Vol. 70.] OF DURHAM MAGNESIAN LIMESTONES. 265 



could not properly be defined as dedolomitization. He enquired 

 whether the Author did not think it possible that the oolite-grains 

 had been originally formed of carbonate of lime and subsequently 

 dolomitized by the concentrated sea-water, so that the calcite that 

 formed the core of some grains was original. 



He understood from Prof. Kendall that the first suggestion that 

 the cavities in the Magnesian Limestone might be explained by the 

 removal of gypsum in solution was made by Mr. Ernest Guv. 1 



The Author, in reply, said that the concentric material sur- 

 rounding the nuclei in the oolites is always practically pure 

 dolomite ; in fact, it is among the purest dolomite that occurs 

 in the formation. No trace of originally calcareous oolites was 

 noticed. The nuclei of oolites are sometimes dolomite and some- 

 times calcite, and this calcite may in some cases be of secondary 

 origin. The evidence is in favour of the dolomite of the oolites 

 having been deposited as such. 



Referring to the use of the word dedolomitization, the only 

 term that he could suggest to define the process of washing- 

 out of powdery dolomite was mechanical dedolomitization. 



He thought that the former presence of sulphates in the English 

 Magnesian Limestone may have been suspected by many observers, 

 but he could not say by whom the idea was first expressed. 



[Sulphate-bearing Magnesian Limestones were found many years 

 ago in sub-Triassic borings in the Tees Valley, and have been more 

 recently recorded in connexion with the sinkings to the concealed 

 coalfield of Yorkshire and Nottinghamshire. The analogy between 

 these gypsiferous dolomites and those at the surface can scarcely 

 have escaped the attention of earlier observers. In Germany the 

 Zechstein is still gypsiferous in many surface-exposures. — C. T. T., 

 Hay 26th, 1914.] 



1 Trans. Leeds Geol. Assoc, pt. xvi (1911) pp. 10-15. 



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