DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



107 



eastern side, the agglomerate can be seen to abut against the truncated ends of the flat 

 beds of the plateau-basalts, which are of the usual bedded compact and amygdaloidal 

 character. There can be no doubt, therefore, that the vent has been opened through these 

 basalts. But it will be observed that the latter belong to the lower part of the volcanic 

 series. These lowest sheets are exposed on the slope, resting upon yellowish and spotted 

 grey sandstone, with seams of jet and a reddish breccia, which, lying in hollows of the 

 quartzites, quartz-schists, and mica-schists, form no doubt the local base of the Jurassic 

 rocks of the district. Hence, the vent, though younger than the older sheets of the 

 plateau, may quite well be contemporaneous with some of the later sheets. 



An interesting feature at this locality is the peculiar grouping of some of the large 

 dykes in the area around the agglomerate. They run in the direction of the vent, and 

 one or other of them may represent the fissure or fissures on which the volcanic orifice 

 was blown open to the surface. Another notable element in the geological structure of 

 the ground is the vast amount of intrusive material, both in dykes and sheets, which has 

 been erupted. The intrusive sheets of Ben Hiant form the most prominent eminence in 



d' d c d a 



Fig. 30. — Section of agglomerate Neck at Maclean's Nose, Ardnamurchan. aa, Quartzites and schists; b, bedded basalts 

 lying partly on the schists and partly on patches of Jurassic sandstones that occupy hollows of the older crystalline 

 rocks ; c, agglomerate ; dd, dykes and veins traversing the agglomerate ; e, dolerite sheets of Ben Hiant. 



this part of Ardnamurchan. Reserving them for description in a later part of this 

 memoir, I will only remark here that they partly overlie the agglomerate, and are there- 

 fore to some extent at least younger than the vent. They belong to that late stage in 

 the history of the basalt-plateaux when the molten material, no longer getting ready 

 egress to the surface, forced its way among the rocks about the base of the bedded basalts, 

 and more especially on the sites of older vents, which were doubtless weak places, where 

 it could more easily find relief. 



By far the largest mass of agglomerate in any of the Tertiary volcanic areas is that 

 which occurs on the north side of the main valley of Strath, in Skye.* Unfortunately, 

 it has been so seriously invaded by the eruptive rocks of the group of the Red Hills, that 



* This extensive mass was not separated from the " syenite " of the Red Hills by Macculloch. Von Oeynhausen 

 and Von Dechen noticed it as a conglomerate with quartz pebbles, but did not realise its volcanic nature (Karsten's 

 Archiv, i. p. 90). In my map of Strath (Quart. Jour. Geol. Soc, xiv. plate i.) I distinguished it from the rock of the 

 Red Hills, but no name for it appears in the legend of the map, nor is it referred to in the text. Its character as a true 

 volcanic agglomerate was recognised by Professor Judd, Op. cit., p. 255. See Plate II. of the present memoir. 



