310 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



out-flow of the mineral springs, which must then have been in 

 full operation. There were probably similar conglomerates 

 formed at the same time westward and northward, but if so, 

 they have since been entirely removed by denudation. 



1 0. It is probable that the general schorlaceous and stan- 

 niferous impregnation of the " granitic penumbra," and even the 

 early stanniferous concentrations into cavities, took place before 

 the elevatory strains had been relieved by the rupture of the 

 stratified rocks; and at great depths below the present sea-level. 

 The impregnation of the rock masses with sulphide solutions of 

 copper, lead, and zinc, had also taken place in all probability, 

 perhaps at the time of the basic intrusions above referred to, 

 but the concentration of these substances into the re-opened or 

 later-produced fissures seems to have been effected at much later 

 periods for the most part. The " roots of the mountains " have 

 since been exposed by the subsequent elevation and denudation 

 above referred to.*' 



11. We have seen that the earliest tin deposits are cut 

 through by the newer elvans, whose formation was followed 

 successively by that of our metalliferous fissure-groups, V to 

 VIII. Then the oldest cross-courses, many of them containing 

 lead and iron, were formed and filled (Group IX) and these were 

 followed by the newer E.W. Copper lodes in which tin is nearly 

 or entirely absent ; then came the newer caunters containing 

 little besides quartz, and by this time we have probably arrived 

 at post- cretaceous times. 



That there were N.8. faults of this period is well-known. 

 Thus at Combe Beacon near Combe St. Nicholas, in the Blackdown 

 Hills, is a N.S. fault which carries down the chalk. 



Passing to the south at Wanbrook the lias and greensand 

 are brought into juxtaposition by a displacemment of above 200 

 feet vertical. Other N.S. faults in the same district are known 



* We have clear indications of the great amount of this denudation in Mr. 

 Sorby's observations and experiments already alluded to, and also in the wide 

 extent of the great plane of marine denudation which extends for hundreds of 

 miles around Cornwall, the present peninsula being a mere central strip of the 

 land which formerly existed. 



The full consideration of this immense period is foreign to our present subject, 

 which is limited to that portion of it during which mineral veins were being 

 formed. 



