370 SIR A. GEIKTE ON THE TERTIARY [May 1 8 96,. 



scanty, we are not left wholly to conjecture. A consideration of 

 the general topographical features of the wide region of the Inner 

 Hebrides, from the beginning of the volcanic period onward, will 

 convince us that, in spite of the effects of prolonged basalt-eruptions, 

 the persistent now of the drainage of the Western Highlands must 

 have taken a westerly direction. It was towards the west that the 

 low grounds lay. Though the long and broad valley which 

 stretched northwards from Antrim between the line of the Outer 

 Hebrides and the West of Scotland was gradually buried under a 

 depth of 2000 or 3000 feet of lava, the volcanic plain that over- 

 spread it probably remained even to the end lower than the 

 mountainous Western Highlands. Hence the rivers, no matter 

 how constantly they may have had their beds filled up and may 

 have been driven into new channels, would nevertheless always 

 seek their way westward into the Atlantic. 



On Canna and Sanday we have the traces of a river which poured 

 its flood-waters across the lava-fields in that part of the volcanic 

 region, while the basalts were still from time to time streaming 

 from vents and fissures. JSTot more than 14 miles south-east stands 

 the Scuir of Eigg, with its buried river-channel and its striking 

 evidence that this river likewise flowed westward, though at a far 

 later time, when the basalt-eruptions had ceased and the volcanic 

 plain had been already deeply trenched by erosion, but when the 

 subterranean fires were not yet quenched. 



When one reflects upon the enormous denudation of this region y 

 to which further reference will be made in the sequel, one is not 

 surprised that many connecting-links should have been effaced. 

 The astonishment rather arises that so continuous a story can still 

 be deciphered. Even, however, had the original record been left 

 complete, it would have been exceedingly difficult to trace the suc- 

 cessive mutations of a river-channel during long ages of volcanic 

 eruptions. Such a channel would have been concealed from view 

 by each lava-stream that poured into it, and would not have been 

 again exposed save by the very process of erosion that destroys 

 while it reveals. 



While, therefore, there is not and can never be any positive 

 proof that in the fluviatile records of Canna, Sanday, and Eigg 

 successive phases are registered in the history of one single stream, 

 I believe that this identity is highly probable. It was a river 

 which rose among the mountains of Western Inverness-shire, and 

 had already taken its course to the sea before any volcanic eruptions 

 had begun. It continued to flow westward across the lava-floor 

 that gradually spread over the plains. Its channel was constantly 

 being filled up by fresh streams of basalt, or deflected by the uprise 

 of new cinder- eones. But, fed by the Atlantic rains, it maintained 

 its seaward flow until the general subsidence which carried so much 

 of the volcanic plain below the sea. Yet the higher part of this 

 ancient watercourse is no doubt unsubmerged, still traversing the 

 schists of the Western Highlands as it has done since older Tertiary 

 time. It may, perhaps, be recognized in one of the glens which 



