92 



MK. W. A. BiCHABDSOJs* O^ THE fETKOLOUY fVol. lxxix, 



situations, and are easily recognizable. The veins m-dj be fractured, 

 often along the conic surfaces, and in folds the fibres may be bent 

 in such a way that the ends of the top fibres point up the dip. 



When examined microscopically, the caleite-fibres have bluntly- 

 pointed terminations, and are rudely circular in cross-section, for 

 crystal faces are rarely if ever developed (fig. 6). In convergent 

 light cross-sections show the emergence of the optic axis, indicating 

 that the fibres are elongated in the direction of the vertical axis, 

 as is usual in fibrous calcite. 1 The mean angle at the pointed ends 

 of the fibres is nearly equal to that between the rhombohedral 

 cleavages of calcite, and the close relation between the fusiform 

 shape of the fibres and this cleavage is sketched in fig. 7. Opaque 



Fig. 6. — Thin section of beef ^ Charniouih ; X 50. 



Fig. 7.— Sketch of 

 single calcite- 

 fibre, showing 

 fusiform. shape 

 and rhombohedral 

 cleavage. X 50. 



particles of marl are sporadically distributed throughout the slide, 

 but there is no sign of sheaths of marl surrounding the fibres or 

 separating the cones. At the parting there is a certain amount 

 of granulation, doubtless the result of external pressures. No 

 optical sign of strain was observed, and even the glide-planes 

 parallel to e (110) rarely appear. 



Cone-in-cone structure has engaged the attention of many 

 workers. H. C. Sorby 9 compared cone-in-cone with oolitic 

 structure, and considered that the former was produced by radial 



1 L. J. Spencer, Min. Mag-, vol. xi (1896) pp. 184-87. 



- Rep. Brit. Assoc. (Aberdeen, 1859) pt. 2, p. 124; and 'The Geologist' 

 vol. ii (1859) p. 485. 



