218 ANHYDRITE-MASS IN THE MAGNESIAN LIMESTONE. [Junei9I3, 



more purely marine conditions prevailed, is largely due to conditions 

 of deposition, the Middle and Upper beds were deposited iu a sea 

 more completely isolated from the oceans, and undergoing con- 

 centration, but never complete desiccation, until perhaps the 

 deposition of the salt-measures. This view is entirely supported 

 by evidence deducible from the nature of the fauna. 



(9) The distribution of the fauna in the Middle Limestones was 

 largely influenced by the quantity of sulphates present in the sur- 

 rounding waters in the Permian Sea. The highly fossiliferous 

 Shell-Limestone indicates an area of comparatively sulphate-free 

 deposition, where the fauna consequently flourished and influenced 

 the sedimentation by its remains. The present distribution of the 

 Shell-Limestones indicates the possibility that current-action in the 

 Permian Sea was the determining cause of the comparative absence 

 of sulphates. 



(10) The increasing deposition of sulphates prevailing towards 

 the top of the Shell-Limestone brought about a gradual extinction 

 of the fauna, with a progressive dwarfing of those forms which 

 survived, until the Uppermost Shell-Limestones yield a fauna 

 which, though still typically Middle, yet approaches in character 

 that of the brachiopod-free Upper Limestones. 



EXPLANATION OF PLATE XXII. 



Map of the Hartlepool area, showing the position of the anhydrite and the 

 Magnesian Limestone, on the scale of three-quarters of an inch to the 

 mile, or 1 : 84,480. 



Discussion. 



Dr. J. W. Evans commented on the interest and importance of 

 the occurrence of anhydrite and gypsum in the Magnesian Lime- 

 stone, either disseminated through it, or forming lenticles of 

 various dimensions, in striking similarity to what was met with 

 in the same formation in Germany. On the reading of Dr. Woola- 

 cott's valuable pamphlet on the rocks of Tyneside before the 

 Geologists' Association last May, and afterwards during the excur- 

 sion to that district, the speaker had insisted that the true explana- 

 tion of the brecciated structure of the Magnesian Limestone in the 

 neighbourhood of Sunderland was the former presence of gypsum, 

 representing original anhydrite, which was afterwards dissolved 

 out; and that the cellular and porous condition, which likewise 

 resulted from the solution of the gypsum, favoured the formation 

 of the concretions for which the locality is famous. In times 

 when the rock was saturated with moisture, the excess of car- 

 bonate of lime over carbonate of magnesia would be dissolved, 

 to be precipitated again in concretionary structures in times of 

 desiccation; while the residue, containing lime and magnesia, 

 approximately in the proportions of dolomite, would resist solution 

 and remain as that fine powdery material which is found in the 

 cavities when they are first broken open. This had been only a 

 speculation, but the Author had shown that it was founded on fact. 



