part 3] CHELLASTON GYPSUM BRECCIA. 185 



the action of gravity. 1 In this connexion it may he recalled 

 that in my estimation the gypsum of the Chellaston pots retained 

 a soft and sticky consistency for a long time. The folds in the 

 contorted layers are about 1 foot long and from 6 to 8 inches in 

 amplitude. 



The ' bird's-eye ' may be due to segregation of white gypsum set 

 up at many different spots in a soft grey gypsum that had been 

 deposited in unbroken sequence with no definite laminar or strati- 

 form arrangement (compare, for instance, clay with shale). The 

 absence of any sign of angularity in the white portions, or of broken 

 cover, is against the theory that ' bird's-e} r e ' is due to brecciation. 

 The white fragments may be compared with the rounded lumps at 

 the bottom of the Chellaston pillars, and in the coarsestone. Bird's- 

 eye occurs sometimes in the pink and white rock of other areas : 

 for example, Cocklakes and Aston Glebe. 



Fig. 5. — Diagrammatic section showing the structure of 



the gypsum at Kirlcby Thore Quarry. 



Feet. 

 ME--^ — 1 



t "1 



;-15to 17 





](i) & (iv)= horizontal bands ; (ii) = ' bird's-eye ' ; (hi) — folded bands.] 



The ' cap ' and the ' black post ' are similar one to the other : 

 that is, mixed, stratified sediment and gypsum, the latter often in 

 stellate clusters arranged along the bedding-planes. The cap is 

 undisturbed, and any signs of disturbance in the overlying 

 marls can be fully explained by collapse of - these beds into the 

 solution-channels and potholes formed by subsoil water in the cap. 

 In Derbyshire such disturbances in the overlying beds have 

 probablv been accentuated bv the differential shrinkage of the 

 more irregular seam during consolidation. 



At Thistle Plaster Quarry, a short distance to the west, the same 

 seam is 16 feet thick. There is little or no 'bird's-eye* here, and 

 most of the colour-bands are horizontal. This is consistent with 

 the theory that the bed was laid down in successive thin sheets, 

 for one part might suffer contortions under the action of gravity, 

 or even disturbance by current action, while another, a little 

 farther off, might be unaffected. Disturbances in the marls and 

 drift overlying the seam are again obviously due to the action of 



1 B. Smith, ' Ball or Pillowform Structures in Sandstones ' G-eol. Mag. dec. 6, 

 vol. iii (1916) pp. 146-56. 



