00 Mr. W. a. riChakdsoK on the petrologi* [vol. lxxix, 



nodule), probably because such planes present surfaces of rela- 

 tively iow cohesion. Moreover, the mean direction of the fibres is, 

 in all cases, normal to the bedding-planes. The surfaces of the veins 

 are generally free from marked irregularities, although bed 73 g 

 (described above by Dr. Lang, p. 60) is exceptional, exhibiting coarse 

 projections from its upper surface. There is, indeed, a tendency 

 for some fibres on the upper surfaces to project above the general 

 level ; but the lower surfaces of the veins are always smooth. 



A single vein rarely has a greater thickness than 2| inches, and 

 the top is commonly, but by no means invariably, thicker than the 

 root. Where calcareous nodules occur along the same plane as a 

 ' beef '-vein, the latter splits, passing above and below the nodule. 

 Both parts of the vein, however, preserve the normal structure, 

 and possess a parting. The vein below a concretion is thinner than 

 one above, and beneath very large nodules the vein may either die 

 out, or be absent altogether. This behaviour would be explained 

 if feeding solutions were travelling vertically downwards, for big 

 nodules would shelter the region immediately below them. 



The calcareous deposits at Charmouth present a remarkable 

 resemblance to the gypsum -beds of East Nottinghamshire. Not 

 only do the fibrous veins exhibit similar structure, but in both 

 localities the veins show the same relation to concretionary nodules. 

 This similarity of behaviour doubtless indicates similarity in the 

 processes of deposition. Accordingly, the following brief state- 

 ment of the problem of origin is presented, and the reader is 

 referred to an earlier paper of mine for a more detailed discussion 

 of problems common to both the deposits. 1 



(1) Crystallization of calcite-fibres was initiated along a plane of rupture, 

 developed by the conditions of stress prevailing at the time of deposition. 

 The crystal fibres grew outwards in both directions from that plane, a record 

 of which is preserved in the parting. 



(2) Fibrous habit suggests rapid crystallization from solutions, which 

 probably became suddenly supersaturated on relief of pressure. 



(3) The conditions of stress, which resulted in rupture, can only be referred 

 to a contraction of the deposit under the general desiccation that attended its 

 final consolidation. No open horizontal crack of the extent of these veins 

 could have existed, and crystallization must have kept pace with contraction. 



(4) The normal attitude of the fibres to the stratification, the slight 

 upward projection of some top fibres, and the sheltering effect of the nodules 

 on the region immediately below them are points which suggest that the 

 ' beef ' crystallized from solutions, the movement of which relative to the 

 matrix was vertically downwards. 



With regard to the relative age of the ' beef '-veins, there is 

 the following evidence : — ■ 



(a) The time of formation of the veins was later than that of the calca- 

 reous nodules, as is shown by the splitting of the veins by the nodules, and 

 the sheltering effect that the latter exerted. And, even if the concretions be 

 regarded as contemporaneous with sedimentation, that phase of calcium- 

 carbonate deposition which is represented by the ' beef ' is secondary. 



(b) The ' beef '-veins are earlier than the Lias folding and faulting, for they 

 also are involved in the movements. 



1 Min. Mag. vol. xix (1920) pp. 77-95. 



