200 WORK OF THE OCEAN 



insoluble parts of the shells of pelagic life. The nodules and crystals 

 are secondary products, the materials for which were derived from 

 the decomposition of the sediments which gave rise to the clay. 

 Eolian dust, or the materials derived from it by chemical alteration, 

 is doubtless a constituent of the red clay. 



It is significant that deposits corresponding to those of the 

 deep sea have not been identified with certainty among the rock 

 formations of the land. If such deposits are absent from the land, 

 as they seem to be, their absence must mean that the continents 

 have never been beneath deep seas. That large parts of them have 

 been beneath shallow sea-water is abundantly attested. 



Map work. See Plates CXXX-CLIV, Professional Paper 60, U. S. Geological 

 Survey, and Laboratory Manual, The Interpretation of Topographic Maps, Exercise 

 XV. " 



