part 3] CHELLASTON GYPSUM BRECCIA. 1S7 



calcareous shales and thin limestones that underlie or abut against 

 them. Thus, as in the case figured (tig. 0, p. 186), slickensides 

 may be developed at the contact. The purest gypsum is said to be 

 found behind the masses of limestone that appear to swell up from 

 the floor or descend from the roof. In such cases it is obvious- 

 that the miners have broken through foulstone into a 'pot.' 



In the floor the shales contain nodules of gypsum varying in size 

 from that of a pea upwards. These may be compared with the 

 rounded nodules at the bottom of the Chellaston pots. 



It is clear that in the process of solidification the enclosing 

 shales and limestones pinched the seam together chiefly between 

 the pillars, and after solidification earth-movements have affected 

 all the beds, for they are bent into the form of a shallow anticline. 

 There has consequently not only been readjustment in the seam 

 itself, but the seam as a whole has undergone differential movement 

 relative to the enclosing beds, resulting in the creation of planes of 

 weakness at the top and bottom of the seam. These are now filled 

 with fibrous gypsum, which, according to the miners, occurs as a 

 continuous sheet both above and below the seam ; but. where the 

 seam is thickest and purest, the fibrous gypsum is thinnest, and 

 vice versa. This is what might have been expected. 



IV. Suggested Nomenclature : 'Pillars,' 'Balls,' and 



' Domes/ 



In the foregoing account 1 have used the term ' pillar ' only in 

 reference to those isolated masses of gypsum that are shaped 

 more or less like bowls, and it seems well that it should be thus 

 restricted. There are other isolated masses of different shape that 

 have been variously denominated bullets, balls, etc., which should 

 be distinguished from pillars when possible. 



The rounded, ellipsoidal or bi-convex masses might be called 

 ' nodular masses ' or ' balls,' while those which are the inverse of 

 pillars and are shaped like inverted bowls or plano-convex lenses- 

 resting on their flat faces, might be termed ' domes.' The term 

 ' ball ' could be used in all cases where both shape and origin are 

 indefinite. 



There is no difficultv in understanding how the nodular masses 

 of different sizes may have been formed in series or as isolated 

 units, when we recall the formation of isolated pillars. In 

 Nottinghamshire I have noticed ' domes ' occurring in isolation 

 above the main gypsiferous horizon. 1 The surrounding marls 

 appear both to abut against their convex backs, and to arch partly 

 over them. 



It seems probable that the 'pillar' habit is due to segregation of 

 pure gypsum in successive layers of sediment during a period 

 of increasing mineral deposition ; while the plano-convex ' dome ' 

 habit is due to segregation of pure gypsum during a period of 

 decreasing mineral deposition. 



1 ' Geology of the Melton Mowbray District' Mem. Geol. Surv. 1909, p. 15. 



