410 



DR. C. GILBERT CULLIS. 



It is at first very thin, and constitutes only a minute proportion of the mass ; but 

 traced downwards it increases rapidly in thickness and soon becomes an abundant 

 and conspicuous element in the rocks (Plate F, figs. 4, 5, 6). With occasional 

 interruptions and many variations in its detailed constitution, it continues to the 

 bottom of the boring. 



Mineralogically it is at first composed entirely of calcite. At greater depths it 



sometimes consists of calcite throughout 

 (Plate F, figs. 4, 5), sometimes of regularly 

 alternating layers of calcite and dolomite — 

 the dolomite layers being thin in comparison 

 with those of calcite, and destitute of the 

 fibrous structure (figs. 44, 48 ; Plate F, 

 fig. 6) — and sometimes of dolomite 

 throughout (figs. 45, 49). Occasionally it 

 may be uniformly clear and transparent, 

 but usually it exhibits a very marked 

 concentric structure produced by bands — 

 from two or three to as many as fourteen 

 or fifteen in number — of different degrees 

 of transparency, which impart to it a 

 markedly agate-like appearance. Asso- 

 ciated with it, in the cavities in which 

 it occurs, are numerous floors of "mud" 

 (fig. 46), and there is repeated evidence 

 that the darker layers of the deposit owe 

 their comparative opacity to the inclusion 

 of this finely divided silt within their 

 substance. The deposit appears to have 

 been formed in waters which were alter- 

 nately clear and turbid with suspended 

 matter, which settling slowly down accu- 

 mulated in the irregularities of the cavity 

 floors. Its occurrence as the innermost lining 

 of the cavities proves it to be the latest 

 addition to the rocks, by deposition from 

 solution ; its introduction, moreover, did not take place until after solvent action had 

 affected many of the corals and other aragonite organisms, for it is frequently found 

 lining or filling spaces which were formerly occupied by these. In this respect it difiers 

 from the similar fibrous deposits seen in the first few feet of the boring, where the 

 organisms of aragonite are as fresh and solid as those of calcite (Plate F, figs. 1 

 and 2). Moreover, there can be little doubt that, in many cases, the rocks in which it 



Fig. 44. — Main Boring. Core 205a. 

 830 feet, x 100. 



Depth 



Cavitj', in dolomitised rock, containing banded 

 encrusting deposit consisting of three layers, 

 the first and third of which are of calcite, the 

 second of dolomite. The dolomite layer may 

 represent part of the original deposit converted 

 into dolomite, or, as seems more probable, it 

 may have been deposited directly as such. The 

 encrusting material, the introduction of which 

 was apparentl.v subsequent to the dolomitisation 

 of the rock, is separated from the boundaries of 

 the original organisms by a layer of well-formed 

 water-clear dolomite crystals (see also fig. 48 

 and Plate F, fig. 6). 



