[144] Op. cit. ii. (1838), p. 343.
[145] Trans. Northumberland and Durham, ii. (1868), p. 30.
[146] Proc. Roy. Soc. xxiii. (1875), p. 543.
[147] Quart. Journ. Geol. Soc. xl. (1884), p. 209.
The geological age of the great series of Tertiary volcanic rocks has only been determined district by district, and at wide intervals. That some part of the Antrim basalts is younger than the Chalk of that region was clearly shown by Berger, Conybeare and Buckland. Portlock, however, referred to the occurrence of detached blocks of basalt which he supposed to be immersed in the Chalk near Portrush, and which inclined him to believe that "the basaltic flows commenced at a remote period of the Cretaceous system."[148] Macculloch showed that the corresponding basaltic plateaux of the Inner Hebrides were certainly younger than the Oolitic rocks of that region. But no nearer approximation to their date had yet been made when in the year 1850 the Duke of Argyll announced the discovery of strata containing fossiliferous chalk-flints and dicotyledonous leaves, lying between the bedded basalts of Ardtun Head, in the Isle of Mull.[149] In the following year these fossil leaves were described by Edward Forbes, who regarded them as decidedly Tertiary, and most probably Miocene. This was the first palæontological evidence for the determination of the geological age of any portion of the basalt-plateaux, and it indicated that the basalts of the south-west of Mull were of older Tertiary date. Taken also in connection with the occurrence of lignite-beds between the basalts of Antrim, it suggested that these volcanic plateaux were not due to submarine eruptions, as the earlier geologists had supposed, but were rather the result of the subærial outpouring of lava at successive intervals, during which terrestrial vegetation sprang up upon the older outflows.
[148] Report on the Geology of Londonderry, p. 93. There can be no doubt that this was an error of observation. The Antrim basalts are all certainly younger than the Chalk. The supposed "lumps of basalt" were probably the ends of veins intruded into the Chalk, and perhaps partially disconnected from the main parts of the veins. Such apparently detached masses of intrusive rock are not infrequent occurrence in connection with the Tertiary intrusive sills. An example will be found represented in [Fig. 321].
[149] Brit. Assoc. Report, 1850, Sections, p. 70; and Quart. Jour. Geol. Soc. vii. (1851), p. 87.
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.
[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 I furnished evidence that this action was prolonged through a vast interval of time, during which great subærial denudation of the older lavas took place before the outflow of the younger.[152] Later in the same year, in an address to the Geological Section of the British Association, I reiterated these views, and more particularly emphasized the importance of the system of dykes, which in my opinion was possibly the most striking manifestation of the vigour of Tertiary volcanic action.[153] In 1871, after further explorations in the field, I gave a detailed account of the structure which had led to the mistake as to the age of the Tertiary volcanic rocks of the Western Islands; and in a description of the island of Eigg, I brought forward data to show the enormous duration of the Tertiary volcanic period in the west of Britain.[154]