Yol. 52.] BASALT-PLATEAUX OF NORTH-WESTERN EUROPE. 351 



junction with the sedimentary layers, and has probably indurated 

 them. 



This intercalation of a shaly and coaly band among the lavas 

 can be followed northward along the coast. In some places it has 

 been invaded by dykes, sills, and threads of basalt on the most 

 remarkably minute scale, of which I shall give some account in a 

 later part of this paper (see fig. 21, p. 375). North of Tianavaig 

 Bay — that is, about | mile nearer to the Portree vent — a per- 

 ceptible increase in the amount of volcanic material is observable 

 among the shales and leaf-beds. Not only are lapilli of basic 

 pumice abundant, but the volcanic detritus has accumulated here 

 and there in sufficient amount to form a band of dull greenish- 

 brown tuff. 



I have already alluded to the characteristic fact that the inter- 

 stratifications of sedimentary material among the basalt-plateaux 

 frequently terminate upward in leaf-beds, thin coals, or layers of 

 shale, full of indistinctly preserved remains of plants, and some 

 further striking illustrations of this feature will be described from 

 the river-shingles and other evidences of water-action during the 

 volcanic period in the islands of Canna and Sanday. There cannot 

 be any doubt that the vegetation thus preserved was terrestrial. 

 It probably grew not far from the sites where its remains have 

 been preserved. Leaves and seeds would naturally be blown or 

 washed into pools on the lava-fields, and would gather there among 

 the mud and sand carried by rain from the surrounding ground. 

 Such a topography and such a sequence of events point to intervals 

 of longer or shorter duration between the successive outpourings of 

 basalt. It was probably during one of these intervals of quietude 

 that the crater of the Portree volcano became a maar, and was 

 finally silted up. 



There is one last example of a volcanic vent, of which a description 

 may here be given. It occurs at the eastern end of the island of Canna. 

 A portion of it projects from the grassy slopes, and rises vertically 

 above the beach as a picturesque crag in front of the precipice of 

 Compass Hill (PI. XVI.). But the same rock may be traced south- 

 ward to the Coroghon Mdr, and north-westward in the lower part 

 of the cliffs to a little beyond the sea-stack of An Stoll. It has 

 thus a diameter of at least 3000 feet. "Westward it passes under the 

 conglomerate to be afterwards described, and its eastern extension 

 has been concealed by the sea. 



The materials that fill this vent consist of a typical agglomerate 

 composed entirely, or almost entirely, of volcanic detritus. The 

 imbedded blocks vary up to 8 feet in diameter or even more. 

 They are chiefly fragments of various basalts and andesites, generally 

 vesicular or amygdaloidal. Some of these, which have evidently 

 been broken off already consolidated lavas, are angular or sub- 

 angular in shape, and their steam-holes are cut across by the outer 

 surfaces of the stones. Where filled with calcite, zeolite, etc., the 

 amygdules so exactly resemble those of the bedded basalts of the 



