378 PROFESSOR J. W. JUDD. 



Ldthothamnion, which are now shown to play such an important part in the building 

 up of coral-reef rocks, undergo rapid changes in their mineral skeleton after the 

 death of the organisms. It is not improbable that the vegetable matter, so 

 intimately united with the mineral substances, yields carbon dioxide during decay and 

 decomposition, and that a solvent action goes on by which mineral matter is being 

 continually carried away. 



As we shall show, subsequently, there is good evidence that, when mixtures 

 of the calcium and magnesium carbonates are slowly acted upon by weak solvents, 

 the calcium carbonate is dissolved and carried away faster than the magnesium 

 carbonate, leaving the whole, or nearly the whole, of the latter constituent 

 behind. By this process of " leaching out" of the calcium carbonate from the mixed 

 carbonates, the percentage of magnesium carbonate in the mineral skeletons must 

 continually rise. It is by this post-mortem action that we may account for the high 

 percentage of magnesium carbonate in the skeletons of Ilalimeda, Lithotliamnion , 

 &c., which has been discovered by Payen, Walther, Hogbom, and others, and has 

 been thought by some authors to indicate a special capacity in these calcareous algae 

 for secreting the magnesium carbonate. 



That this change, which takes place, perhaps, with especial facility in the case of 

 the calcareous algae, goes on in the case of other calcareous skeletons, we have abundant 

 proof Professor Liversidge found that, while in fresh coral the proportion of 

 magnesium carbonate was very small, yet, in a coral limestone from Duke of York's 

 Island, it amounted to 1'808 per cent. Hogbom, while proving that fresh corals 

 contain in every case much less than 1 per cent, of magnesium carbonate, found in a 

 coarse reef-rock 1'64 per cent. ; in a reef-rock, with fragments of gastropods, 213 per 

 cent. ; in a white lagoon-mud, 179 per cent. ; in a terra-cotta coloured lagoon-mud, 

 4"04 per cent. ; while, in a coral-mud from the Java seas, 372 per cent, of 

 magnesium carbonate was found present to 2774 per cent, of calcium carbonate 

 (= 11-12 percent.).* 



Very significant, too, is the fact which has been pointed out by Sir John Murray 

 and Mr. Irvine, that in certain specimens of the giant-clam (Tridacna gigas, L.) a 

 much higher percentage of magnesium carbonate was found in the outer and older 

 portions of their umbos than in the internal and more recently formed sheU-layers. t 



((') The Action of Solvents upon Mixtui-es of tlce Calcium and Magnesium 



Carbonates. 



There has been much diversity of opinion among chemists as to the relative 



solubilities of the calcium and magnesium carbonates. From the behaviour of the 



two salts during ordinary analytical operations, it is generally taken for granted that 



the magnesium carbonate is much more soluble than the calcium carbonate ; but, as 



* Loc. cit., p. 270. 



T See 'Niiliiral Scieme,' vol. 7 (1895), p. 22. 



