144 DYNAJVIICAL GEOLOGY. 



2. Animals. — Kacliolarians or Ehizopods, having siliceous shells, make 

 Radiolarian ooze in the deeper parts of the ocean. Sponges contribute 

 much silica in the shape of spicules and skeletons to deposits from shallow 

 depths to the lowest. Globigerinse with other Rhizopods make Globigerina 

 ooze, especiall}^ between depths of 1500 and 2900 fathoms, but not in the 

 higher latitudes. Pteropods are a characteristic of other bottom deposits 

 between 500 and 1400 fathoms. 



But besides these characteristic species there are also the solitary Corals, 

 the Echinoderms, Mollusks, and various other abyssal species, giving variety 

 to the fossils of the sea-bottom. There are also at the bottom the relics of 

 the great variety of pelagic species which after death escape feeders and 

 sink to the bottom. Murray says that, in the course of the Challenger cruise, 

 over 600 Sharks' teeth (genera Carcharodon, Oxyrliina, and Lamyia) and 100 

 ear-bones of Whales (genera Ziphias, Baloenoptera, Balcena, Orca, and Del- 

 phinus), along with 50 fragments of other bones, were obtained in one haul 

 of the dredge in the central Pacific. The locality was, however, not in 

 either of the organic oozes mentioned, but in the " Red ooze " ; and the 

 phosphatic nature of bone was its protection from carbonic acid. Along 

 the course of the Gulf Stream and of its abundant life, the bottom deposit is 

 largely earthy or " terrigenous," and sometimes contains stones of considera- 

 ble size which were distributed by the floating ice of the Glacial period. 



The blue and gray muds of the sea-bottom, which are common in the 

 Pacific, are due to the volcanic dust, cinders, and pumice with which it has 

 been sprinkled by the ocean's aerial volcanoes, and to their decomposition ; 

 and Phillipsite (page 136) is found chiefly in the areas of the Red ooze. 



2. Deposits from Littoral Species. 



The process of limestone-making by shells and corals is essentially 

 the same in its more important steps, and therefore only the latter is 

 here considered. 



CORAL FORMATIONS. 



Coral formations are made from the calcareous secretions of coral-making 

 polyps, with large contributions from the shells and other relics of the 

 littoral fauna. 



Coral formations, while of one general mode of origin, are of two 

 kinds : — 



1. Cored islands. — Isolated coral formations in the open sea. 



2. Cored reefs. — Banks of coral, bordering other lands or islands. 



The positions of the coral-reef seas and the causes of limitation are 

 explained on pages 46, 56, and illustrated on the chart, page 47. 



The exclusion of corals from certain tropical coasts is owing to different 

 causes : — (1) Cold extratropical oceanic currents, as in the case of western 

 South America (see chart). (2) Muddy or alluvial shores, or the emptying 

 of large rivers ; for coral-polyps require clear sea water and generally a solid 



