While Forbes brought forward palæontological 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.[150] The Duke of Argyll, in the paper which he on the same occasion communicated 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. [Footnote 150: _Quart. Journ. Geol. Soc._ vol. vii. (1851), p. 104.] 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.[151] 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 late part of the Jurassic period. In that same 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, 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 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