DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 181 



In trying to reconstruct the topography of the ground, at the time when the streams 

 of pitchstone flowed, there are several little pieces of evidence which help us. Among the 

 water-worn blocks of the buried river-bed, which are wrapped up in a gravel of basalt 

 debris, there occur fragments of the volcanic rocks of the plateau, also of red (Cambrian) 

 sandstone, quartzite, clay-slate, and white (Jurassic) sandstone. All these rocks are 

 found in place at higher elevations to the north, but in no other direction. There is there- 

 fore good reason to suppose that the river drained some wooded region lying to the north 

 of the present island of Eigg, of which the red sandstone mountains of Ross-shire, the 

 white sandstone cliffs of Skye and Eaasay, and the quartzite and slate uplands of western 

 Inverness-shire are surviving fragments. That the stream, in that portion of its channel 

 preserved under the Scuir, flowed from east to west, may be inferred from the angle at 

 which the tributaries meet the main stream, and also from the fact that the old river-bed 

 at the east end of the Scuir is considerably higher than at the west end. 



The direction of the flow of the stream may afford some indication of that of the 

 pitchstone currents. There can be little doubt, I think, that the lava flowed down the 

 river-valley. Its successive streams are still inclined from east to west. The vent of 

 eruption, therefore, ought to be looked for, not towards the west, but towards the 

 east. Nowhere within the Tertiary volcanic region is there any boss of pitchstone or any 

 mass the shape or size of which is suggestive of an actual vent of discharge to the 

 surface. In the island of Eigg no boss of any kind exists, save those of granophyric 

 porphyry already referred to. But none of these affords any satisfactory links of 

 connection with the rock of the Scuir. More probably the vent lay somewhere to the 

 east on ground now overflowed by the sea. The pitchstone veins of Eigg may represent 

 some of the subterranean extrusions from the same volcanic pipe, and, if so, its site could 

 not be far off. But no other relic of its activity now remains. The lofty picturesque 

 ridge of the Scuir, now so prominent an object in the West Highlands, once occupied 

 the bottom of a valley worn out of the basaltic tableland. Prolonged and stupendous 

 denudation has destroyed the connection with its source, has cut down its ends into 

 beetling precipices, has reduced the former surrounding hills into gentle slopes and 

 undulating lowland, and has turned the bottom of the ancient valley into a long, narrow, 

 and high crest. In this worn fragment we see the only evidence which now remains, 

 that towards the close of the protracted volcanic history of the Tertiary period, streams 

 of acid lavas flowed out over the wasted surface of the basalt-plateaux. 



V. SUMMARY. 



In this final section of the paper, I shall briefly sketch what seem to me to have been 

 the leading features in the history of Tertiary volcanic action in the British Islands. 



1. The region within which this activity manifested itself, during Tertiary time in 

 Britain, cannot be very strictly defined, but if it is restricted to those parts of the country 

 where igneous rocks, probably of that age, now appear at the surface, we find that it 



VOL. XXXV. PART 2. 2 A 



