Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/424

 the passage of the great system of divergent dykes across faults of every age and through the different geological formations up to and including the chalk — these are facts which make it sufficiently evident that in the north-western part of the British area, along the great hollow stretching from Ireland northwards between the chain of the Outer Hebrides and the Scottish mainland, volcanic action was abundantly manifested in miocene times. "Whether the eruptions took place wholly within the miocene period, or whether they extended beyond it, into later ages, remains yet uncertain. That the time during which the eruptions continued was of enormous duration, is shown by several considerations : — (1) The plateaux are made up of many successive sheets, each of which marks at least one, and sometimes more separate eruptions. In Antrim these sheets rise one over another for a thickness of sometimes 900 feet ; and how much thicker they may have been cannot now be deter- mined, seeing that the upper part of the series has been removed by denudation. In Mull there is a visible thickness of more than 3000 feet of volcanic beds ; yet there, too, the upward continuation of them has been worn away, and there are now no means of measuring what the original total thickness may have been. (2) There occur among the basalts intercalated layers of tuff, clay, and coal, indicating pauses between the eruptions of long enough duration for the growth and accumulation of vegetable matter sufficient, when com- pressed, to form two or three feet of coal*. The leaf-beds of Mull likewise indicate long and tranquil intervals between the outflow of successive sheets of basalt. (3) But the most striking evidence of the long continuance of this volcanic period is furnished in the island of Eigg, by the occurrence of ancient hollows worn by river-action out of the basalt plateaux, and subsequently filled by the outpouring of fresh lava. The latter bears thus the same relation to the more ancient eruptions that the later coulees of Auvergne do to the old denuded basaltic plateaux of that region.

But not only have the last-erupted rocks been worn away, and all evidence removed as to the time when volcanic action ceased to manifest itself in our area ; denudation has since then been so constant and so potent, that even of the whole mass of erupted matter only disconnected fragments remain. Out of the great basaltic tablelands long and wide valleys have been carved to a depth of

regularly intercalated with the oolitic strata. As pointed out in a subsequent page, I have now learned, however, after continued surveys of the other islands, that this intercalation is deceptive, and that the basalts of Skye are only a prolongation of the miocene basalts of Mull.

All the coal associated with the volcanic rocks of the Inner Hebrides is of contemporaneous, that is of miocene, date. The so-called oolitic coal of Skye I have now no doubt is of this age ; and hence the intercalated strata shown to occur in the volcanic series of Skye on the map of Scotland published by Sir Roderick Murchison and myself, and meant to indicate the strips of coal and associated strata, must be regarded not as oolitic, but as miocene. Their size was necessarily greatly exaggerated, with the view of expressing the bedded character of the igneous rocks.
 * Such beds of coal occur in Mull (see Proc. Roy. Soc. Edin. vol. vi. p. 72).