Vol. 70.] OF DURHAM MAGNESIAN LIMESTONES. 257 



A separation of calcareous material along lines, presumably of 

 tension or weakness, in the Magnesian Limestone frequently leads 

 to an appearance of breceiation. The process in an incipient stage 

 can be seen in many thin sections of dolomitic rocks, showing no 

 trace of breceiation, when stained with Lemberg's mixture, as thin 

 calcareous lines passing in various directions. The calcareous ribs 

 thus induced seem to act as nuclei for further segregation, and 

 gradually to divide up and isolate angular fragments of the rock, 

 which by the withdrawal of interstitial calcite become reduced to 

 an incoherent dolomitic powder. In some cases, I have seen cal- 

 careous ribs, obviously of later origin than the main development 

 of the cellular structure, passing straight through both the spongy 

 cellular material and a dolomitic fragment embedded in it. 



Many, therefore, if not all of these structures may, in my opinion,. 

 be classed under the category of pseudo -breccias. 



An alternative view that these angular structures are of clastic 

 origin deserves notice. At the outset, it will be clear that the rock 

 whence the} 7- were derived must have been in a very different con- 

 dition of hardness, through impregnation with gypsum or calcite, 

 from its present condition. The smallest pressure now suffices 

 to reduce to powder either the original unbrecciated bedded rock 

 adjacent to it, or the fragments enclosed in the cells. 



A possible mode of origin is some such process as the following. 

 In the Magnesian Limestone found by boring beneath the thick 

 anhydrite-bed at Hartlepool already mentioned, occurred angular 

 breccias of a hard, dense, highly-dolomitic and gypsiferous rock. 

 The fragments were slightly separated, and cemented by secondary 

 gypsum derived from hydration of the overlying anhydrite. This 

 secondary gypsum might become replaced by calcareous material 

 derived from other parts of the rock. A subsequent complete 

 leaching-out of more soluble constituents, including all the gypsum 

 from the enclosed fragments, would result in a calcareous matrix 

 enclosing angular masses of more or less compact dolomite, similar 

 to that which occurs in the typical cellular rock. 



VI. The Insoluble Residues. 



The residues of so problematical a formation as the Magnesian 

 Limestone merit a special investigation, and only a few points can 

 be mentioned here. 



The comparatively large residue found in the dolomite embedded 

 in anhydrite (No. XLIX) is difficult to account for. in view of its 

 absence in the adjacent anhydrite, 1 and would seem to indicate 

 that much of the quartz in the Magnesian Limestone is of non- 

 detrital origin. Certainly, the quartz-grains enclosing pyrite- 



1 Several lumps of anhydrite were dissolved up without any residue being: 

 found. 



