Dol omit izat ion in tJie Carboniferous Limesione. 109 



limestone between Manystones and the region around the Eainster 

 Rocks, less than 1 mile distant. 



Faunal evidences prove that the bottom of the basal dolomite in the 

 district as a whole is not a true stratigrajjhical plane of separation. 



Evidences obtained from exposed vertical transitions also tell 

 against the theory of contemporaneous origin. In a field west of 

 Manystones, several yards of the " basal dolomite " are seen resting 

 on unaltered limestone. The junction, though sharply marked, does 

 not conform accurately to the bedding. At certain places irregular 

 metasomatic patches invade the limestone below. Thin sections of 

 the limestone in contact with dolomite do not show any dolomite 

 rhombohedra, hence the metasomatism, though irregular, ceases 

 abruptly. 



The small exposure near the roadway north of the Rainster Rocks 

 shows a few feet of dolomite sharply separated from lunestone 

 below. Thin sections of the limestone in contact with dolomite 

 show that here dolomitization, imperceptible megascopically, has 

 affected the limestone on a minute scale. Well-marked dolomite 

 rhombohedra occur in both organisms and matrix, the latter being 

 partly recrystallized (PI. 2, Fig. 2). The degree of magnesia tion in the 

 limestone immediately below the basal dolomite is thus varied in 

 the area, and ranges from megascopically apparent epiphyses to the 

 entire absence of magnesium carbonate as shown by microscopical 

 sections and chemical analyses. Iri the workings of the Golconda 

 Mine ^ the basal dolomite is persistently separated from the lime- 

 stone below by a sharp boundary which conforms to the bedding. 



A striking feature which lessens the value of evidence from the 

 vertical passage seen in these workings is the occurrence of a " way- 

 board " of clay separating the dolomite and limestone. The inference 

 that the limestone below may have been protected by this clay 

 from metasomatism appears legitimate. In a cutting at Longclifie 

 station and in quarries near LongclifEe pseudo-interbedding effects 

 similar to those of the exposure near Manystones are seen. 



The higher vertical transition of the upper dolomites into lime- 

 stone is not seen anywhere in the whole district between Roystone 

 and Wirksworth. It is to be regretted that the passage material 

 has either been removed by denudation as at Harborough and 

 Rainster, or has been let down by faulting as at Griffe Wood and 

 north-west of Longcliffe. Towards Wirksworth, where the sequence 

 ascends into higher D^, lateral transition has occurred. 



Lateral Boundaries. — The eastern boundary of the Harborough 

 and Carsington dolomitic mass is partly faulted and partly of the 

 nature of abrupt lateral transition into limestone. In the region 

 of the Grifie Wood a sharp straight line with dolomite on one side 

 and limestone on the other marks the position of the Griffe Fault. 

 I have not found any traces of magnesia tion in the limestone thus 



^ For facilities to examine these workings T am indebted to the courtesy 

 of the manager, Mr. E. Weightman. 



