894 DR. C. GILBERT CULLIS, 



small proportion at the surface, on the one hand, and at the depth of about 50 feet, 

 on the other. 



This rise in the proportion of magnesium carbonate in the first 50 feet of the boring 

 was ascertained by chemical means. It could not have been discovered by means of 

 the microscope ; unstained sections show nothing that can be identified as dolomite, 

 while sections treated with Lemberg's solutioji stain uniformly, as they would if 

 composed of calcium carbonate only. Well-defined dolomite is not present in 

 these rocks ; although magnesium carbonate has been introduced into them in con- 

 siderable amount, it has, so far, foiled to cause the molecular rearrangements necessary 

 for the formation of the crystidlised mineral. The horizon is one of interest and 

 importance, inasmuch as it aifords evidence that partial dolomitisation, at least, may 

 be effected at no greater depth than 25 feet from the surface of the sea, and that 

 magnesium carbonate may enter into the constitution of the rocks of coral-reefs, to 

 the extent of 16 per cent., without causing the formation of individualised dolomite. 



The rocks of the first few feet of the boring consist only of the original organisms 

 of calcite and aragonite, and the products of their wear and tear, but limestones of 

 this simple original character constitute but a small proportion of the rocks of the 

 boring, and include only those quite recently deposited. As the cores are examined 

 in descending order, various secondary characters make their appearance owing to 

 the addition of new matter to the rocks, or the modification, in various ways, of that 

 of which they were primarily composed. Of these changes, three, which are the 

 most important to be observed in the first 637 feet of the boring, will be briefly 

 described : — 



a. The deposition of secondary calcite and aragonite from solution. 

 h. The crystallisation of the finely-divided calcareous detritus. 

 c. The disappearance of aragonite. 



a. The Deposition of Secondary Calcite and Aragonite f7-om Solution. 



In the most recent rocks of the boring, a large proportion of the interspaces 

 between the various organisms (and a still larger projiortion of those within them) are 

 seen to be quite empty ; but as successively deeper cores are examined, these empty 

 cavities are observed to be the sites of deposition of secondary calcite and aragonite. 



Of these two minerals, calcite is the one which is deposited more frequently and in 

 greater relative amount. Normally, it would seem, calcite is deposited upon calcite 

 organisms, and aragonite upon aragonite organisms ; but this is not invariably the 

 case, for not only is calcite found in all cavities bounded by calcite organisms, but it is 

 also found in a considerable proportion of those bounded by aragonite organisms. 

 Aragonite, on the other hand, is never deposited upon organisms of calcite, and only 

 upon those of aragonite which are composed of weU-marked crystalline elements, these 

 elements' seeming to exercise a determining influence upon the calcium carbonate being 



