

 and the silicification of various organisms.* In some parts of the 



* Dana's Geology, p. 489. 

 21 



