472 



MESOZOIC EKA— AGE OF REPTILES. 



Origin of Chalk. — A material so unique must have been formed 

 under peculiar conditions. Eecent investigations have shown that 

 chalk is a deep-sea ooze. In all the deep-sea soundings and dredgings 

 recently undertaken, it is found that the sea-bottom between the depths 

 of 3,000 and 20,000 feet, where not too cold, is a white ooze, consisting 

 mainly of Ehizopod shells (Globigerina, Eadiolaria, etc.) and Coccoliths, 

 Coccsopheres, etc., through which are scattered silicious shells of Dia- 

 toms. These shells are in every stage of change : some living, or at 

 least still retaining sarcode ; some perfect, though dead and empty ; 

 some broken ; most of them completely disintegrated into an impalpa- 

 ble mud. From the great abundance of one genus of Ehizopods, this 

 calcareous mud has been called Globigerina ooze. In deep-sea bottoms, 

 therefore, chalk is now forming. Also, strange to say, many Sponges, 

 and Starfishes, and Echinoids, and Crustaceans, very similar to those 

 found in the chalk of Cretaceous times ; have been brought up from 

 present deep-sea bottoms. 



Fig. 762.— Shells of Living Foraminifera: a, Orbulina universa, in its perfect condition, showing the 

 tubular spines which radiate from the surface of the shell; b, Globigerina bulloidefl, in its ordi- 

 nary condition, the thin hollow spines which are attached to the shell when perfect having been 

 broken off; c, Textularia variabilis; d, Peneroplis planatns; e, Rotalia concamerata; /, Cristel- 

 laria subarcuatula. (Fig. a is after Wyville Thomson; the others are after Williamson. All the 

 figures are greatly enlarged.) 



There seems little doubt, therefore, that chalk is a deep sea-bottom 

 formation. The flint- nodules have been formed by a subsequent process 

 similar to that which gives rise to other nodules (p. 188). The silica, 

 which in the ooze was at first scattered, is slowly aggregated into pure 



