376 PROFESSOR J. W. JUDD. 



The first of these is said to have been quite fresh, the second was in fragments, 

 and may have undergone some alteration. 



The proportion of magnesium carbonate combined with the calcium carbonate of 

 Lithotli amnion, Halimeda and other calcareous algse has been very variously stated 

 by different authors. 



In a specimen of Halimeda from Labuan, Hogbom found only a trace of magnesium 

 carbonate ; in a Penicillus from the West Indies, he obtained the same result.* A 

 RiviolaHa yielded "69 of magnesium carbonate to 8 4 "8 8 of calcium carbonates 

 ("81 per cent.), while an undetermined calcareous alga from the Java seas gave '04 

 of magnesium carbonate to 32"72 of calcium carbonate (•12 per cent.). 



The very fresh tuft of Halimeda opuntia which was grown in six weeks at 

 Funafuti (see p. 146) supplied the means of obtaining a very exact determination of 

 the proportion of the two carbonates in a specimen which could have undergone little 

 if any change. Dr. Skeats' analysis of this specimen gave "60 of magnesium 

 carbonate to 8 6 "50 of calcium carbonate ('7 per cent, of the total mineral matter). 



(6) Changes in the Proportions of (he Calcium and Magnesium Carbonates ivhich 

 Occur after the Death of Calcareous Organism,s. 



While there is good reason for accepting as correct the conclusion of Foechhammer 

 that living organisms, which secrete calcium carbonate in their skeletons, take up 

 magnesium carbonate to the extent of, at most, only about 1 per cent, of the calcium 

 salt, there is much evidence that many skeletons of dead organisms contain a con- 

 siderably higher proportion of the magnesium salt, and the same is true of the muds, 

 sands and rocks formed by the disiiitegration of the remains of these organisms. 



In his ' Flora,' Payen gives an analysis of Halimeda showing 5 '50 of magnesium 

 carbonate to 90" 16 of calcium carbonate, t which works out as 5 "7 5 of magnesium 

 carbonate in the total mineral matter. A specimen of dead Halimeda fronds from 

 the Funafuti lagoon, which was analysed by Dr. Cullis for me, gave 1"39 of 

 magnesium carbonate to 98 "32 of calcium carbonate, that is, the magnesium salt 

 formed 1 '4 per cent, of the whole mineral matter present. 



A mass of Halimeda fronds, dredged from the face of the reef, off Tutanga, at a 

 depth of between 50 and 60 fathoms, was analysed by Dr. Skeats with the following 

 result :■ — Magnesium carbonate was present to the extent of 4*0 ])er cent, to 93*59 

 of calcium carbonate, that is, the magnesium carbonate formed 4'1 per cent, of the 

 total mineral matter. 



The estimations made by Dr. Skeats of the composition of the dredgings made on 

 the floor of the lagoon of Funafuti, consisting of 90 per cent, and upwards of 

 Halimeda fronds, showed from 2 to 6 per cent, of magnesium carbonate as being 

 present. Similar results were yielded by the analysis of the materials in the Lagoon 



* Lor. (it., p. 273. 



t Payen's ' Flora,' vol. 1, p. 71. 



