﻿Vol. 66. ,] ORE-DEPOSITION IN LEAD AND ZINC VEINS. 313 



scattered through the secondary mineral. The fluorspar has heen 

 followed by quartz, which occurs as scattered grains or aggregates, 

 and at times the quartz crystals form pseudomorphs after cubic 

 fluorspar. 



Specimens from Rotherhope show a mass of crystallized fluor- 

 spar, the cracks in which are occupied by infiltrated quartz. 

 Strings of galena have advanced into the fluorspar by replacement 

 of the quartz, and also by further replacement of the fluorspar. 

 The replacement of fluorspar by galena is also seen in specimens 

 from other districts ; the reverse change was not observed. 



The characteristic metasomatic processes seen under the micro- 

 scope are, then, as follows : — 



1. Eeplacement of limestone by fluorspar. 



2. Do. of limestone by galena. 



3. Do. of fluorspar by galena. 



4. Do. of limestone and fluorspar by quartz. 



5. Do. of limestone by blende. 



Galena is much more active than blende in processes of replace- 

 ment. Quartz and calcite do not replace the sulphides to any 

 extent, but quartz freely replaces both calcite and fluorspar. 



The metasomatic formation of fluorspar is a striking feature. 

 "While the greater part of this mineral has probably been deposited 

 in open spaces, its practical restriction to limestones or calcareous 

 rocks clearly indicates that the calcium has for the most part been 

 derived from the country-rock, the mineral being deposited at 

 times by direct replacement, and at other times after more or less 

 transport. 



It is important to observe that galena always follows fluorspar 

 in replacement, a fact which is of value in determining the order 

 of deposition of the vein-minerals. 



Y. Paragenesis. 



The relations and order of deposition of the ores are easily studied 

 in the field, more especially in banded comby veins and in solution- 

 cavities. The results, however, are frequently conflicting and 

 inconclusive, and the relations may differ in different districts, 

 while anomalous orders of deposition are to be seen in one and the 

 same vein. These anomalies are due to the complex history of the 

 ores in the limestone-districts and to continued re-arrangement and 

 redeposition by circulating waters since the ores were first trans- 

 ported from a deep-seated source. The problems cannot be satisfac- 

 torily approached from field-evidence alone, the relations of the ores 

 in cavity-fillings being often notoriously unreliable. Calcite and 

 quartz show no definite order: they have been deposited both before 

 and after the sulphides, and their relations to the other minerals are 

 therefore variable. In this connexion, mention should be made of 

 some brecciated ores found in several mines. At Foxdale the main 

 vein in the lowest levels is frequently brecciated, the ore consisting of 



