DEEP-SEA FORMATIONS. 



141 



and finer clays were carried subsequently to longer distances, 

 and finally to depths o£ nearly six or seven hundred fathoms. 

 The finest shore deposits are not known to extend beyond four 

 hundred miles. Along; the course of currents fine silt will be 

 deposited on the farther slope of ridges over which they pass, 

 as in the Windward Passage, or on the sea-face of the conti- 

 nental slope off Hatteras, at the foot of the falls of the Gulf 

 Stream, if I may so call that slope. Greensand appears to-day 

 to be deposited in the eddies of the Gulf Stream. Rounded 

 pebbles with conchoidal fractures are found in the West Indies 

 at considerable depths. Their shape is probably due to the 

 action of the comparatively warm water of those depths upon 

 the fragments of rocks disintegrated from their original layers. 

 Manganese nodules and incrustations are met only in deep 

 water. In the vicinity of coral islands extensive deposits of 

 comparatively coarse limestones often run to great depths, as 

 off the western Florida Bank ; quite large pieces of such lime- 

 stone were brought up by the "Blake" from a depth of about 

 fifteen hundred fathoms. NuUipores extend to a depth of one 

 hundred and fifty-five fathoms. We find large forests of the 

 larger kinds of bryozoa at a depth of from one hundred to two 

 hundred fathoms. Deposits of sand, of groves of bryozoa, or 

 of nuUipore limestone, can therefore be found within the limits 

 of the deep-sea fauna. 



The plateaux of limestone which form the submarine base of 

 the Florida and Yucatan banks are examples of deep-water 

 limestone deposits, the like of which we know to have been 

 formed in the West Indies during cretaceous and tertiary times, 

 and to have been elevated to considerable heights, as, for in- 

 stance, in Cuba, Jamaica, San Domingo, Barbados, and other 

 West India Islands. 



By deep-sea deposits of past ages we of course mean those 

 deposits in wdiich animals now characteristic of the deep-sea 

 fauna have been found ; that is, such deposits as do not in our 

 day seem to play any important part in modifying the outline 

 of our continents ; such deep-sea deposits consist exclusively of 

 globigerina, diatom, biloculina, and radiolarian ooze, and of the 

 abyssal red clay. 



