﻿294 me. a. if. finlayso:n t on the [May 1910,. 



The Hercynian Epoch of ore-deposition was ushered in by earth- 

 movements and deep-seated intrusions, the latter being exposed 

 only in Cornwall and Devon. The igneous masses, on cooling, gave 

 off gas-aqueous emanations which rose into the fissures, carrying 

 with them in solution the various ores which had been segregated 

 from the magma. There were first deposited, in the deepest zones 

 and in close proximity to the intrusions, ores such as cassiterite,. 

 with chalcopyrite, pneumatolytic minerals, and some of the rare 

 metals. As the solutions rose higher, they were largely depleted 

 of tin with its special solvents and carriers. Gold and silver, 

 chalcopyrite, pyrite, arsenopyrite, and pyrrhotite, with some galena 

 and blende, were then deposited. Gradually the solutions were 

 robbed of the less soluble ores, and carried a preponderance of the 

 more mobile lead and zinc ores, which attained a maximum 

 development in the Carboniferous Limestone zone. As the solutions 

 circulated in still higher zones, they became impoverished and 

 diluted by admixture with descending surface-waters, and the 

 scattered ore-occurrences in the Lower Mesozoic rocks represent the 

 last stage in the sequence of events which began at the end of 

 the Carboniferous. 



The zones of deposition of the ores, as deduced from an examina- 

 tion of the British occurrences, are, then, as follows :— 



1. The deep pneumatolytic zone, with tin, tungsten, etc., and some 



copper. 



2. The intermediate zone, with copper, gold, and some lead and 



zinc. 



3. The upper zone, containing chiefly lead and zinc. 



The conception of successive vertical zones of ore-deposition 

 has been elaborated by other workers along different lines. 

 Mr. Waldemar Lindgren has mapped out the zones by a con- 

 sideration of the physical conditions attending deposition at different 

 depths, 1 and Mr. J. E. Spurr has obtained similar results from an 

 examination of the relations between ' petrographic and metallo- 

 graphic provinces.* 2 His classification is given below : — 



' 1. The pegmatite zone, containing tin, molybdenum, tungsten, etc. 



2. The free gold-auriferous pyrite zone. 



3. The cupriferous pyrite zone. 



4. The galena-blende (galena usually argentiferous) zone. 



5. The zone of silver and also much gold, usually associated with 



metals which combine with them to make substances which are 

 highly mobile, and account for the relatively elevated position 

 of the zone, i. e. antimony, bismuth, arsenic, tellurium, and 

 selenium.' 



This last zone is not represented in the British area. Otherwise 

 it will be seen that Mr. Spurr's subdivisions are quite applicable to 

 the British Hercynian province, and are in general agreemeut with 

 the subdivisions deduced from an examination of this province by 

 itself. 



1 ' Economic Geology,' vol. ii (1907) p. 105. 



2 Ibid. p. 781. 



