DEEP-SEA FORMATIONS. 141 
and finer elays were carried subsequently to longer distances, 
and finally to depths of 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. Nullipores 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 nullipore 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 which 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. 
