﻿296 mr. a. ]vi. finlayson on the [May 1910, 



EXPLANATION OF PLATE XXIII. 



Metallogenetic map of the British Isles, on the scale of 80 miles 

 to the inch. 



Discussion. 



The President (Prof. W. J. Sollas) remarked that the chief 

 novelty in the Authors communication was to be found in the 

 asserted Armorican age of many ore-deposits hitherto regarded as 

 Caledonian, and expressed the opinion that the evidence was hardly 

 equal to the burden laid upon it. In Glendalough, lead-veins 

 occurred in association with Caledonian granite ; and, pending 

 further investigation, it still seemed probable that most of the 

 Wicklow veins were of Caledonian age. Apart from the rare 

 outstanding instances discussed by the Author, the absence of 

 metallic deposits around Tertiary granite in the British Isles had 

 always seemed a very remarkable fact. 



Prof. Watts stated that, although he had always associated many 

 of the chief metalliferous deposits in Britain with the Caledonian 

 movement, he had been reluctantly compelled to accept the evidence 

 adduced by the Author that the bulk of it was associated with the 

 Hercynian movement. The Author had visited all the principal 

 mining localities in the country, and had based his conclusions 

 partly on evidence gathered on the spot and partly on published 

 accounts of the mines. 



Mr. D. A. Macalister complimented the Author, and referred to 

 his establishment of a connexion between Hercynian movements 

 and the metalliferous lode-deposits of this country. It was difficult 

 to assign Assuring to a particular period, and, as it was certain that 

 Assuring and faulting along Armorican lines of folding had taken 

 place as late as Tertiary times, he asked the Author what limits he 

 would put to Assuring in this direction definitely connected with the 

 Armorican movement. It appeared best to restrict the term to the 

 time during which the movements were actually bringing about 

 their orogenic effects. The termination of the Hercynian and 

 Yariscan movements in Central Europe and in the Armorican of 

 Prance and South- Western Britain was marked by intrusions of 

 granite, some of which gave rise to tin and copper deposits. Since 

 the intrusion of granite in Cornwall and Devon there had been 

 no disturbances, except those of Assuring and slight overthrustiug. 

 The tin and copper lodes were roughly parallel with the folding of 

 the sediments, but as they traversed the granite and were Ailed 

 with minerals of pneumatolytic origin, they were evidently conAned 

 to the period immediately following the consolidation of the granite 

 or to Armorican-movement times. But in the case of the lead-lodes 

 there were three points which negatived a similar conclusion — 

 {a) They traversed the Armorican folding at right angles ; (b) the 

 minerals were genetically distinct from those formed by pneuma- 

 tolysis ; and (c) they were of later age than the tin and copper 

 lodes. The only grounds for putting them in the Armorican period 

 would be that no mountain-building movements had occurred in 



