170 THKEE CRUISES OF THE " BLAKE." 



On the character of the bottom upon which these animals die 

 will depend the chance of their being preserved, and while the 

 accumulations of dead limestone carcasses may go on indefi- 

 nitely at a depth of one hundred to one hundred and fifty fath- 

 oms, the same mollusks, when thrown upon rocks, or upon a 

 gravelly or sandy beach which is exposed to a greater or less 

 action of the sea, would soon be reduced to an impalpable 

 powder, and be unrecognizable. 



The fine muds and ooze deposited at considerable distances 

 from shore form beds admirably adapted for the preservation 

 of the most delicate pelagic or deep-sea types which may hap- 

 pen to become imbedded in them. The coarser sands and 

 pebbles may form a conglomerate, in which the firmer kinds of 

 invertebrates will occasionally be partially preserved. 



When the deposition of a deep-sea mud is rapid, the form of 

 the body of fishes and the soft part of mollusks is often re- 

 tained, but in littoral flats, where large quantities of gas are 

 evolved, and the decomposition of animal matter takes place 

 rapidly, few well-preserved fossils will be found. 



