674 PRINCIPLES OF STRATIGRAPHY 



nental plateau off the Atlantic coast of North America. The pla- 

 teau or shelf itself is covered with shore-derived arenaceous de- 

 posits. Dredgings from these were examined by Bailey Willis, who 

 states that they "are indeed finer near the eastern edge, yet are dis- 

 tinctly granular and incoherent." 



The black muds of the Ordovicic with their world-wide distri- 

 bution of graptolites seem to offer an older illustration of this type 

 of sedimentation. The graptolites are often arranged so as to in- 

 dicate current action, and it appears that these muds (Deep Kill, 

 Norman's Kill, Utica) were distributed by these currents and de- 

 posited on the ocean floor under their pathway. 



Abyssal Deposits. 



In the abyssal district of the ocean, beyond the one thousand- 

 meter line, current-borne detrital material is relatively unimpor- 

 tant. Terrestrial material borne by currents is not unknown, how- 

 ever, as in the case of the deep-sea vegetal deposits already referred 

 to. Even rock waste is carried by currents to realms of greater 

 depths where these are not far from shore, while terrestrial material 

 carried by floating icebergs or rocks held by the roots of floating 

 trees or sea weeds may come to be deposited at any depth. Among 

 other deposits of the littoral district which pass over into the abys- 

 sal, greensand has already been noted, as extending to a depth of 

 700 fathoms. The characteristic deposits of this district, exclusive 

 of the marine derelicts, may be divided, according to their origin, 

 into the pelagic and the terrigenous. To these must be added the 

 meteoric or extra-terrestrial materials, and the subcrustal or vol- 

 canic. 



Abyssal Deposits of Pelagic Origin. 



Pelagic deposits are those which have descended to the bottom 

 from the pelagic district of the ocean, i. e., the upper hundred- 

 fathom layer of water of the open ocean. They comprise chiefly 

 the shells and skeletal parts of pelagic animals and plants, such as 

 Foraminifera, radiolaria, pteropods, ostracods and diatoms, the 

 shells and other hard parts of pelagic molluscs, and the skeletal 

 parts of fish, especially the teeth of selachians, and, further, the 

 car bones of whales, and other hard parts of this as well as other 

 pelagic vertebrates. The young stages of organisms, such as mol- 

 luscs, brachiopods and echinoderms, may also be included in these 



