Vol. 52.] BASALT-PLATEAUX OF NOBXH-WEBTEBJI EUROPE. 3Q~) 



The proofs of transport are admirably exhibited by the pieces of 

 Tonidon Sandstone, epidotic grit, quartzite, and other hard rocks, 

 none of which occur in situ except at some distance from Canna. 

 These stones are often not merely rounded, but so well smoothed 

 and polished as to show that they must have been rolled along for 

 some considerable time in water. 



3. The lenticular character and rapid lithological variations of 

 the strata, both laterally and vertically. The coarse conglomerates 

 die out as they are followed along their outcrop and pass into finer 

 sediment. They seem to occur in irregular banks, which may not 

 be more than 200 feet broad, like the shingle-banks of a river. 

 The coarser sediment generally lies in the lower part of the sedi- 

 mentary group. But cases may be observed, such as that shown in 

 fig. 16, p. 361, where the fine sediment laid down upon the bottom 

 conglomerate has subsequently been overspread by another inroad of 

 coarse shingle. Such alternations are not difficult to understand, if 

 they are looked upon as indicating the successive floods and quieter 

 intervals of a river. 



For these reasons I regard the platforms of sedimentary material 

 intercalated among the basalts of Canna and Sanday as the successive 

 flood-plains of a river which, like the rivers that traverse the lava- 

 deserts of Iceland, flowed perhaps in many separate channels across 

 the basalt-fields of the Inner Hebrides and was liable to have its course 

 shitted from time to time by fresh volcanic eruptions. That this 

 river came from the east or north-east and had its source among the 

 western highlands of Inverness-shire may be inferred from the nature 

 of the stones which it has carried for 30 miles or more along its bed. 

 And that it crossed in its course the tract of Torridon Sandstone, 

 of which a portion still remains in Rum, is manifest from the 

 abundance of the fragments of that formation in the conglomerates. 



'With the remarkable exception of the section on Dun Beag, to be 

 immediately referred to, no trace of any eroded channel of this 

 river through the lavas of the great volcanic plain has been preserved. 

 Possibly frequent invasions of its bed by streams of basalt from 

 different vents hindered it from remaining long enough in one course 

 to erode anything like a gorge or canon. But, in any case, the main 

 channel of the river probably lay rather to the east of the present 

 islands of Canna and Sanday, on ground which is now covered by 

 the sea. The banks or sheets of boulder-conglomerate undoubtedly 

 show where its current swept with great force over the lava-plain, 

 but the manner in which these coarser materials are so often covered 

 with fine silt suggests that the sedimentary materials now visible 

 were deposited rather on the low grounds over which the stream 

 rushed in times of flood. Pools of water would often be left after 

 such inundations, and in these depressions silt would gradually 

 accumulate, partly carried in suspension by the river, partly washed 

 in by rain, while drift-wood that found its way into these eddies, 

 and leaves blown into them from the trees and shrubs of the sur- 

 rounding country, would remain for some time afloat, and would 

 be the last of the detritus to sink to the bottom. Hence, no doubt, 



