384 PROFESSOR J. W. JUDD. 



known to play such an important part in building up of coral-reef rocks. The former 

 of these organisms, a purple seaweed, secretes the calcium carbonate in the form ot 

 calcite ; the latter, a green siphonaceous form, has its skeleton composed of aragonite. 



It is now quite certain, both from the results of experiment and observation, that 

 in this solution of the carbonates — alike in organic and in inorganic sediments — the 

 calcium carljonate is, as a rule, much more rapidly removed than the magnesium 

 carbonate. The eifect of this continual leaching oat of the calcium carbonate is to 

 leave behind residues which become progressively richer in the magnesium carbonates. 

 This appears to be true of all oceanic deposits containing carbonates, though there is 

 evidence that certain organisms, like the corals and other forms consisting of aragonite 

 or containing much organic tissue, and especially the calcareous algae, undergo 

 solution, and consequently the leaching-out process, more rapidly than others. 



How far the enrichment of a material with magnesium carbonate by the constant 

 leaching out of the calcium carbonate may proceed has not Ijeen exactly determined. 

 But the analyses of different forms of Lithothamnion by Hogbom show that sj)ecimens 

 of that organism, which probably contained originally not more than 1 per cent, 

 of magnesium carbonate, may have the calcium carbonate leached out till the 

 proportion of magnesium carbonate rises to 14 per cent, (see p. 377). It is obvious 

 that in this leaching-out process the density and solidity of the mass of carbonates 

 must be greatly reduced, and the appearance of many of the remains of these 

 calcareous algae indicates that much of the mineral substance of their skeletons has 

 been removed by solution. 



In the cores from the borings at Funafuti we have seen that, over a very 

 considerable area, there is evidence that down to the depth of 15 to 25 feet there is a 

 toleral)ly rapid rise in the percentage of magnesium carbonate up to 16 per cent, and 

 then a decline at nearly the same rate down to 50 feet. In the case of the Main 

 Boring two maxima of 16 "4 and 16 per cent, of magnesium carbonate can be detected 

 at depths of 15 and 25 feet respectively, with a minimum of 12 per cent, between 

 them. In the two borings made by Professor Sollas, although we have not the 

 means of tracing so accurately the rate of rise and fall of the magnesium carbonate 

 percentage, we have clear evidence that a similar condition of things exists. 



It seems not improbable that the enrichment of the rock in magnesium carbonate 

 up to 16 per cent, in the upper part of the cores may be entirely due to the leaching 

 out of the calcium carbonate. Of this we are able to supply an interesting confirmation 

 from the microscopic examination of the cores by Dr. Hinde and Dr. Cullis. The 

 degree of mineralisation in this part of the core is described by the former as being 

 slight, and Dr. Cullis, whom I asked to look very carefully in this part of the core 

 for indications of dolomite, assures me that he has failed entirely, either by microsco- 

 pical or chemical tests, to detect any trace of the formation of crystals of that mineral 

 in this part of the core. 



From these facts, then, it seems not unfair to conclude that in the upper part of 



