﻿Yol. 66.] METALLOGENY OF THE BRITISH ISLES. 283 



other, have been sufficiently worked out to give a basis for the 

 determination of the metallogenetic epochs. It is necessary here T 

 however, to pass over the minor periods of crust-movement with 

 their attendant vulcanicity. Thus the complex events of pre- 

 Cambrian time may be regarded as condensed into the disturbances 

 and intrusions immediately preceding the Cambrian subsidence. 

 Likewise the several Palaeozoic epochs of vulcanicity will be repre- 

 sented by the two major periods : that is, Caledonian and Hercynian. 

 "With this understanding, the major epochs of disturbance in the 

 geological evolution of the British area are as follows : — 



1. Pre-Carubrian (Huronian of Marcel Bertrand l ). 



2. Post-Silurian (Caledonian). 



3. Post-Carboniferous (Hercynian). 



4. Post-Cretaceous and early Tertiary. 



To the pre - Cambrian Epoch belong the older granites and 

 gneisses, with their associated rocks, which accompanied the fold- 

 ing and uplift of the pre-Cambrian rocks of the Highlands. The 

 Caledonian Epoch, with the elevation and folding of the Lower 

 Palaeozoic formations, was marked by the intrusion of a widely 

 distributed series of granite masses, forming a denned petrographic 

 province, and including the ' newer granites ' of the Scottish High- 

 lands and the Grampians, the Galloway and other granites of the 

 Southern Uplands, the granites of the Lake District and the Isle of 

 Man, the Newry and Leinster granites, the Donegal and Gal way 

 granites, and probably the hornblende-granite of Leicestershire. 

 The Hercynian Epoch was marked by uplift and disturbance of the 

 Devon o- Carboniferous as well as the older rocks/while intrusions 

 of this age are exposed in the Cornish area. Finally, the uplift at 

 the beginning of the Tertiary was followed by an outburst of 

 vulcanicity, with its centre in the north-west of the area. 



III. Metallogenetic Epochs in the British Area. 



It is now necessary to consider each of these epochs in turn, in 

 order to examine the evidence as to the age and relations of the 

 various groups of ore-deposits in the area. 



Pre-Cambrian Epoch. — A deposit belonging to this division 

 is the tin-bearing magnetite occurring as a segregation in the 

 Archaean granite-gneiss of Cam Chuinneag (Ross-shire). 2 Its 

 content of tin oxide (up to 17 per cent.) marks it as an unusual if 

 not a unique mineralogical occurrence, but genetically it belongs to 

 the type of the magnetic iron-ore segregations of Scandinavia and 

 of the Adirondacks. There are doubtless other deposits in the 

 metamorphic complex of the Scottish Highlands which should be 

 referred to the pre-Cambrian, but our knowledge of the scattered 

 ore-occurrences of this area is very incomplete. 



1 ' Sur la Distribution Geograpkique des Roches Eruptives en Europe ' Bull. 

 Soc. Geol. France, ser. 3, vol. xvi (1888) p. 576. 



2 ' Summary of Progress Geol. Surv.' for 1903 (1904) p. 58. 



