Geikie — British Tertiary Volcanic Rocks. 317 



as regards Mull and the adjoining islets this opinion was erroneous, 

 that the enormous volcanic accumulations of these islands belonged 

 in reaKty 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 Ter- 

 tiary period. 



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 claystones, 

 clinkstones, and porphyries belonging to the trachytic group. In 

 addition to these lava-form rocks, masses of coarse volcanic agglome- 

 rate 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 3,000 and 4,000 feet in 

 thickness. Throughout this enormous mass of bedded igneous rock, 

 layers of ash, often abounding in Chalk-flints, are interstratified, and 

 in one part of the cliffs of Inimore of Carsaig a bed of flints twenty- 

 five feet thick lies between the dolerites. Thin lenticular 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 that the Tertiary coal 

 of the Western Islands will ever come to be of commercial im- 

 portance. 



Proofs of the long continuance of volcanic action among these 

 islands are afforded by the great thickness of the successive sheets 

 of igneous matter, which in one moimtain alone — Ben More — reach 

 a depth of 3,185 feet without revealing either the actual bottom or 

 top of the series. Another and striking piece of evidence on this 

 subject is given by the well-known Scuir of Eigg. That island 

 consists of nearly horizontal sheets of dolerite, like those of Mull, 

 resting unconformably upon Oolitic rocks. After their eruption, 

 they must have been long exposed to the wasting agencies of the 

 atmosphere. A valley was cut out of them, and its bottom was 

 watered by a river, which brought down coarse shingle and sand 

 from the distant Cambrian mountains of the north-west. These 

 changes must have demanded a lengthened lapse of time, yet they 

 took place during an interval in the volcanic history of the island. 

 The igneous forces which had been long dormant broke out anew, 

 and poured several successive coulees of vitreous lava down the river- 

 bed. In this way the channel of the stream came to be sealed up. 

 But the same powers of waste which had scooped out that channel 

 continued their operation. The hills which had bounded the valley 



