Geology and Mineralogy. 241 



Speaking of the mass resulting from the leaching-out of the 

 calcium carbonate with its enrichment in magnesium, the author 

 adds : " Now this mass in a coral reef is everywhere permeated 

 and acted upon by sea-water containing a very notable propor- 

 tion of magnesium, principally in the condition of chlorides and 

 sulphates. May not these materials enriched by the magnesium 

 carbonate exercise an attractive action on the magnesium salts 

 of the ocean waters, giving rise to double decomposition and the 

 gradual replacement of a part of the calcium in the carbonates 

 by magnesium." . . . u It by no means follows that, because the 

 dolomite crystals are found only at considerable depth, the action 

 to which the formation of the crystals was due took place only 

 at this depth. The action may possibly have taken place at or 

 near the surface and the rock have subsided after its alteration. 

 At the same time it may be noted that all the rocks now at short 

 distances from the surface in Funafuti show no dolomite crystals 

 and contain only such an amount of magnesium carbonate as may 

 be accounted for by the leaching-out process." 



The author adds in closing : "From what has been said, it will 

 be apparent that while the investigations that have been carried 

 on upon materials obtained in the vertical borings of Funafuti 

 and also in specimens obtained from upraised reefs in the Indian 

 and Pacific oceans, show that the dolomitization of coral-reef 

 rock, first demonstrated by the researches of Dana and Silliman, 

 really takes place sporadically over very wide areas, the exact 

 conditions under which the operations occur still call for careful 

 investigation both by observation and experiment." 



The mineralogical changes in the cores from the Funafuti bor- 

 ings have been carefully investigated by Dr. Cullis and the results 

 are described with many excellent illustrations in Section XIV 

 of the Report. The discrimination between the three constit- 

 uents of the coral rock, calcite, aragonite and dolomite, was aided 

 by the use of methods of staining, one of which (after Meigen) 

 served to separate the aragonite from calcite and dolomite, the 

 other (Lemberg) the dolomite from the other species. Speaking 

 generally, it was found that aragonite occurs in the upper cores 

 only and dolomite only in the lower ones (below 637 feet), while 

 calcite, which is the sole constituent of the middle cores, occurs 

 with aragonite above and with dolomite below; aragonite and 

 dolomite were in no case found associated. 



The microscopic examination of the cores down to a depth of 

 637 feet shows the original rock unchanged in the first few feet 

 only; below this point a greater or less degree of alteration has 

 gone on. The changes noted in the first 637 feet include the 

 deposition of secondary calcite and aragonite from solution, the 

 former generally and the latter always in continuity with the 

 same mineral in the original organisms; also the crystallization 

 of the finely divided calcareous detritus and finally the gradual 

 disappearance of the aragonite. As already stated, no individual- 

 ized dolomite is found in the upper cores even where partial 



