396 



DE. C. GILBERT CULLIS. 



Fig. 26. 



-Main Boring. Core 13. Depth 10-20 feet. 

 X 100. 



fine calcareous " mud," which is absent from those that are perfectly clear, and which 

 must have been intermittently present in the waters from which the deposit was 



precipitated. In no case are the elements 

 of this fibrous deposit disposed in continuity 

 with the structural elements of the 

 organisms invested ; it is as though their 

 deposition had proceeded too rapidly»for 

 the necessary molecular adjustments to be 

 made. 



The distribution of this type of deposit 

 is remarkable. In the first of the borings 

 made under the direction of Professor 

 SoLLAS, it has been detected at various 

 points between 30 and 80 feet ; in the 

 second at a number of depths from about 

 20 feet down to the bottom of the boring 

 (72 feet). In the Main Boring it occurs, 

 and is abundant between the depths ol 

 20 and 30 feet, and is also seen in small 

 amount for a little distance above and 

 below these limits. In other words it 

 occurs at that horizon, in this upper part 

 of the boring, at which the proportion of 

 magnesium carbonate is unusually high. 

 A similar, if not identical deposit is 

 conspicuous in the rocks of the lowest 

 300 feet of the boring (figs. 44, 45, 46, 48, 49, 67, 68, 69, and Plate F, figs. 4, 5, 6), 

 rocks again in which the proportion of magnesium carbonate is high. 



The other type of secondary calcite consists of crystals which, though actually of 

 smaU dimensions, are usually relatively large and well developed compared with those 

 of the fibrous deposit. They are clear and transparent, and their apices, which project 

 into the cavities which they line, often exhibit well-defined crystal faces (figs. 29, 33, 

 36) ; moreover, whenever they invest a calcite organism which possesses a marked 

 crystalline texture, they are disposed in optical continuity with its structural elements 

 (figs. 30, 39, 40 ; Plate F, figs. 3, 4). This more perfectly crystallised type of secondary 

 calcite, the characters of which suggest that it may have been formed more slowly 

 than the fibrous type, is abundant in the boring from below 30 feet down to 637 feet, 

 and occurs occasionally also at greater depths. 



These various deposits of secondary calcite and jtragonite furnish a crystalline 

 cementing material by which the original organisms and organic fragments of the 

 rock are bound together, and the cavities, within and between them, more or less 

 completely filled up. 



Coral with cavities containing calcite and arago- 

 nite. The cavities on the left are filled with 

 secondary aragonite, the crystals of which are 

 continuous with the adjacent coral fibres. The 

 cavities which arc only partially filled contain 

 secondary calcite of the fibrous encrusting 

 type, the elements of which, in all cases, spring 

 perpendicularly from the cavity walls. 



