678 PRINCIPLES OF STRATIGRAPHY 



Older Deposits That Hove Been Considered of Deep-sea Origin. 



The question, Are there any deep-sea deposits among the sedi- 

 ments in the older geological series, has been variously answered 

 in the past. Sir John Murray held that abyssal sediments were not 

 represented among the known sediments, except perhaps in such 

 instances as the radiolarian ooze of Barbados. Chalk has in the 

 past frequently been cited as an example of a deep-sea deposit of 

 Cretacic time, and this opinion is still defended by some. (Supan- 

 91 :5'55.) There seems to be, however, a growing recognition of 

 the fact that this formation is of comparatively shallow-water ori- 

 gin, the organisms composing it being benthonic rather than pelagic. 



Many older radiolarian cherts or radiolarites have been referred 

 to deep-sea origin. Their almost constant interpolation between 

 beds of shallow-water origin has, however, thrown doubt upon this 

 interpretation. Of comparatively deep-water origin are believed to 

 have been the massive limestones of the Trias of the eastern Alps 

 and of some of the Jurassic deposits of the Alpine region. This 

 applies especially to the Aptychus beds of the Upper Jurassic, some 

 of which seem to have no other remains than the opercula of am- 

 monite shells, the so-called Aptychi. These opercula are believed 

 to have sunk to the bottom on the death and decay of the pelagic 

 ammonite, while the shell continued to float and eventually was de- 

 posited in other sediments of generally shallower water origin. 



Whether these deposits will eventually prove to have such an 

 origin, or whether they, too, may not be of shallow-water origin, 

 must for the present remain undecided. 



In North America the black muds of the Upper Devonic have 

 been regarded as of deep-water origin (Clarke-21), but formed 

 under conditions similar to those obtaining in the Black Sea. The 

 same origin has been suggested by Pompeckj (71) for the dark 

 Liassic shales with saurian remains found in Wiirttemberg and in 

 England. In both cases, however, the deposits may with equal if 

 not greater certainty be classed as of shallow-water origin. 



Concretions of the Deep Sea. 



Concretions are a characteristic feature of the deep sea. Though 

 not clastic, they may be noted here for the sake of completeness. 

 Foremost among these are manganese concretions, which are widely 

 distributed on the abyssal ocean floor. Sometimes they occur in 



