400 



DK. C. GILBEET CULLIS. 



which may be called secondary " mud," is usually easily distinguished from the truly 

 contemporaneous (primary) " mud " by its mode of occurrence and by certain 

 peculiarities in its texture (fig. 34). 



The conversion of the fine calcareous detritus into secondary aragonite and 



calcite, together with the deposition of 

 secondary aragonite and calcite, which has 

 already been described, cause the rocks to 

 become gradually more and more crystal- 

 line in character as the boring is descended. 



(c) The Disa2')pearance of Aragonite. 



From the surface down to a depth of 

 about 100 feet the cores are largely com- 

 posed of well-preserved aragonite, but near 

 this depth signs begin to appear of changes, 

 which ultimately lead to the complete dis- 

 appearance of this mineral. The processes 

 by which this result is brought about are 

 two, the conversion of aragonite into 

 calcite, and the removal of aragonite by 

 solution. The former process will be con- 

 sidered first. 



The earliest sign of this approaching 

 change is the cessation of the deposition of 

 secondary aragonite in cavities already 



Fig. 34.— Main Boring. Core 204. Depth 373- 

 378 feet, x 100. 



" Coral-reef sand " with interstitial spaces con- 

 taining primaiy " mud," ciystalline calcite, and 

 secondary " mud." The primary " mud " — to the 

 right of the central organism — is seen to come 

 into direct contact with the organisms ; but the 

 secondary "mud " is separated from them by a 

 layer of deposited crystals. 



partly filled with it, and the deposition of 

 calcite instead (figs. 35, 36, 56). The calcite is seen to occupy the more central 

 portion of the cavities, and rests upon and encloses the apices of the aragonite crystals 

 in such a manner as to leave no doubt that it is a product of deposition and not of 

 the conversion of pre-existing aragonite. The direct deposition of calcite upon the 

 secondary aragonite, in this maimer, points to the supervention of conditions which 

 were no longer suitable for the precipitation of calcium carbonate as aragonite ; such 

 as continued to be thrown down being deposited as calcite. This condition is 

 immediately followed by the breaking down of the secondary aragonite, and its 

 recrystallisation as calcite, with the result that the cavities now contain the latter 

 mmeral only. But for a time the pritnary aragonite of the organisms withstands this 

 conversion. Eventually, however, it also is attacked, and passes over into calcite. 



The recrystallisation of the aragonite organisms results in the complete loss of that 

 organic texture which was originally characteristic of them, only the bolder features of 

 form and structure being preserved, and it is by these alone that they can be identified, 



