Vol. 69.] IN THE MAGNESIAN LIMESTONE AT HARTLEPOOL. 189 



pseudomorphic aggregates found in several beds of Magnesian 

 Limestone at the surface. 



: The gypsiferous limestones underlying the anhydrite represent 

 an intermediate stage between the above-described rock and the 

 completely gypsum-free rock as it is exposed at the surface. They 

 are in a condition comparable with the limestones underlying the 

 Red Beds south of the West Hartlepool fault, and in other areas 

 where the Magnesian Limestone has been partly protected. Re- 

 moval of the protective covering, and exposure to atmospheric 

 agencies, with consequent leaching-out of the gypsum and oxidation 

 of the ferrous compounds, would bring about a complete change in 

 the nature and appearance of these rocks. If accompanied by 

 pressure of overlying strata, the consequent collapse and degra- 

 dation of a bed, such as D, would probably complete the 

 obliteration of the already scanty traces of organic remains. 



The anhydrite cores have now been lying in the open air for 

 24 years, and are considerably weathered and pitted, the mineral 

 first becoming changed to gypsum and then dissolved away. In 

 situ, however, the hydration seems to take place much more rapidly 

 than the solution of the gypsum. The result is that at the base 

 the anhydrite mass was found more or less completely changed 

 to gypsum for 4 or 5 feet. 



Both in this part, and in the underlying gypsiferous limestones, 

 some instructive features which seem to throw light on the present 

 condition of the formation were observed. They can be well 

 observed if the specimens are left under running water for some 

 days, in order to remove the superfluous gypsum. The following 

 may be mentioned : — 



(1) Fragments from the lower part of the anhydrite-bed enclosing streaks of 

 Magnesian Limestone. The anhydrite is almost completely changed to gypsum 

 throughout its mass. On the weathered surfaces glistening crystals of anhydrite 

 are seen in layers or irregular masses standing out from the more rapidly 

 dissolving gypsum, while the Magnesian Limestone is becoming liberated in soft, 

 friable fragments taking on in part a yellow powdery character, very different 

 in appearance from the streaks of rock in the undisturbed anhydrite above. If 

 left in water the mass disintegrates, throwing down the fragmentary Magnesian 

 Limestone. The complete removal of sulphates from a thick bed such as this 

 suggests an origin for some of the irregular masses of soft comminuted breccia 

 which are seen at various horizons in the Magnesian Limestone. 



(2) Masses of brecciated limestone recemented with gypsum. The brecciation 

 of the Magnesian Limestone evidently begins before the complete removal 

 of gypsum, and before the rock is chemically altered, no doubt through the 

 strains set up in the mass of the formation. The hard, brittle, unfossiliferous 

 rock (B) is found in part very much broken up into angular fragments of 

 all sizes down to extremely minute powdery particles. The fragments are 

 more or less displaced vertically and are slightly separated, the intervening 

 spaces being occupied by secondary gypsum cementing the otherwise un- 

 disturbed brecciated fragments. After having been placed under running 

 water for some time the mass becomes disintegrated. 



(3) Brecciated fragments of hard brown limestone associated with gypsum. 

 Aggregates of dolomite crystals are seen on the surfaces of the fragments 

 as the gypsum is dissolved away, and also lying loose in the secondary gypsum. 



