354 SIR A. GETKIE ON THE TERTIARY [May 1896, 



III. The Rivers of the Volcanic Period. 



Many years ago I communicated to this Society an account of an 

 ancient river-channel which, during the volcanic period, had been 

 eroded on the surface of the basalt-plateau, and of which a small 

 portion had been preserved under a stream of pitchstone-lava that 

 had flowed into and buried it. 1 This watercourse, now marked 

 by the picturesque ridge of the Scuir of Eigg, was shown to have 

 been excavated by a stream which came from the north-east or 

 east, and to be younger, not only than the plateau-basalts of the 

 district, but younger even than the dykes which cut these basalts. 

 Yet that it belonged to the volcanic period was proved by the 

 manner in which it had been sealed up and preserved under the 

 black glassy lava of the Scuir. 



Within the last two years I have met with other and more 

 abundant evidence of river- action in the same region of the Inner 

 Hebrides. This evidence, however, belongs to an earlier part of 

 the volcanic period. It reveals that a powerful river, flowing west- 

 ward from the Highland mountains, swept over the volcanic plain, 

 while the sheets of basalt were still being poured forth, and while 

 volcanic eruptions were taking place from cones of slag. 



This interesting record is preserved in the islands of Canna and 

 Sanday. The gravels and silts of the river are there found inter- 

 calated between the basalts, mingled with volcanic detritus, probably 

 ejected from the active vent already described. On visiting these 

 islands for the first time last year, I found so much that was new 

 to me in regard to the history of Tertiary volcanic action, and 

 which demanded a careful survey, that I returned to the locality 

 this summer and remained in Canna until I had mapped that 

 island and its dependencies upon the Ordnance Survey sheets on 

 the scale of 6 inches to a mile. 



Macculloch, in his account of Canna and Sanday, took notice of 

 the intercalation of beds of conglomerate among the basalts. He 

 regarded these detrital rocks as having been arranged under water 

 and as marking pauses in the deposition of the sheets of ' trap.' 

 He likewise gave two diagrams in illustration of the relations of 

 the conglomerates, but he expressed no definite opinion as to the 

 origin of these rocks, though in one passage he seems to have 

 inclined towards the belief that they were formed in the sea. 2 

 Since his time, so far as I am aware, no fresh light has been thrown 

 upon the subject. 



The conglomerates are best developed at the eastern end of 

 Canna, where the cliffs present the structure illustrated in fig. 15. 

 At the base, and passing under the level of the sea, lies the agglo- 

 merate (a) of the vent already described. This rock has a some- 

 what uneven upper surface, which rises in places about 150 feet 

 above high-tide mark. Here and there it shades off upward into 



1 Quart. Journ. Geol. Soc. vol. xxvii. (1871) p. 303. 



2 ' Description of the Western Islands,' vol. i. (1819) pp. 449, 457, pi. xix. 

 figs. 2 & 3. 



