﻿132 PROF. E. W. SKEATS ON THE [Feb. I905, 



In this case as much as 16 per cent, of magnesium-carbonate 

 is present in the rock, and no dolomite- crystals can be traced. 

 Prof. Judd gives a different explanation of the presence of so 

 much magnesium-carbonate in the rock — an explanation depending 

 on the relative solubilities of the two carbonates in sea-water. 

 He quotes Gustav Bischof's and Hogbom's experiments in support 

 of the view that under atmospheric pressure calcium- 

 carbonate is much more soluble than magnesium-carbonate. The 

 skeletons of calcareous organisms contain in the living state a small 

 amount of magnesium -carbonate, sometimes reaching 1 per cent. 

 in amount. If, then, the rock composed of such organisms is 

 attacked by water bearing carbon-dioxide in solution, calcium- 

 carbonate will be dissolved more quickly than magnesium-carbonate, 

 and the percentage of magnesium-carbonate in the remainder will 

 as a consequence be raised, and may eventually reach 16 per cent. 

 In this way, Prof. Judd explains the composition of the Funafuti 

 rocks between the limits of 20 to 50 feet below the surface of the 

 water. 



This process of differential solution is one which no doubt 

 coral-limestones undergo to a greater or smaller extent, and it is 

 probably the correct explanation of the origin of the all but 

 structureless limestones containing magnesium-carbonate in in- 

 sufficient amount for the production of dolomite-crystals. Very 

 extensive solution and removal of calcium-carbonate is needed, 

 however, before the percentage of magnesium-carbonate in the 

 residual rock is appreciably raised. Assuming the original lime- 

 stone to contain 1 per cent, of magnesium-carbonate, an amount 

 which is probably near the superior limit for the fresh organisms 

 composing the rock, and further, assuming that only the calcium- 

 carbonate is dissolved by carbonated water, 80 per cent, of the original 

 rock must be removed by solution before the magnesium-carbonate 

 in the remainder reaches 5 per cent., 90 per cent, must be dissolved 

 before the magnesium-carbonate reaches 10 per cent., and over 

 93 per cent, before the magnesium-carbonate reaches 16 per cent. 

 So extensive a removal of the original substance of the rock would 

 largely destroy the structure of the organisms that it contained. 

 In the case of the limestones from Christmas Island, Niue, and 

 elsewhere, examined by me, in which magnesium-carbonate was 

 present in the rock in amounts up to 11 per cent., the structure 

 of the contained organisms was in general wonderfully preserved ; 

 and not only was there no evidence of solution in the rock, but 

 on the contrary secondary calcite and secondary aragonite were 

 deposited upon the organisms to a considerable extent. I was, 

 in consequence, forced to the conclusion that in these limestones 

 the magnesium-carbonate was introduced into the rock from the 

 sea-water, resulting in the partial replacement of calcium- by 

 magnesium-carbonate. 



Prof. Judd suggests that the conversion of a magnesian limestone 

 containing about 15 per cent, of magnesium-carbonate, into a rock 

 containing 40 per cent, or more of that carbonate, may be due 



