THE WORK OF THE OCEAN 331 



The various nodules and crystals in the clay are believed to be 

 secondary products, the materials for which were derived from the 

 decomposition of the sea sediments. Eolian dust, or the materials 

 derived from it by chemical alteration, is doubtless a constituent of 

 the red clay. 



It is a significant fact 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 really 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. The topographic features of shores find ample illustration 

 on maps. The Topographic Sheets of the U. S. Geological Survey listed on 

 page 335 of the junior author's Physiography afford good illustrations. So 

 also do Plates CXXX to CLIV of Professional Paper 60, U. S. Geological 

 Survey. 



