404 DR. C. GILBERT CULLIS. 



carbonate, as they do down to 637 feet, calcite is the only mineral which enters into 

 their composition. Of that part of the boring which lies between these two depths 

 (220 feet and 637 feet) a comparatively small proportion only consists of solid cores ; 

 the greater portion is either entirely unrepresented or is represented only by loose and 

 incoherent material, which resembles unconsolidated " coral-reef sand." Such cores as 

 do occur consist in great measure of calcite organisms, with only very occasional remains 

 of aragonite organisms, and these always recrystallised into calcite, and usually 

 showing signs of having been to some extent dissolved in the process of transforma- 

 tion. Large masses of dense and solid coral rock, such as occur so abundantly in the 

 highest parts of the boring, are here practically non-existent. As regards the loose 

 fragmental material, a careful examination furnishes the following facts. The grains 

 are irregular in shape and of no uniform size ; they consist, for the most part, of 

 calcite organisms, or broken portions of such ; aragonite organisms in their original 

 mineralogical state do not occur, those which are found being mineralised into calcite. 

 The particles are often surrounded more or less completely by a ragged fringe of 

 calcite crystals, from the presence of which it may be inferred that they were at 

 one time consolidated by a crystalline cement. No rounding of the grains is 

 apparent ; on the contrary, the broken crystals which so frequently occur upon them 

 are often delicate and sharply pointed, and, moreover, cleavage fragments of calcite, 

 with sharp unworn edges, are quite common, suggesting the action of some violent 

 disruptive force. 



These facts point to the conclusion that the rocks of these middle parts of the boring 

 have suffered much by that solution of aragonite which is seen to set in at a depth of 

 about 100 feet, and to proceed so rapidly with descent. Originally, no doubt, these 

 rocks consisted, as those above them still do, of mixtures of calcite and aragonite in 

 various proportions, but the latter mineral has entirely disappeared from them, a 

 certain small proportion having been changed in situ into calcite, but by far the greater 

 proportion having been carried away in solution. Such of them as were originally 

 composed mainly of calcite have been but little affected by this solvent action, and 

 are still sufficiently coherent to yield solid cores. But those which contained a 

 greater portion of aragonite have been rendered so porous by the removal of that 

 mineral, that the operations of boring have reduced them to a fragmental and incoherent 

 condition, and in this state the materials of which they were composed have been 

 brought to the surface. The amount of solid rock wliich has been removed in this 

 manner is, in these parts of the boring, far in excess of that which remains as 

 the insoluble calcite residue. 



[S) The Mineralogical Characters of the Rocks from ()37 feet down 



TO 1114 feet. 

 The rocks included between these limits are, for the most part, dolomites containing 

 some 40 per cent, of magnesium carbonate, and composed of the one mineral dolomite 



