Vol. 52.] BASALT-PLATEAUX OP NORTH- WESTERN EUROPE. 369 



may have originally fallen from the basalts against which the 

 conglomerate now reposes. The far- transported stones are also of 

 considerable size, pieces of granite and gneiss frequently exceeding 

 a foot in length. The well-rounded pebbles of foreign materials 

 have been washed into the interstices between the large volcanic 

 blocks. 



It is, I think, tolerably clear that the wall of basalt against which 

 this conglomerate has been laid down is one of erosion. The beds 

 of basalt have here been trenched by some agent which has likewise 

 scooped out the soft underlying shales, and even cut them awav from 

 under their protecting cover of basalt, as shown in figs. 18 & 19. 

 There can be little hesitation in regarding this agent as a water- 

 course which for some considerable interval of time continued to dig 

 its channel through the hard basalts. There is not room enough 

 between the basalt-wall of the Dun Beag and the opposite cliffs of the 

 shore (where no trace of this conglomerate is to be seen) for any 

 large stream to have found its way. I do not, therefore, seek to 

 identify this relic of an ancient waterway with the channel of 

 the main river which deposited the conglomerate-bands of Canna 

 and Sanday. More probably it was either a mere torrential chasm 

 or a tributary stream, draining a certain part of the volcanic plateau 

 and allowed to retain its channel long enough to be able to erode it 

 to a depth of nearly 50 feet. Erosion had reached down through the 

 underlying tuffs to the slaggy basalt below, but before it had made 

 any progress in that sheet its operations were brought to an end 

 at this locality by the floods that swept in the coarse shingle and 

 by the subsequent stream of basalt, of which a mere outlying 

 fragment now forms the upper third of the stack (e in fig. 18). 



The ravine or gully of the Dun Beag probably lay within reach of 

 the floods of the main river, as may be inferred from the number 

 and size of the far-transported rocks in its conglomerate. The 

 conditions of deposition remained little changed during the process 

 of filling up with detritus, except that the largest blocks of rock 

 were swept into the chasm in the earlier part of its history, while 

 much smaller and more waterworn shingle was introduced towards 

 the close. 



Denudation, which has performed such marvels in the topography 

 of the "West of Scotland since older Tertiary time, has here obliterated 

 every trace of this ancient gully, save the little fragment of one of 

 the walls which survives in the stack of Dim Beag. When in the 

 course of centuries this picturesque obelisk shall have yielded to 

 the action of the elements, the last leaflet of one of the most 

 interesting chapters in the geological history of the Inner Hebrides 

 will have been destroyed. 



The question naturally arises, What was the subsequent history 

 of the river which has left so many records of its floods entombed 

 among the basalts of Canna and Sanday ? In particular, can any 

 connexion be traced or plausibly conjectured between it and the 

 river-bed preserved under the Scuir of Eigg ? 



In dealing with this subject, though the evidence is admittedly 



