"Vol. 52.] BASALT-PLATEAUX OF NORTH-WESTERN EUROPE. 359 



faasic lavas, probably mingled with true volcanic dust. The coarser 

 parts display only the feeblest indication of stratification ; indeed, 

 in a limited exposure the rock might be regarded as a tumultuous 

 agglomerate. But the manner in which the deposit is intercalated 

 with, and sometimes overlies, green tuffs and shales, together with 

 the waterworn condition of its stones, shows that it has not been 

 accumulated in a volcanic chimney, but has been thrown down by 

 some powerful body of water, with probably the co-operation of 

 volcanic discharges. 



While the composition of the conglomerate suffices to indicate 

 that it was accumulated at a time when some volcano was active 

 in the immediate neighbourhood, singularly convincing proofs of the 

 work of this vent are to be seen in the form of intercalated sheets 

 of lava. Thus on Eilean a' Bhaird the boulders of the conglomerate 

 are overlain and wrapped round by a sheet of rudely prismatic 

 basalt, with lines of vesicles arranged in the direction of the 

 bedding. A similar relation can be traced along the beach between 

 Canna House and the wooden pier, where successive sheets of basalt 

 have flowed over the conglomerate (see fig. 16, p. 361). 



But, besides coarse volcanic detritus, the sedimentary platform 

 represented by the lower conglomerate of Compass Hill includes 

 other deposits of which good sections may be examined all round 

 €anna Harbour. Beds of fine, well-stratified, dull green tuff pass by 

 an admixture of pebbles into fine ashy conglomerate or pebbly sand- 

 stone, and by an increase in the proportion of their fine detritus 

 into volcanic mudstone and fine shales. The shales vary from a 

 pale grey or white tone into blackish grey, brown, and black. 

 They are well stratified and are frequently interleaved with layers 

 of fine tuff. The darker bands are carbonaceous, and are not in- 

 frequently full of ill-preserved vegetation. Indeed, leaves and stems 

 in a rather macerated condition are of common occurrence in all the 

 shaly layers. Here and there, especially in some ashy shales in 

 front of Canna House, I observed a recognizable Sequoia. The 

 mudstones are dull green, close-grained, shattery rocks composed of 

 fine volcanic detritus, and pass both laterally and vertically into 

 shales, tuffs, and conglomerates. They suggest showers of fine 

 dust or streams of volcanic mud. They, too, contain fragmentary 

 plants. 



It is a noteworthy fact, to which reference has already been made, 

 that the sedimentary intercalations among the Canna basalts gener- 

 ally end upward in carbonaceous shales or coaly layers. The strong 

 •currents and overflows of water, which rolled and spread out the 

 coarse materials of the conglomerates, gave way to quieter conditions 

 that allowed silt and mud to gather over the water-bottom, while 

 leaves and other fragments of vegetation were blown or washed 

 into these quiet reaches. Good illustrations of this sequence in the 

 case of the lower conglomerate-zone of Canna may be studied along 

 the shores of Sand ay, from the Catholic Chapel eastward. The 

 fine pebbly sandstones, tuffs, and shales, which there overlie the 

 coarse conglomerate, are surmounted by dark brown or black 

 carbonaceous shale with lenticles of matted vegetation that pass 



