RATE OF DEPOSITION 209 



the base on which oceanic deposits have been laid 

 down, and therefore all fragments of rock from a greater 

 depth than 100 fathoms should be carefully preserved 

 for study. Along some continental coasts fragments 

 have been dredged which indicate an outcrop of 

 tertiary strata. Along some coral atolls solid cal- 

 careous rocks have been torn from larger masses, and 

 off both coral atolls and volcanic islands fragments of 

 igneous rocks thickly covered by peroxide of manganese 

 have been broken from steep slopes, indicating an 

 outcrop of volcanic rocks. 



In the North Atlantic, from a depth of 1,460 fathoms, 

 the telegraph engineers have brought up, by means 

 of the " Lucas Grapnel," fragments of gneiss which had 

 been broken off larger masses. These may have come 

 from outcrops of bare rock in place, but more probably 

 they were broken from large boulders of gneiss trans- 

 ported to the position by icebergs. 



We have likewise very few indications of the thick- 

 ness of the deposits now covering the floor of the 

 ocean, or of the rate at which these are now forming. 

 In the North Atlantic, telegraph engineers think there 

 are reasons for supposing that about 1 inch of Globi- 

 gerina Ooze accumulates in ten years. The rate at 

 which this type of deposit accumulates where cold and 

 warm currents meet at the surface, is probably greater 

 than elsewhere ; and the rate at which terrigenous 

 deposits are laid down near the embouchures of large 

 rivers is evidently greater than that of any other type 

 of deposit. The deep Red Clay of the Pacific Ocean 

 far from land appears to accumulate at an exceedingly 

 slow rate. 



The materials which make up oceanic deposits are 

 now fairly well known, but are worthy of much more 

 detailed study. They consist of inorganic and organic 

 elements : i 4 



