454 



PRINCIPLES OF STRATIGRAPHY 



common in the chalk, and diatoms, Radiolaria and sponge spicules 

 probably were as plentiful in the chalk as in the modern deposits, 

 but these siliceous structures have since been dissolved, and the sil- 

 ica redeposited as flint. Among the clastic admixtures quartz is also 

 common; indicating the neighborhood of land. 



The principal species of the chalk is Tcxtularia globulosa (Fig. 

 io6, a), which lives to-day in the estuary of the Dee near Chester, 

 but of course forms no extensive foramini feral deposits there. 

 (Murray-67:^;/.) 



In the island of Malta, Oligocenic limestones (Aquitanien) oc- 

 cur which are made up largely of pelagic Globigerina, and nearly 



Fig. 106. Preparation of wliilc chalk, showing the foraminifera, etc. (After 

 Zittel.) A— Chalk from Sussex, Eng. B— Chalk from Farafrah, 

 Lib3'an desert. Both x 60. a — Tcxtularia globulosa; b — Rotaiia 

 (Discorbina) marginata. C — dried residue of milky chalk water 

 with coccolilhs. x 700. 



40 per cent, of the species of this rock still live in the neighboring 

 Mediterranean. These beds further contain phosphate concretions 

 and green sands such as characterize the modern sea bottom in 

 depths from 500 to 2,000 meters, and also sharks' teeth similar to 

 those found by the Cliallcngcr in the greater oceanic depths. The 

 percentage of lime in the Globigerina deposits of Malta ranges 

 from 95 fo 98.6. These deposits appear to have been formed in 

 the greater depths of a mediterranean rather than in the open sea. 

 Similar foraminiferal deposits have been obtained from the Plio- 

 cenic of Sicily and Calabria. 



In this connection it must be noted that foraminiferal limestones 

 are not necessarily deep-sea or even submarine deposits, for the 



