Revieivs — The Atoll of Funafuti. 227 



Volcanic rock is entirely absent. The proportion of magnesium 

 carbonate between the depths of 10 and 20 feet rises considerably, 

 and then falls away again. Professor Judd shows that under 

 certain conditions calcium carbonate is more soluble than magnesium 

 carbonate, and he thinks that "down to 637 feet the degree of 

 enrichment of the rock by magnesium carbonate may be probably 

 ascribed to the leaching out of calcium carbonate," and this accounts 

 for the friability of the upper part of the core. 



The presence of the large percentage of magnesium carbonate in 

 the lower part of the boring is considered by Professor Judd to be 

 due to some such segregation as has produced flints in the Chalk, 

 and the iron disulphide nodules of other formations. The material 

 of a reef " is everywhere permeated and acted upon by sea-water, 

 containing a very notable proportion of magnesium, principally in 

 the condition of chlorides and sulphates. May not these materials, 

 enriched by the magnesium carbonate, exercise an attractive action 

 on the magnesium salts of the ocean waters, giving rise to double 

 decomposition and the gradual replacement of a part of the calcium 

 in the carbonates by magnesium ? " 



Section xiii consists of some remarks by H. C. Sorby, LL.D., F.R.S., 

 concerning the production of aragonite and dolomite in the coral 

 rock. Dr. Sorby thinks that there " may be special conditions not 

 fully understood under which carbonate of lime may crystallise as 

 aragonite at such a temperature as would be met with in coral 

 rock." Obviously this must be so, for Dr. Cullis has found secondary 

 aragonite in the cores. Dr. Sorby was not able by artificial means 

 to replace calcium carbonate by dolomite ; he only succeeded in 

 replacing it by magnesium carbonate. 



The last section is the account, by C. Gilbert Cullis, D.Sc, F.G.S., 

 of the mineralogical changes observed in the Funafuti borings. 

 Near the surface the cores consist of calcite and aragonite according 

 to the composition of the skeletons of the organisms of which it is 

 made. Tlie magnesium carbonate and other chemicals present in 

 the cores are not perceptible in a microscope-section as crystals. 



The first change that occurs as a greater depth is reached is that 

 the cavities in the organisms composing the cores become filled with 

 secondary calcite and aragonite. Next the secondary aragonite 

 becomes converted into calcite, and finally the primary aragonite 

 also becomes similarly converted. Thus at 220 feet the cores 

 consist entirely of calcite. From 637 feet dolomite begins to 

 replace the calcite, and from 6-50 to 820 feet pure dolomite is present. 

 From 820 to 875 feet, and again from 1,050 to 1,070 feet, calcite is 

 again present with the dolomite. Apart from this the core from 650 

 to 1,114 feet, the lowest point reached, is of pure dolomite. 



This section is excellently illustrated with figures of microscope- 

 slides of sections from different depths, and also by diagrams 

 illustrating the mineralogical changes. 



The volume is illustrated by six plates, a number of woodcuts, 

 and charts of the atoll. Accompanying it is a portfolio with 

 geological maps and sections of the islands. Printed in large, clear 



