200 Proceedings of the Royal Physical Society. 



rock have been made, nor does evidence exist in favour 

 either of the first or of the second of these propositions. It 

 is certain that the veins were formed after the metamorphism 

 of the original sandstones and argillaceous shales into 

 quartzites and schists. These rocks were metamorphosed 

 before the Old Eed Sandstone period, as breccias containing 

 fragments of schist are found at places where the basement 

 conglomerate of the Lower Old Eed rests in violent uncon- 

 formability on the upturned edges of the metamorphic series. 

 The great faults which traverse the Highland area are very- 

 old ; some of them are later than the Carboniferous period, 

 but most, if not all, are Pre-Miocene, as the great basalt 

 dykes which emanate from the Miocene volcanic plateau of 

 the Hebrides run across them, or sometimes become deflected 

 and follow their course for short distances, showing the fault 

 fissures to have been in existence at the time of the irruption 

 of the basalt. One of these dykes crosses the vein at the top 

 of the Sron nan Golan (see Map). There are abundant proofs 

 that an enormous thickness of rock has been denuded from 

 the Highland area since Miocene times. Some of the dykes 

 cross the country from shore to shore, passing through the 

 tops of mountains over 2000 feet in height, and situated at 

 no great distance from the sea coast. This shows that the 

 valleys themselves have been produced by denudation since 

 the time of the irruption of the basalt, and that in all pro- 

 bability a considerable thickness of rock has also been re- 

 moved from the tops of even the highest hills. If, then, there 

 has been such a vast denudation, the metalliferous veins 

 now laid bare must have been formed at great depths 

 below the surface, where heat and slow chemical pro- 

 cesses could go on quietly, producing the results we wit- 

 ness to-day. It is thus obvious that the parts of the original 

 veins now exposed, whether produced by infiltration from 

 above, below, or the sides, have been formed at depths so 

 great that it is a matter of indifference from which quarter 

 the solutions came. The gaping fissures must have been 

 quite full of liquid, as water is believed to occur in all deep- 

 seated regions, no matter how high the temperature ; and • 

 wherever the water had free room to circulate, there it has 



