406 DR. C. GILBEET CULLIS. 



magnesium carbonate. The rise in the proportion of this constituent is therefore very 

 sudden ; but it is probably not quite so abrupt as these facts would imply, for 

 number of small fragments, the precise position of which, in the boring, is uncertain 

 but which are known to belong hereabouts, and most likely come from between the 

 two cores in question, have given intermediate values. Sections of these fragments 

 show some to consist of calcite only, others of calcite and dolomite. One of the 

 former kind was analysed by Dr. Skeats, and gave 3*82 per cent, of magnesium 

 carbonate, while of three of the latter kind, two gave 11 "67 per cent, and 16 "2 1 per 

 cent, respectively, and the third 2972 per cent. 



The cores following below that in which dolomite is first seen show a gradual 

 increase in tlie proportion of magnesium carbonate, until a maximum of 40 per cent, 

 or thereabouts is reached, at a depth of 650 feet. 



When sections cut from a number of these cores are examined in descending order, 

 this increase is seen to be represented by the gradual growth and extension of the 

 areas composed of dolomite at the expense of those of calcite. So long as the 

 proportion of magnesium carbonate falls short of its maximum, areas of unaffected 

 calcite may still be discovered, but, as the maximum is approached, such areas become 

 smaller and less numerous, and by the time it has been actually attained they have 

 all disappeared, calcite having everywhere given place to dolomite. It is interesting 

 to trace the various stages by which this progressive "dolomitisation is efi'ected. 

 Sections taken from the depth of 637 feet consist entirely of calcite, partly of primary, 

 partly of secondary origin. The rock here is very porous, and the interstices are 

 lined with acute scalenohedral crystals of calcite, which are often disposed in 

 continuity with the structural elements of the organisms by which the cavities are 

 lined (fig. 39). In sections taken only a foot lower down dolomite is seen to be 

 present as well as the primary and secondary calcite. The rock is less porous than 

 that immediately above, the cavities being now in part occupied by crystals of 

 dolomite which rest upon and enclose the projecting apices of the scalenohedra of 

 secondary calcite (fig. 40 ; Plate F, fig. 3). If these dolomite crystals could be 

 removed, the remainder would be essentially identical with the rock at 637 feet. It 

 would appear, therefore, that the dolomite Is, in part at least, an addition to the mass 

 by direct deposition, and not always a substitution product for pre-existing calcite. 

 As sections from successively deeper cores are examined, the lining of scalenohedral 

 calcite, which intervenes between the unaltered organisms and the subsequently 

 deposited dolomite, is seen to disappear by conversion into dolomite, which latter, by 

 a process of inward invasion, ultimately comes into direct contact with the organisms 

 themselves (Plate F, fig. 3), and the sites of aragonite organisms, which in the cores 

 above are composed of secondary calcite, are also rapidly usurped by dolomite. At 

 this stage the rock consists only of primary calcite and of dolomite, all secondary 

 calcite having been changed into dolomite. It appears, therefore, that the secondary 

 calcite of these rocks is more readily converted Into dolomite than the primary calcite. 



