MINERALOCtICAL changes observed in cores of FUNAFUTI BORINGS. 408 



done so only imperfectly.* It is of great service in the microscopic examination of the 

 rocks of coral reefs in which crystallisation of the organisms has occurred. But for 

 its presence the identification of the original constitution of many such rocks 

 would be quite impossible ; but revealing, as it does, the shape of the various 

 constituent bodies of which the mass was, in the first place, com^josed, this becomes in 

 many cases a matter of no great difficulty. It is obviously of greatest use in those 

 rocks which have undergone the most extreme crystallisation, and in which, as a 

 consequence, the primary characters have been most completely obscured. 



The second of the two processes by which aragonite disappears from the boring, 

 namely, its disintegration and removal in solution, is one that produces results which 

 are not only appreciable under the microscope, but are also obvious in the cores when 

 examined with the naked eye. It is expressed first by the appearance, in the substance 

 of the aragonite organisms, of numerous small and irregular holes, which impart 

 a porous or spongy character to it (fig. 59). Later, these holes are seen to become 

 gradually larger, and eventually an empty cavity, retaining merely the form of the 

 original organism, alone remains. These empty spaces render the cores highly 

 cavernous, and light in weight ; many of them, which were formerly occupied by coral, 

 show an interesting phenomenon which will be briefly described, since it explains the 

 peculiar mode of preservation of the corals in certain parts of the boring. If a 

 section be cut of a rock containing a mass of coral more or less surrounded by 

 fine calcareous detritus, the peripheral cavities of the coral, which are in 

 communication with the exterior, are seen to be " mud " filled, while those which, 

 because of their more internal position, are inaccessible to suspended matter, are 

 either empty, or contain crystals of secondary aragonite deposited from solution. 

 When such a mass is attacked by solvent action, the whole of the aragonite, both 

 primary and secondary, is dissolved out, but the " mud " of" the peripheral cavities, 

 consisting, as it does, of calcite, remains unaffected. The result is that an empty 

 space is produced, from the walls of which casts in " mud " project inwards for a 

 certain distance, forming reversed presentations of the form of the external parts of 

 the coral (fig. 60), This, which may be spoken of as the phenomenon of reversal, is 

 of very common occurrence in the boring, and especially in its lowest portions. 



It has already been stated that the disappearance of aragonite begins at about 

 100 feet down in the boring. As the cores from below this depth are examined 

 in descending order, it is seen to proceed more and more rapidly. At a depth of 

 150 feet or so practically all aragonite is gone, only occasional patches of the mineral 

 in an advanced state of recrystallisation, or of disintegration, being encountered at 

 greater depths, and none, up to the present, having been detected at depths exceeding 

 220 feet. 



Below this depth, therefore, and so long as the rocks consist essentially of calcium 



* Skeats: 'Bull. Mus. Comp. Zool. (Harvard),' vol. 42, June, 1903, pp. 109, 110. 



3 F 2 



