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. [Footnote 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.] [Footnote 149: _Brit. Assoc. Report_, 1850, Sections, p. 70; and _Quart. Jour. Geol. Soc._ vii. (1851), p. 87.]