196 MR. C. T. TRECHMANN ON A ilASS OF ANHYDRITE [June I913, 



In some thinly-bedded limestones they are arranged in layers 

 on the planes of bedding, as though some crystalline substance 

 had been thrown down rapidly from a concentrating solution as 

 formless plates and aggregates on the sea-bed, to become covered 

 up by the next layer of sediment, and only to be removed again 

 by solution long after the solidification of the deposit. I find 

 that the rock which encloses these structures is always of a highly 

 -magnesian character (45 per cent, of magnesium carbonate). 



These structures can be compared with crystals of anhydrite 

 that occur in several pieces of unaltered Magnesian Limestone in 

 the lower part of the anhydrite-bed at Hartlepool. In this case 

 the pieces have been cut through by the boring-tool, giving a 

 -surface which has been exposed to the atmosphere for 24 years. 

 The anhydrite has been changed to gypsum and dissolved away, 

 .the result being a fretted surface of limestone with stellate spaces 

 similar to, but generally on a smaller scale than, those in the 

 Magnesian Limestone at the outcrops. 



Stellar aggregates of gypsum are seen in masses of that material 

 dredged from Hartlepool Harbour. They are obscure structures, 

 and may be pseudomorphic after anhydrite. 



J. W. Kirkby * refers to the pitted surface of several of the 

 limestones of the Brotherton Beds and small-grained dolomites 

 -of South Yorkshire, which seems to be an analogous structure ; 

 and makes some remarks upon the curious false bedding of parts 

 of the Magnesian Limestone. He doubtfully refers the former 

 structures to 



* some peculiar concretionary action, analogous to that which gave the Upper 

 Limestone of Durham its remarkable structures.' (Op. cit. p. 291.) 



(3) The apparent former presence of lenticular inclusions of 

 soluble substance. This is well seen in some of the thinly-bedded 

 limestones of the Upper Middle Division, exposed on the shore south 

 of Blackball Rocks. In the cliff-section, the strata present a series 

 of undulations, and are very loosely compacted along the planes of 

 bedding. On the foreshore, however, the beds are seen in the 

 form of low irregular domes 2 of more or less circular outline, 

 measuring as much as 10 feet in diameter. Several examples are 

 broken into by the sea, and present every appearance of having 

 •enclosed a lenticular mass of some easily-removed material, either 

 powdery limestone or some soluble constituent. J. W. Kirkby 3 

 notices that in the flaggy limestones of the Brotherton Beds of 

 South Yorkshire 'the surface-planes are generally a little apart.' 



Whether the sulphates in the Magnesian Limestone were 

 originally all deposited as anhydrite, or partly as gypsum, is not 

 precisely an easy question to decide. The evidence seems to be 

 in favour of complete anhydrite deposition, such as might be 

 expected in a sea charged with magnesian salts. 



1 Q. J. a. S. vol. xvii (1861) p. 290. 



2 This feature was first noticed by my friend, Mr. Gr. T. McKay, M.Sc. 



3 Op. jam cit. pp. 289-90. 



