and its Bearing on the Coral Reef Problem. 193 



definite channels, long after the formation of the limestone, 

 and usually after it has been elevated above sea level. 



The important questions as to the distribution of dolomites 

 among geologically recent coral limestones and the conditions 

 Under which replacement can take place beneath the sea remain 

 for discussion. 



The distribution of dolomites in cored islands. 



The Evidence of Funafuti. — When Dr. Cnllis made a 

 mineral ogical and chemical examination of the core of the 

 main boring at Funafuti, it was at first confined to the upper 

 698 feet from the surface, the depth reached by Professor 

 David's Expedition. 



In this part of the bore the mineral distribution appeared to 

 be regular, aragonite being practically limited in its occurrence 

 to the upper part above 150 feet, calcite occurring alone 

 between the lower limit of aragonite and a depth of about 635 

 feet, while below this depth to the bottom of this part of the 

 core at 698 feet, dolomite was represented almost to the exclu- 

 sion of calcite. It appeared at that time as if there existed 

 three zones of aragonite, calcite, and dolomite, respectively, 

 whose formation and limits could most easily be explained as 

 being conditioned by depth beneath the sea surface, that is, by 

 pressure. 



But even at that time, before the deeper cores were avail- 

 able for examination the idea that dolomitization was produced 

 only at considerable depth and considerable pressure, was ren- 

 dered doubtful by the fact that in the upper part of the arago- 

 nite zone, at depths of 15 to 25 feet only below the top of the 

 bore, although the mineral dolomite was not present yet intro- 

 duction of MgC0 3 and replacement of CaC0 3 , had gone on to 

 the extent of 10 to 16 per cent. This relatively high magne- 

 sian content in the coral limestone near the surface was not a 

 localized and sporadic occurrence but was evidently widespread 

 beneath the surface of the atoll, since in both of the shallow 

 bores put down by Prof. Sollas, one of which is a mile and 

 a quarter from the main boring, a similar rise in the magne- 

 sian content of the coral limestones was found to occur at 

 about the same depth. When the cores from the deeper part 

 of the boring between 698 feet and the bottom at 1114 feet, 

 came to be examined mineralogically and chemically, it be- 

 came clear that the apparently simple relations of the upper 

 part of the bore did not continue indefinitely. The downward 

 continuation of rocks consisting entirely of dolomite below 698 

 feet was interrupted between the depths of 820 and 875 feet, 

 and again between 1050 and 1070 feet. At 826 feet, a rock 

 containing only 4*83 per cent of MgCO s was met with while 



