LAURENTIAN LIFE II9 



time there were, as in later periods, calcareous or- 

 ganisms composed of aragonite, these may have 

 been destroyed by conversion into dolomite, while 

 others more resisting were preserved, just as a 

 modern Polytrema or Balanus might remain, when 

 a coral to which it might be attached would be 

 dolomitized, or might even be removed altogether 

 by sea-water containing carbonic acid. There is 

 reason to believe that this last change sometimes 

 takes place in the deeper parts of the ocean at 

 present. This would account for the persistence 

 of Eozoon and its fragments, when other organisms 

 may have perished, and also for the frequent filling 

 of the canals and tubuli with the magnesian carbo- 

 nate. 



The main point here, however, for our present 

 purpose is that, when a calcareous shell or skeleton 

 has been thus infiltrated with a sificate, it becomes 

 imperishable, so that any amount of alteration of 

 the containing limestone short of its absolute fusion 

 would not suffice to destroy an organism once in- 

 jected with silicious matter. Thus the occasional 

 persistence of silicified fossils in highly metamor- 

 phosed limestones is in no respect contradictory to 

 the general fact, that when not preserved by sili- 



