﻿Vol. 66.~] IN THE LEAD AND ZINC VEINS OF GREAT BRITAIN. 311 



The rhyolitic breccia at Trecastell, Conway, shows similar pro- 

 cesses. It is generally much silicified, cavities and cracks in the 

 rock being filled by coarse quartz crystals, while a fine-grained 

 quartz-mosaic replaces the mass of the rock (fig. 3). Galena and 

 blende occur in the breccia, both replacing quartz, and appearing 

 as irregular rounded grains, which may grow to a considerable 

 size. The galena tends to wrap round the fragments of rhyolite, 

 -which act as nuclei (fig. 4, p. 312). The rhyolite is gradually 

 replaced by a quartz-mosaic, and this in turn by galena (fig. 5, 

 p. 312), the replacement of quartz by the ore following closely on 

 the silicification. 



The replacement of limestone by fluorspar was observed in 



several specimens 



Pig. 3.—Metasomatic quartz, forming a fine- f ™ m ^ e ^ U * C1 ° Se 

 grained mosaic, accompanied by quartz filling ^n ne (-N orth Derpy- 

 a cavity, with comb-structure: Trecastell s ^ ire )- One section 



Mine, Conway. ( Transmitted light : xSO.) showed a dark or- 

 ganic limestone, al- 

 most completely 

 replaced by coarse- 

 grained colourless 

 fluorspar. The fluor- 

 spar has sharp 

 crystal-faces where 

 it abuts against the 

 limestone, and small 

 crystals of fluorspar 

 in the limestone it- 

 self are also gene- 

 rally well formed. 

 The process consists 

 in a general aggre- 

 gation of small crys- 

 tals until the whole 

 of the limestone is 

 replaced, the fluor- 

 spar individuals fre- 

 quently growing on 

 one another in crys- 

 tallographic conti- 

 nuity. The fluor- 

 spar is often followed 

 by galena, with its usual granular structure, replacing both the 

 fluorspar and the residual limestone. In the former it appears 

 first as grains and strings on the edges and along cleavage-planes 

 of the fluorspar, gradually forming larger masses. 



Other specimens from the same mine show replacement of 

 fluorspar by quartz. The quartz appears first along cracks and 

 cleavage-planes, and gradually spreads into the fluorspar as a 

 mosaic. One specimen shows a granular fluorspar formed by re- 

 placement of an organic limestone, patches of organic matter being 





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