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ME. A. M. FINLAYSON ON OEE-DEPOSITION [May I9IO, 



In conclusion, while the vein-solutions were at first and pre- 

 dominantly of truly deep-seated origin, it seems necessary to 

 recognize that underground waters of meteoric origin must have 

 played an important part in the distribution and final deposition of 

 the ores in the upper strata of the limestone-tracts. 



importaut deposits which have yielded 



Pig. 2. — Replacement of limestone by quartz 

 and galena: Halhyn Mine (Flintshire). 



IV. Metasomatic Peoce.sses. 



The relative part played by replacement on the one hand, and 

 by the filling of fissures on the other hand, is variable, but on the 

 whole the latter process has been dominant in the districts examined. 

 The vein-structures generally show the characters due to filling, 

 except where local silicification and carbonization have occurred. 

 Replacement has played a minor part in the formation of the 



the bulk of the ores. 

 Interesting metasoma- 

 tic processes are, how- 

 ever, seen under the 

 microscope. 



Specimens of lime- 

 stone from veins at 

 Halkyn show, first, 

 re crystallization with 

 the formation of vein- 

 lets and coarse plates 

 of calcite. This has 

 been followed by the 

 introduction of galena, 

 blende, and quartz, all 

 replacing the calcite. 

 The formation of 

 scattered quartz-gran- 

 ules is characteristic, 

 the quartz being most 

 abundant where sul- 

 phides are forming. 

 Blende occurs in ir- 

 regular rounded grains. 

 Galena appears first in 

 small crystals, uniting 

 into strings which pene- 

 trate the calcite. These 

 gradually form a net- 

 work, and finally a 

 solid mass of granular 

 ore, enclosing a few 

 residual patches of 

 calcite (fig. 2). The silicification appears to be an accompaniment 

 of the formation of the sulphides, and the absence, in the latter, of 

 defined crystal outlines, such as appear when they are deposited 

 in open cavities, is very characteristic. 



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[The quartz appears as scattered grains (white), closely 

 followed by granular galena (black), which gradually 

 replaces the rock, enclosing residual plates of recrys- 

 tallized calcite and quartz-grains. (Transmitted 

 light : X 50.)] 



