LAURENTIAN LIFE I17 



We may now consider shortly the relation of 

 dolomite, or the mixed carbonates of lime and mag- 

 nesia, to the preservation of fossils. The presence 

 of dolomite or magnesian limestone in these beds 

 does not affect the conclusion as to their probable 

 organic origin. This form of limestone occurs abun- 

 dantly in later formations, and is even forming in 

 connection with coral deposits in the modern ocean. 



Dana has shown this by his observations on the 

 occurrence of dolomite in the elevated coral island 

 of Matea in Polynesia,^ under circumstances which 

 show that it was formed in the lagoon of an ancient 

 coral atoll, or ring-shaped island, while he finds 

 that coral and coral sands of the same elevated reef 

 contain very little magnesia. He concludes that 

 the introduction of magnesia into the consolidating 

 under-water coral sand or mud has apparently taken 

 place — "(i) In sea-water at the ordinary tempera- 

 ture ; and (2) without the agency of any other 

 mineral water except that of the ocean " ; but the 

 sand and mud were those of a lagoon in which the 

 saline matter was in process of concentration by 

 evaporation under the solar heat. Klement has 



* " Corals and Coral Islands," p. 356, etc. 



