408 DE. C. GILBERT CULLIS. 



calcite has disappeared, and thence, to the depth of 820 feet, with the rare exception 

 of an incompletely converted organism, nothing but dolomite can be identified in the 

 cores. Throughout the whole of this 170 feet the composition of the rocks is 

 remarkably uniform ; the proportion of magnesium carbonate is about 40 per cent., 

 sometimes a little less, occasionally a little more, but in no part of the boring, either 

 here or elsewhere, does it exceed 43 per cent. 



The proportion of magnesium carbonate in pure dolomite is 4 5 '6 5 per cent. If 

 40 per cent, be taken as the average amount of magnesium carbonate in these most 

 highly dolomitic rocks of the boring, and that be considered to be combined with 

 calcium carbonate in the proportions represented by the formula CaCOg.MgCOg, then 

 they must contain an excess of uncombined and uncrystallised calcium carbonate 

 amounting to 13 per cent, or so. This may be included in the dolomite as an 

 invisible impurity, but possibly it is this material which, in the condition of minute 

 particles, renders a large proportion of the dolomite crystals more or less opaque, 

 and often causes that zoned appearance which is so characteristic of dolomite in 

 sedimentary rocks (figs. 42, 48). 



The dolomites, which for 170 feet in this part of the boring maintain this peculiar 

 constancy in chemical and mineralogical composition, are by no means uniform in 

 their lithological character. Normally, the dolomites of the boring are dense and 

 compact rocks, and a certain proportion of those of this 170 feet are of this nature, 

 but for the 80 feet included between the depths of 660 feet and 740 feet they are of 

 a totally different character, being so soft and friable as to crumble readily under the 

 finger nail, and so porous as to feel, in the hand, scarcely heavier than pumice. The 

 reason of these peculiar characters becomes evident under the microscope. When 

 the rocks below 650 feet are examined in descending order, it is observed that many 

 of the organisms show signs of corrosion by solution. The number of the organisms 

 so affected increases with the depth, and also the proportionate part of each organism 

 removed by solution ; and at the depth of 660 feet the organisms present the 

 appearance shown in fig. 41. Solution has here removed much of their original 

 substance, and especially certain parts which seem to be more readily dissolved than 

 the rest, with the result that they are now little more than sponge-like skeletons of 

 what they were originally. The undissolved parts are composed of dolomite, and 

 each organism is invested by a very thin layer of tiny dolomite crystals. These are 

 often very perfect in form, appearing in sections as minute isolated or nearly isolated 

 rhombohedra. The majority of them contain a central zone or nucleus of the same 

 form as the crystal itself, composed of a white opaque or semi-opaque material 

 (fig. 42). The outer part of the crystal, which is transparent, is probably composed 

 of pure or nearly pure dolomite. The central kernel, however, probably owes its 

 opacity to the presence of calcareous impurity, which represents the excess of calcium 

 carbonate which the rocks contain over and above that present in pure dolomite. 

 As the middle of this 80 feet of soft dolomite is approached, signs of solution 



