Dolomitization in the Carhoniferous Limestone. 63 



If considerable leaching has occurred in dolomitic material of 

 uniform composition, then 



percentage of MgCO, _ ^ constant (approximately), 

 porosity 



This holds true whether later infiltration has introduced calcite, 

 iron oxide, etc., or not, since the introduction of such materials 

 decreases both the proportion of MgCOg and the porosity. In the 

 . case of the dolomite of Masson and Upperwood, 

 percentage ofMgCO., 

 porosity 

 gives a fairly constant figure, and the inference is that the 

 dolomitization was fairly uniform prior to leaching. But in the case 

 of the Gratton to Winster dolomitic limestones having a variable 

 proportion of MgCO,,, a similar constant is not obtained though the 

 various specimens are obviously porous and give considerable, 

 porosity results, hence leaching in originally variable dolomitic 

 material is indicated. 



The field relations and inherent structures of both the Winster 

 and Matlock materials prove conclusively the subsequent nature 

 of the dolomitization. Palseontological features yield no satisfactory 

 evidence. The results of chemical analyses support the theory of 

 subsequent alteration in the case of the Winster area, but not so 

 definitely with regard to the Matlock material. That magnesian 

 ground waters of different dates have produced the metasomatism 

 is, I think, the most satisfactory solution, since marine influences 

 or those of pneumatolysis are not indicated by the evidence. While 

 such appears to be the mode of magnesiation, the actual source of the 

 magnesian factor remains problematical. The Permian Magnesian 

 Limestone, which probably extended much further westwards than 

 its present outcrop, may have supplied the magnesium, but 

 suggestions of this kind can be only of a speculative nature. 



EXPLANATION OF PLATE IV. 



FIG. 



I. — Dolomitic limestone, escarpment east of Winster. X 23. Generally 

 rather allotriomorphic. Coral structures (left) show larger dolomite 

 crystals than those in the matrix. Other parts of this slide exhibit this 

 feature reversed, larger crystals in the matrix. 



2. — Transition limestone, quarry opj)osite the Miners' Standard, Winster. 

 X 21. Sponge spicules form a considerable part of the rock. 



3. — Dolomitic limestone, D2 beds, Gratton Dale. X 23. The matrix (left) 

 consists largely of dolomite having zonal inclusions of limonite, while the 

 coral structures (right) are in dolomite free from such limonite inclusions. 

 RecrystaUized calcite occurs in the spaces within the coral. 



4. — Transition limestone JugholeWood, West of Matlock. X 23. Matrix 

 mainly non-recrystajlized. 



(To be continued.) 



