46o 



PRINCIPLES OF STRATIGRAPHY 



elusion that these rocks are of shallow-water or lagoon origin, 

 (Dixon and Vaughan-25 :5='i'.) 



The Onondaga coral reefs of western New York are succeeded 

 and sometimes replaced by very cherty limestones, and it is not im- 

 possible that the chert layers in the so-called Corniferous limestone 

 represent shallow-water radiolarites accumulated in the lagoon be- 

 hind the coral reefs. 



Recent Diatomaceous Ooce. (Fig. 109.) This is a phytogenic 

 deposit, but is best considered in this connection. It is most abundant 

 in the ocean in the higher latitudes of both hemispheres, and origi- 

 nates from the phytoplankton of the colder and less saline waters 

 of the polar regions. The diatom frustules of the ooze obtained in 

 the southern hemisphere belong to a number of genera (Navicula, 

 Coscinodiscus, Fragillaria, Synedra, Asteromphalus, Rhizosolenia, 



Fig. 109. Diatom ooze, x 200. (After C. Chun.) 1-5. Coscinodiscus; 6 

 Asteromphalus ; 7. Fragilaria antarctica ; S, 9. Synedra ; 10. Rhiz- 

 osolenia; II. Chaetoceras ; 12. Navicula (?) ; 13, 14. Dictyochi and 

 Radiolaria. 



etc.), and associated with them are fragmentary frustules, Radio- 

 laria, sponge spicules, lime particles, etc. 



These sediments form a circumpolar belt in the southern hemi- 

 sphere, and likewise form a band across the northern Pacific. The 

 southern area comprises nearly 22 million square kilometers. In the 

 tropical regions diatomaceous sediments also occur. They have 

 been found to constitute a veritable tripolite or infusorial earth at 

 depths from 2,700 to 5,200 meters under the Peru stream between 

 Callao and Galapagos. Between the Marian and Philippine Islands 

 the floor of the Pacific is covered in patches by the frustules of Cos- 

 cinodiscus rex, one of the largest known forms, wnth a diameter of 

 0.8 mm. This occurs in depths ranging from 4,500 to 6,000 meters. 

 Altogether about 2^ million square kilometers of the ocean bottom. 



