72 Proceedings of the Royal Society 
of the western region, extending from the south of Antrim to the 
north of Syke. The districts more especially dwelt upon were the 
islands of Mull, Eigg, and Staffa. After alluding to the writings 
of previous geologists upon these tracts, more particularly to the 
discovery by the Duke of Argyll of tertiary leaves under basalt at 
Ardtun Head, in Mull, the author remarked that up to this time 
the great mass of volcanic rocks in the Western Islands has been 
usually regarded as of Oolitic age — an opinion in which he himself 
had shared. His object in the present communication was to show 
that as regards Mull and the adjoining islets this opinion was 
erroneous, that the enormous volcanic accumulations of these 
islands belonged in reality to the Miocene period, and that, in all 
likelihood, the long chain of basaltic masses, extending from the 
north of Ireland along the west coast of Scotland to the Faroe 
Islands, and beyond these to Iceland, was all erupted during the 
same wide interval in the Tertiary periods. 
The nature of the volcanic products was first sketched. It was 
shown that the two great classes of recent lavas — the basaltic and 
the trachytic — were well represented among the Western Islands, 
and that the basaltic series was on the whole the older, since it 
was found to pass under massive sheets of pale grey and blue clay- 
stones, clinkstones, and porphyries belonging to the trachytic 
group. In addition to these lava-form rocks, masses of coarse vol- 
canic agglomerate occurred, along with beds of tuff and peperino. 
The manner in which these various volcanic rocks occur in Mull 
and Eigg was next described. It was shown that the leaf-beds of 
Ardtun, which are known by their fossil contents to be of Miocene 
age, lie near the bottom of the whole volcanic series, and that 
above them comes a series of trap-beds between 3000 and 4000 feet 
in thickness. Throughout this enormous mass of bedded igneous 
rock layers of ash, often abounding in chalk-flints, are interstrati- 
fied, and in one part of the cliffs of Inimore of Carsaig a bed of 
flints twenty-five feet thick lies between the dolerites. Thin lenti- 
cular seams or nests of coal likewise occur, but these only occupy 
small pond-like hollows of the original surface of the trap beds, 
and are overlaid directly with trap. They are sometimes excellent 
in quality, and occasionally three feet in thickness ; but they 
rapidly die out in every direction. There is thus no probability 
