38G PROFESSOR J. ^V. JUDD. 



that, not oaly are many of the organisms reduced to the condition of casts, but that 

 in the hollow spaces of the rock, new chemical deposits, encrusting the sides of 

 hollow spaces, and sometimes filling them up, have been formed. 



In seeking to account for the pseudomorphic changes which have taken place in 

 this coral-rock, it is obvious that we are limited to one of three hypotheses, or to a 

 combination of two or more of them. These are as follows : — 



(a) The solution of calcium carbonate at a greater rate than of the magnesium 

 carbonate, by which the percentage of the latter salt will gradually rise. 



(6) The introduction of magnesium carbonate from Avithout, filling up the cavities 

 of the rock, or entering into combination with the calcium carbonate, so as to 

 produce a double salt. 



(c) An interchange of the two bases between the rock and the surrounding 

 sea-water, whereby a portion of the calcium in the carbonate passed into solution 

 and is replaced by magnesium, thus tending to form the double salt of dolomite at 

 the expense of calcite or aragonite. 



We have already considered the first of these alternative hypotheses, and have 

 shown that there is good ground for concluding that a process of leaching out of 

 calcium carbonate does go on in the waters of the ocean, especially in the case of 

 certain classes of organisms, and under particular conditions. But we have also seen 

 that there is a probable limit beyond which this action cannot go on without entirely 

 destroying, or rendering unrecognisable, the organic or other structures in the mass. 

 We have suggested that this limit may probably be nearly reached when the 

 percentage of magnesium carbonate has been raised from 1 to 14 or 1 6 per cent. 



It is clear then that we must look to another cause to explain the much greater rise 

 in the proportion of magnesium carbonate which takes place in the lower portions of 

 the core (637-1114 feet) up to 40 per cent, and upwards. Without affirming that 

 a direct deposition of magnesium carbonate from sea water is not possible, most 

 chemists will be disposed to regard it as more probable that a portion of the 

 calcium in the calcium carbonate has been gradually replaced by magnesium. 



What are the exact conditions under which these interesting chemical changes 

 take place is a problem of great interest, l)ut at the same time one of which it is 

 very difficult to suggest a satisfoctory solution. I have consulted Dr. Sorby, who 

 has for so many years directed his studies to this and similar questions. He has 

 been good enough to examine a series of slides from the different cores, and has 

 written a note on the subject.* 



At the same time I have arranged that Dr. Cullis, following the methods so 

 admirably inaugurated by Dr. vSoRBY,t and aided by modern methods of micro- 

 chemical analysis, should make a very thorough and detailed study of all the cores 



* See Section XIII (p. 390). 



t ' Quart. Journ. Geol. Soc, Proc' (Anniversary Address for 1879), vol. 35, pp. 46-92. 



