398 



DR. C. GILBERT CULLIS. 



h. Tlie Crystallisation of the Fine Calcareous Detritus. 



Deposition from solution is not the only process by which secondary calcite and 

 araoonite are formed. Another chano-e is observed as the cores are studied in 

 descending order which also results in the formation of these minerals. This is not, 

 however, one which afPects the empty spaces of the rock, but those which contain 

 finely divided calcareous detritus. 



If a section, taken from the first two or three feet of the boring, be examined under 

 the microscope, the contents of these cavities are seen to be quite fresh and unaltered ; 



viewed by transmitted light they appear 

 as dark, more or less opaque areas, owing to 

 their finely comminuted condition (fig. 51). 

 But in sections from deeper cores some of 

 them are seen to become more or less 

 transparent, owing to incipient crystallisa- 

 tion. The resulting clarification is at first 

 local (fig. 32), showing that crystallisation 

 commences at a number of points simul- 

 taneously, but by the extension of crystal 

 growth from these starting points it 

 eventually affects the whole cavity, which, 

 with its crystallised contents, now closely 

 resembles those which have been completely 

 filled by deposition from solution (fig. 55). 

 Indeed, in cores from some little way down 

 it is not always easy to say by which of 

 these two processes the crystalline contents 

 of any particular cavity may have 

 originated. Generally, however, the pro- 

 ducts of crystallisation of calcareous detritus 



Fig. 31.- 



-Main Boring. Core 92. Depth 70-80 feet. 

 X 100. 



Coral showing the crystallised contents of origin- 

 ally " niiul "-filled cavities. The substance of 

 the coral and the " mud " were traversed by 

 boring organisms, whose tubes became subse- 

 quently filled with " mud," which has resisted 

 crystallisation. 



are less transparent than those of deposition 

 from solution, owing to the presence of minute particles which have remained 

 uncrystallised ; often, too, tliey may be distinguished by the presence of the 

 unobliterated tube-like structures of boring organisms (fig. 31). 



In the process of " mud " crystallisation, as in that of deposition from solution, the 

 crystals formed, whether of calcite or of aragonite, sometimes exhibit an arrangement 

 in continuity with the elements of the adjacent organisms. " Mud" lying within, or 

 between, aragonite organisms, becomes converted into crystals of aragonite, each of 

 which is continuous with an organic fibre (figs. 32, 35, 37, 54), while that occurring 

 within or between calcite organisms is changed into crystals of calcite, often similarly 

 disposed. 



