Vol. 70.] OF DURHAM MAGNESIAS LIMESTONES. 253 



A certain segregation of gypsum is apparent in these rocks, where 

 it often occurs in small isolated masses. 



Edward Wilson l quotes two evidently reliable analyses of 

 Magnesian Limestones associated with gypsum, at depths of 154 

 and 193 feet respectively below the salt-bed : one is dolomitic, 

 and the other calcareous. The latter may be in the concretionary 

 series ; but, in any case, it seems to point to a calcareous segre- 

 gation prior to the removal of sulphates from the rock. 



In every case in these borings, the rock seems to have been hard 

 and compact through impregnation with gypsum, and no mention 

 of powdery or ' marly ' material is made in the records. 



The formation of botryoidal concretions can scarcely be taking 

 place at the present day, but those of the second type may very 

 probably be still in process of formation, and certainly mechanical 

 dedolomitization is proceeding apace. 



Under existing conditions, the separation of the calcite and 

 dolomite in the Magnesian Limestone seems to be entirely a 

 question of differential solution of these two minerals. I have 

 not been able to trace the slightest evidence of any leaching-out 

 of magnesium carbonate from the dolomite of the rock, leaving 

 calcite behind it. However, the possibility of small quantities 

 of dolomite having been removed by solution is not to be 

 denied. The process is doubtless assisted primarily by the inherent 

 tendency of the constituents of this and other ancient sedimentary 

 rocks to revert to a highly - crystalline condition. A certain 

 loosening of dolomite-grains would take place in such a rock as 



indicate that heat generated by the movement had some effect in assisting the 

 segregation of calcite. It is especially well seen at a small fault cutting the 

 Flexible Limestone, on the coast-section between Ryhope and Sunderland. 

 There is no evidence of contraction, as the bed maintains a uniform thickness ; 

 but the dolomite is gradually replaced by calcite, which first appears as 

 calcareous granules along the lines of beading, associated with a very fine 

 incoherent yellow powder. The granules become more numerous and finally 

 coalesce, until they form a hard mass still retaining the bines of bedding and, 

 to some extent, the fissile nature of the original flexible rock. This hard 

 calcareous rock becomes highly brecciated at the fault-line, and all 

 trace of bedding is thus finally obliterated. All this change from flexible 

 dolomite to a calcareous brecciated mass takes place within a space of 100 

 feet. The initial and final stages were chemically examined, and the results 

 are tabulated below : — 



Flexible Limestone. Hard calcareous rock. 



Insoluble residue 0*88 1*04 



Fe 2 0, 0-61 0-19 



CaCO, 55-01 88-56 



MgC0 3 43-78 10-92 



Totals 100-28 100-71 



Calcite 2-88 75-55 



Dolomite 95'91 23'93 



1 Q. J. G. S. vol. xliv (1888) p. 763. 



