﻿j>84 MR. A. M. FINLAYSON ON THE [May 1QIO, 



Caledonian Epoch. — The ores of this epoch are likewise con- 

 fined chiefly to the Scottish Highlands. The Caledonian granitic 

 intrusions are widely developed in this area, and the ores are often 

 intimately related to these igneous rocks. The common occurrences 

 ■of molybdenite in the Caledonian granites are an example. The 

 segregations of chromite in the serpentine at Unst, in the Shetlands, 

 are Caledonian or older. The deposits of nickeliferous pyrrhotite 

 and chalcopyrite at Glen Esochossan and Craignure, Loch Eyne, 

 adjoining felsite and quartz-porphyry, are border-segregations or 

 contact-deposits of Caledonian age. 1 The pyritous schists, and 

 fahlbands carrying complex sulphides, which are developed around 

 Loch Tay, 2 in the Cowal district, Loch Fyne, and elsewhere, 

 also probably belong here. 3 Lastly, the alluvial gold of various 

 localities in the Highlands, notably the Helmsdale district in 

 Sutherland, and parts of Forfarshire and Perthshire, has been 

 derived from veins in the older rocks, which veins appear to belong 

 to the Caledonian Epoch. Most of the deposits of the Highland 

 area, in short, are provisionally referred to the Caledonian, but 

 •clear evidence of their age is not always forthcoming. Some of the 

 fahlbands may be older, while there are many occurrences of normal 

 lead and copper veins in the area which are probably Hercynian, 

 including those of Strontian, in Argyllshire, and of Islay. The 

 deposits here referred to the Caledonian are, however, of the 

 ' Scandinavian' facies, typically represented by the segregations of 

 oxide and sulphide ores, the pyritic masses, and the fahlbands, of 

 Norway and Sweden, all of which are, in general, of Caledonian age. 



In the Lake District the wolfram-bearing pegmatites in the 

 Grainsgill greisen belong to this epoch, 4 and a few unimportant 

 occurrences of molybdenite and chalcopyrite, associated with 

 tourmaline, represent this phase in the Caledonian granites of 

 Donegal. 



Lastly, the belt of pyritic masses in the Avoca district (County 

 "Wicklow), together with that of Parys Mountain (Anglesey), is 

 urobably of Caledonian age. The deposits occupy zones of shear- 

 ing and crushing in Ordovician slates and phyllites, and are inti- 

 mately related to a well-marked province of varied intrusive sills 

 and dykes of Caledonian age, adjoining the granitic mass of 

 Leinster on the west. The entire development of igneous rocks 

 and associated earth-movements in this area dates from post- 

 Silurian times, and the pyritic ores were probably concentrated, in 

 the first place, by magmatic differentiation accompanying the igneous 

 outbursts and subcrustal stresses, being afterwards deposited by 

 replacement along shear-zones and fault-slip planes. Structurally 

 and genetically, the Avoca masses are in every way analogous to 

 the cupriferous deposits of the Huelva district in Southern Spain, 

 which are likewise intimately related to a complex series of crust- 

 movements and igneous intrusions. 



1 J. B. Hill, ' Geology of Mid-Argyll ' Mem. Geol. Surv. 1905, p. 151. 



2 C. H. Gustav Thost, Quart. Journ. Geol. Soc. vol. xvi (I860) p. 422. 



"' 3 J. B. Hill, loc. supra cit. ; and W. Gunn (& others), ' Geology of Cowal' 

 Mem. Geol. Surv. 1897, p. 287. 



4 A. M. Finlayson, Geol. Mag. dec. v, vol. iii (1910) p. 19. 



