CALCAREOUS XUDULES. 137 



Mr. Ward writes — '" The nodule appears to consist of a 

 fine sand cemented by crystalline carbonate of lime."' 



Slides of the nodules were prepared parallel to and 

 cutting transversely the equatorial plane. In natural 

 light the slide shows a confused micro-granular structure, 

 the component grains being transparent, opaque, and grey; 

 on rotating the slide, a slight change of tint, due to varia- 

 tion' of absorption, is noticed. With crossed nicols, large 

 areas of the slide extinguish simultaneously, and it is seen 

 that adjacent crystalline aggregates appear to grow into 

 each other, there being no trace whatever of anything like 

 a definite crystal boundary, or of the twinning characteristic 

 of tabular calcite crystals. Minute angular grains of 

 quartz and opaque matter are irregularly dispersed through 

 the calcite aggregate. 



The nodules in which the most perfect geometrical form 

 is developed show little or no crystalline structure on 

 fracture ; bands running parallel to the equatorial plane 

 of the nodule are seen to traverse the broken face, and 

 occasionally the area between two consecutive bands, 

 presents a crystalline appearance very similar to that 

 shown in the larger nodules. The nodules of perfect form 

 are rather less than two inches in diameter, and they pass 

 by stages into smaller ones of roughly spherical shape. 

 These smaller ones show a banded structure, but exhibit 

 when broken no traces of crystallisation. 



When the containing rock is broken the nodules are 

 separated from the cavities in which they repose without 

 the slightest difficulty. After the rock has been blasted, 

 the nodules may be found lying in all directions, with 

 scarcely a trace of the bed-rock adhering to them. The 

 cavities and the exterior of the nodules, especially those of 

 larger size, are frequently stained red. The nodules 

 appear to occur most numerously along the bedding planes ; 

 in one slab, the largest dimension of which was about 

 fourteen inches, seven cavities were counted. 



The bed-rock is in places traversed by narrow irregular 

 fissures containing calcite. 



