THE CITE^rTCAL EXAMTXATTOX OF THK .NfATElJlAES FKO^r FFXAFnTI. .SHf) 



the core the rise in the proportion of the magnesium carbonate may Ije entirely due 

 to the leaching out of the calcium carbonate. 



If this conclusion be accepted as a correct one, we have probably an indication 

 of the limits of the variation in the proportions of the two carbonates which can be 

 bi'ought about, by the leaching-out j^rocess, in materials of this kind. As more and 

 more of the calcium carbonate is removed, the remaining mass must lose its cohesion, 

 form, and minute structure, and after a certain limit has been passed only a small 

 quantity of amorphous material will be left behind. 



From the examples of the Lithothamnion cited by Hogbom and the study of the 

 upper portion of the Funafuti cores, we are probably justified in concluding that the 

 enrichment of a mass of mixed carbonates with the magnesium salt may go on to the 

 extent of 14 to 16 per cent, at least — without, on the one hand, the introduction of 

 magnesium salts from without, or, on the other hand, the entire obliteration of the 

 organic structure of the mass. From a depth of 50 feet to one of 637 feet, in the 

 boring, the percentage of magnesium carbonate varies within small limits 

 (1 to 5 per cent.). These small differences may not improbably be due to the varying 

 proportion of organisms which differ in their susceptibility to the leaching-out process. 

 At all events, all the changes in the percentage of the magnesium carbonate may be 

 easily accounted for in this way. 



It is to be noticed, moreover, that through a considerable portion of this descent, 

 from 50 to 637 feet, the material available for analysis consisted only of the com- 

 minuted fragments, largely made up of remains of foraminifera, washed out of the 

 bore-hole, like sand, during the operation of the diamond-drill. It is doubtful how 

 far the analyses of this comminuted material can be taken as representing the 

 composition of the whole rock passed through. It is not improbable that the parts 

 in which most solvent action had taken place had been reduced to a more friable 

 condition, and these 23ortions, especially rich in the magnesium carbonate, would be 

 carried away in a very finely-divided state. 



But, at a depth of 637 feet, we begin to meet with quite new conditions. Instead 

 of an extremely cavernous and friable rock, which, by the action of the diamond- 

 drill, is reduced to a more or less coarse powder, we get a white, chalky-looking 

 material, which is brought up in fairly solid cores, and is found to contain a high 

 percentage of magnesium carbonate. This proportion rises rapidly, till at 660 feet it 

 reaches the limit of 40 per cent. 



Under the microscope, however, this rock, which has undergone such profound 

 chemical and mineralogical change, is found to consist of precisely the same organisms 

 as those found in the lower portions of the core. Ilcdimeda and Lithothamnion 

 are very abundant, and the usual species of foraminifera and corals everywhere 

 occur in the rock. At a depth of about 763 feet this soft, friable rock passes 

 gradually into one of considerable hardness and density, but from time to time there 

 is a recurrence to the softer and more friable type. In this part of the core we find 



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