Vol. 69.] IN THE MAGNESIAS LIMESTONE AT HARTLEPOOL. 197 



The Hartlepool boring shows that hydration takes place more 

 rapidly than solution of the gypsum, and therefore the expansion 

 following upon the hydration of the anhydrite might cause much 

 of the very curious internal deformation and crumpling on a 

 small scale seen in many of the thinly-bedded limestones ; this 

 would probably be followed by a collapse and movement in the 

 opposite direction on solution of the gypsum. 



All the samples of normal 'unprotected' Magnesian Limestone 

 that I have analysed contain a small quantity of sulphur ; but in 

 no case does it exceed 0*2 per cent, of sulphur trioxide within my 

 experience, nor have I yet determined its state of combination in 

 the rock. 



The changes consequent upon the unstable solubility-equilibrium 

 -of dolomitic carbonates are very apparent in certain beds of the 

 Magnesian Limestone. There can be no doubt that the intensely 

 porous condition in which many of the beds are left after removal 

 of the sulphates lays them open to these changes ; a fact which 

 should never be ignored in studying the endless forms of segregation 

 and brecciation that this formation affords. 



The changes may be conveniently indicated by the following 

 synopsis : — 



Calcareous Constituent, Original Eoek. Magnesian Constituent. 



Hard, skeletal, spongy, cal-"') Normal, bedded, fine- f Yellow powdery material, 1 

 careous structures, ultimately I grained limestones present as irregular len- 

 coalescing into beds of dull, | or ' roestones.' | ticles or pockets, 

 lustreless, crystalline, massive I Often intensely Frequently containing 



rock of saccbaroidal structure | porous and friable. | dusty cavities or calcite- 

 and all shades of colour, )■ Magnesium carbon- «{ lined geodes. (Contrac- 

 ate 40 to 45 per I tion effects ?) 

 cent. | Magnesium carbonate ap- 



Fossils traceable if I proaching 45 per cent, 

 originally present. | Fossils obliterated in the 

 t_ final state. 



(white, brown, bright yellow, 

 ashen-grey, etc.). 

 Magnesium carbonate to 10 

 per cent. 



Fossils always obliterated. 



The powdery magnesian constituent occurs in all stages of asso- 

 ciation with, or complete rejection by, the calcareous segregations, 

 the result being an endless variation both in appearance and in 

 composition of the rock-mass. 



In all fossiliferous sections it can be observed that, in proportion 

 as the segregation of calcareous structures becomes more apparent, 

 the obliteration of the fossils becomes more complete. 



The beds chiefly thus altered prove to be the eastern and 

 western equivalents of the Shell-Limestone, together with certain 

 beds of the Upper Limestones — that is, limestones which on other 

 considerations are shown to have been originally rich in sulphates. 



The Shell-Limestone, for reasons indicated in § V of this paper, 

 is very much less affected; but the change is by no means absent, 

 especially in the upper beds of that division. 



Siliceous residues are generally identical in character in the 

 various stages of such altered rocks, though I have frequently 



1 Generally described as ' marl ' or ' marly limestones,' a too-often repeated 



