200 ME. B. SMITH ON THE [vol. lxxiv„ 



there are obvious signs of movement, especially along the boundaries 

 of the cores of purer gypsum in the pots, and it interlaces the 

 rocks where they have been disturbed owing to the solution of 

 part of the thicker masses of gypsum. 



Wherever a gap appeared, either horizontal, vertical, or oblique,, 

 the fibre-like crystals sprang from one or other, but usually from 

 both walls, until the scar was healed. The fibres generally grew at 

 right angles to the faces of the cracks, and in most cases we can 

 trace the central joining-line where they met from opposite sides, 

 somewhat like the irregular central junction of the prismatic- 

 columns of a basic igneous dyke. 



It is being formed at the present day. In the Subwealden Mine 

 long silky fibres spring from the pit-props and the fallen blocks of" 

 stone, where water has been drawn up by capillary action and has 

 evaporated at the surface. In the Acorn- Bank Mine it is also^ 

 growing from the roof and on the pack-walls. 



Fairly-thick horizontal veins of fibrous gypsum or satin-spar are 

 common in parts of the Keuper Marl, and at first it seems difficult 

 to understand how the rock could be held apart, while tender fibres 

 (like those of the Subwealden Mine) build a solid rampart across- 

 the gap. These horizontal beds, however, are most common where 

 calcareous and dolomitic beds ('skerries') occur in the marl. 

 During the slow desiccation of the marls, shrinkage would occur 

 vertically as well as horizontally, and gaps might be formed and 

 held open by the overlying girder or plate-like skerries, which 

 might sustain a considerable weight of marl without collapsing. 



Again, a slight differential movement in a horizontal direction 

 between an irregular bed of gypsum and its enclosing sediments, 

 caused by anticlinal or synclinal bending of the rocks — as in the 

 Weald — would be expected to open cracks at the top and bottom 

 of the seam. Percolating waters Avould do the rest. 



I venture to throw out the suggestion that fibrous gypsum 

 probably forms most easily in the presence of car- 

 bonate of lime. In the Subwealden Mine, where fibrous gypsum 

 is growing now, the water must be strongly impregnated with the 

 carbonate, and in the Acorn-Bank Mine there are calcareous beds 

 in the roof. The Keuper Marls are highly charged with car- 

 bonates, and secondaiy fibrous gypsum occurs perhaps most freely 

 in and near the calcareous and dolomitic skerry-belts where the 

 gypsum was originally deposited in selenitic plates binding the 

 marl together. 



IX. Conclusions. 



1. The Chellaston Breccia furnishes a concrete case, affording 

 proof that the gypsum of which it is composed was laid down in 

 its present position as such, and has suffered no appreciable 

 alteration or addition since the time of its original deposition 

 and brecciation. There is no evidence that the rock was ever 

 anhydrous. 



2. The brecciation was due to the instability of the cover over- 



