﻿Vol. 66.] 



METALLOGENT OF THE BEITISH ISLES. 



291 



Dec. Gr. 



are small and carry insignificant quantities of ore. It is most 

 probable, therefore, that the invasion of the dykes was accompanied 

 by heated waters and resulted in an invigorated underground 

 circulation, by means of which the ores, which had long previously 



been deposited in the 

 Pig. 3. — Section of Foxdale Vein, veins, were taken into 

 820-fathom stores east, January solution, re-arranged, and 

 1909. re-deposited in their pre- 



sent position alongside of 

 or impregnating the dykes. 

 The phenomenon is, then, 

 of secondary origin. 



Lastly, Mr. J. B. Hill 

 and Mr. J. H. Collins have 

 concluded that the lead and 

 zinc veins of Cornwall and 

 Devon are of Tertiary age. 1 

 The chief reasons advanced 

 for this conclusion are 

 that the vein-fissures trend 

 north and south, parallel 

 to faults and dykes of 

 Tertiary age in the Scottish 

 Highlands, and that cer- 

 tain north-and-south faults 

 in the east of Devon ex- 

 tend into and cut the 

 Chalk. 2 This, however, only 

 proves that post-Cretaceous 

 movement has occurred along the fissures, which latter may be 

 much older. These fissures, moreover, are not ore-bearing. It is 

 clear that the lead and zinc veins of the district are younger than 

 the tin-copper veins which they intersect. This is indicated by 

 the differences in vein-filling rather than by the displacement of the 

 one set of fissures by the other. At the same time, the orientation 

 of the two sets of fissures and their relation to the tectonics of the 

 area indicate that both sets have probably been formed by and 

 during the Armorican stresses. Both sets of fissures, in short, form 

 a conjugate system, and, if not contemporaneous, they are at all 

 events not widely different in age. The ore-deposits of Cornwall 

 and Devon are believed to be all of Armorican age, but deposited 

 as two successive phases, namely, an earlier tin-copper phase and a 

 later lead-zinc phase, appearing when the eruptive after-effects 

 were less intense. 



1 J. B. Hill, ' Plutonic &• other Intrusive Eocks of West Cornwall in their 

 Kelation to the Mineral Ores ' Trans. Koy. Geol. Soe. Cornwall, vol. xii (1901- 

 1905) p. 599 ; J. H. Collins, chapt. v of the ' Origin & Development of Ore- 

 Deposits in the West of England' Journ. Koy. Inst. Cornwall, vol. xiii, pt. 2 

 (1896) pp. 283 et seqq. 



2 H. T. De la Eeche, ' Geology of Cornwall, Devon, & West Somerset * 

 Mem. Geol. Surv. 1839, p. 311. 



Dec. Gr 



[Dec. Gr. 



Q 



Q Brecciated Q 

 ore on x - 

 footwall 



= Decomposed granite ; 

 = Quartz -veins ; the black 

 patches represent galena.] 



