DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 27 



supposed, but pointed to the subserial outpouring of lava at successive intervals, during 

 which terrestial vegetation sprang up upon the older outflows. 



While Forbes brought forward palseontological proofs of the Tertiary age of the 

 volcanic rocks of the south-west of Mull, he at the same time laid before the Geological 

 Society a paper on the Estuary Beds and the Oxford Clay of Loch Staffin, in Skye, 

 wherein, while admitting the existence of appearances which might be regarded as 

 favourable to the view that the intercalated basalts of that region were of much later 

 date than the Oolitic strata between which they might have been intrusively injected, he 

 stated his own belief that they were really contemporaneous with the associated stratified 

 rocks, and thus marked an outbreak of volcanic energy at the close of the Middle Oolitic 

 period.* The Duke op Argyll, in the paper which he on the same occasion communi- 

 cated to the Geological Society, adopted this view of the probable age of most of the 

 basalts of the Western Islands. He looked upon the Tertiary volcanic rocks of Mull as 

 occupying a restricted area, the great mass of the basalt of that island, like that of Skye, 

 being regarded by him as probably not later than some part of the Secondary period. 



It must be granted that the appearances of contemporaneous intercalation of the 

 basalt among the Secondary strata are singularly deceptive. When, several years after 

 the announcement of the Tertiary age of the basalts of Ardtun, I began my geological 

 work in the Inner Hebrides, I was led to the same conclusion as Edward Forbes, and 

 expressed it in an early paper, read before this Society in 1861, on the " Chronology of 

 the Trap-rocks of Scotland."t All over the north of Skye I traced what appeared to be 

 evidence of the contemporaneous interstratification of basalts with the Jurassic rocks, 

 and I concluded (though with some reservation) that the whole of the vast basaltic 

 plateaux of that island were not younger than some later part of the Jurassic period. 

 In that paper the attention of geologists was called to the probable connection of the 

 great system of east-and-west dykes traversing Scotland and the north of England, with 

 the basalt-plateaux of the Inner Hebrides, and as I believed the latter to be probably of 

 the age of the Oolitic rocks, I assigned the dykes to the same period in geological history. 

 But subsequent explorations enabled me to correct the mistake into which, with other 

 geologists, I had fallen regarding the age of the volcanic phenomena of the Western 

 Islands. In 1867 I showed that, instead of being confined to a mere corner of Mull, the 

 Tertiary basalts, with younger associated trachytic or granitic rocks, covered nearly the 

 whole of that island, and that in all likelihood the long chain of basaltic masses, extend- 

 ing 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 Tertiary period. At the same time 

 I drew special attention to the system of east-and-west dykes as proofs of the vigour of 

 volcanic action at that period, and I furnished evidence that this action was prolonged 

 through a vast interval of time, during which great subserial denudation of the older 

 lavas took place before the outflow of the younger. J Later, in the same year, in 



* Quart. Jour. Geol. Soc, vol. vii. (1851) p. 104. t Trans. Boy. Soc. Edm., xxii. (1861) p. 649. 



% Proc. Boy. Soc. Edin., vi. (1867) p. 71. 



