CONDITIONS OF SEDIMENTARY DEPOSITION. 517 



composed of exceedingly fine grained, apparently pulverulent, 

 material ; the best of these are from the Knox dolomite 

 and the Solenhofen lithographic stone. The third variety of 

 limestone consists of the thoroughly crystalline marbles, which 

 contain no unaltered material, and which occur in such field 

 relations that they are known to be completely metamorphosed. 

 Extended study is required to determine the nature of deposition 

 of the first and second types. They may have been organic and 

 have suffered moderate alteration only, but there is a reasonable 

 presumption that they did to some extent crystallize in place 

 from sea-water, and were, to a still greater extent, precipitated 

 from the outspread fans of fresh water, radiating from rivers' 

 mouths, whence they spread as fine silt over the bottom of the sea. 



ORGANIC DEPOSITION. 



Since deposits of this character are composed chiefly of the 

 calcareous or silicious remains of marine organisms, their 

 formation is conditioned primarily by the circumstances con- 

 trolling marine life, and secondarily by the insolubility of the 

 skeletons under circumstances of wide distribution and gradual 

 sinking. 



Favorable conditions. — {a) Warm waters. 



(fr) Clear waters. ) n ,... t , , , ,-r 



v ' \ Conditions favorable to lite. 



(V) Abundant food supply. ) 



{j£) Depths less than 1 500 fathoms. 



(e) Expansion and diffusion of currents in rapidly deepening 

 water. 



For a description of the oceanic deposits and of the biological 

 conditions which promote their accumulation, the reader may be 

 referred to the Narrative of the Challenger Expedition, Vol. I, 

 second part, pages 915 to 926. The oozes which are character- 

 ized by the predominance of remains of globigerina, pteropods, 

 diatoms or radiolaria are there described, and it is shown that 

 the nature of the deposit is determined by the conditions of tem- 

 perature, light and motion which favor the generation of multi- 

 tudes of the minute creatures whose living forms swarm at the 



