EVOLUTION OF LIMESTONE AND DOLOMITE 



335 



rock to dolomitic limestone. The principal boring was driven to 

 a depth of 1,100 feet, through coral and coral rock all the way, 

 and samples of the cores were analyzed every ten feet of the dis- 

 tance. The following table of data, quoted by Bull. 330, U.S.G.S., 

 p. 487, shows first, an enrichment in magnesium carbonate near 

 the surface, then an irregular rising and falling in much smaller 

 amounts, while below 700 feet the rock approaches the dolomite 

 ratio. 



TABLE III 



Magnesium Carbonate in Borings on Atoll of Funafuti 



Depth Feet 



Percentage MgC0 3 



Depth Feet 



Percentage Magnesium 

 Carbonate 



4 

 13 

 15 



20 

 26 



55 

 no 



159 

 200 

 250 



423 



7.62 

 16.40 



H-99 

 16.0 



5-85 

 2. n 

 0.79 



2. 70 

 4-9 



295 

 400 

 500 

 598 

 640 

 698 



795 



898 



1000 



1114 



In the analyses of coral rock borings from an artesian well at 

 Key West, Florida, reported by George Steiger, 1 there is no apparent 

 relation between the content of magnesia and depth as shown by 

 the borings on the atoll of Funafuti. 



TABLE IV 

 Lime and Magnesia in Borings at Key West 



The dolomitization of organic calcareous deposits in the sea 

 by secondary replacement as well as by leaching seems to be too 

 well established to admit of serious doubt. The degree of dolo- 



1 George Steiger, Bull. 228, U.S.G.S. (1904), 309. 



