214 THE SEDIMENTARY ROCKS 



The frequent association of dolomite with gypsum gives additional 

 probability to this view. A similar process has been observed in 

 the lagoons of coral reefs at the present time (p. 170), and it has 

 been shown that dolomitization takes place much more readily 

 when the CaC0 3 is in the form of aragonite, as is the case in the 

 shells and tests of many marine animals. 



Green Sand is not strictly a calcareous deposit, but has a natural 

 connection with that series of rocks. Green sand is seen by the 

 microscope to be largely composed of internal casts of foraminiferal 

 shells in the mineral glauconite (p. 22). The dead foraminiferal 

 shells which lie upon certain areas of the ocean floor are gradually 

 filled up with glauconite, and then the shells are dissolved, leaving 

 the grains of the mineral, which retain the form into which they 

 were moulded. This process is still going on, and has been ob- 

 served at several points (p. 175). 



b. Siliceous Accumulations 



The siliceous deposits of organic origin are very much less com- 

 mon and less extensively developed than the calcareous, because 

 of the relatively small amount of silica which is in solution in 

 ordinary waters, and of the- comparatively few organisms which 

 secrete shells or tests of it. Nevertheless, these beds are of suf- 

 ficient importance to require mention. 



Infusorial Earth is a fine white powder composed of the micro- 

 scopic tests, or frustules of the minute plants called diatoms. The 

 fineness and excessive hardness of the particles make this an 

 excellent polishing powder. Beds of this earth occur in both 

 marine and fresh-water deposits. At Richmond, Virginia, is a 

 celebrated deposit of this kind. 



Siliceous Oozes are exceedingly rare as rocks of the land ; they 

 consist of the tests of Radiolaria, such as are now accumulating 

 in the deeper parts of the ocean (p. 179). The only land areas 

 in which such deposits have been found occur in certain of the 

 West Indian Islands (Barbadoes, Cuba, and others). 



Flint or Chert occurs in nodules or beds, especially in marine 

 limestones, though it is also found among the sands and clays of 



