366 SIR A. GEIKIE ON THE TERTIARY [May 1 896, 



the carbonaceous character of the hardened silt in the upper part of 

 each intercalation of sediment. 



If we were to look upon the volcanic materials in the con- 

 glomerates as derived from the subaerial disintegration of the fields 

 of basalt, we should be compelled to admit a very large amount of 

 erosion of the surface of the volcanic plain during the period when 

 the river flowed over that tract. It would be necessary to suppose 

 not only that there was a considerable rainfall, but that the dif- 

 ferences of temperature, either from day to night or from summer 

 to winter, were so great as to split up the lavas at the surface in 

 order to provide the river with the blocks which it has rolled into 

 rounded boulders. I do uot think, however, that such a deduction 

 would be sound. If we compare the materials that have filled up 

 the eruptive vent at the eastern end of Canna with the great majority 

 of the blocks in the coarse conglomerates, we cannot fail to note 

 their strong resemblance. The abundance of lumps of slaggy lava 

 in the river-shingle corresponds with their predominance in the 

 agglomerate of the vent. The boulders of basalt, dolerite, and 

 andesite which crowd the conglomerates need not have been derived 

 from the action of atmospheric waste on the lava-fields, but might 

 quite well have been mainly supplied by the demolition of one or 

 more volcanic cones of fragmental materials. 



That such has really been the chief source of the blocks in the 

 conglomerates I cannot doubt. At the eastern end of Canna we 

 actually detect a volcanic cone partly washed down and overlain by a 

 pile of river-shingle. There were probably many such mounds of slag 

 and stones along lines of fissure all over the lava-fields. The river 

 in its winding course might come upon one cone after another, and 

 during times of flood, or when its waters burst through any tem- 

 porary barrier created by volcanic operations it would attack the 

 slopes of loose material and sweep their detritus onward. At the 

 same time, the current would carry forward its own natural burden 

 of far-transplanted sediment, and hence on its old flood-plains, buried 

 and preserved under sheets of basalt, we find abundant pebbles of 

 the old Highland rocks which it had borne across the whole breadth 

 of the basaltic lowland. 



But the destruction of volcanic cones was probably not the only 

 source of the detritus that now forms the conglomerates of Canna 

 and Sanday. I have shown that these conglomerates pass laterally 

 into tuffs, and are sometimes underlain, sometimes overlain, with 

 similar material. It is quite obvious that their deposition was 

 contemporaneous with volcanic action in the immediate neighbour- 

 hood, and that at least part of their finer sediment was obtained 

 directly from volcanic explosions. In wandering over the coast- 

 sections of these remarkably coarse deposits, I have been impressed 

 with the enormous size of many of the stones, their resemblance to 

 the ejected blocks of the agglomerate and the distinction that may 

 sometimes be made with more or less clearness between their rather 

 angular forms and the more rounded and somewhat waterworn 

 aspect of the other boulders. It seems to me not improbable that 



