Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/436

 parts. The abundant vertical joints only rarely assume a prismatic or rudely columnar arrangement. Hence, though it would often be difficult or impossible to discriminate the intrusive from the interbedded rock in hand-specimens, the distinctions between them are well maintained when we have a cliff-section before us. This similarity and difference become readily intelligible when we regard the two forms as in reality and originally proceeding from the same source, their distinction being due to the different conditions under which they were respectively consolidated ; and we then perceive why the intrusive sheets should he chiefly at the base of the interbedded series. The former are portions of the Tertiary lava which, unable to force their way to the surface, escaped laterally along the lines of least resistance. The increasing mass of the great overlying sheets of the plateau would oppose more and more the rise of the fluid lava, save from the main volcanic vents. Such portions of the latter as were driven up through cracks would often meet less resistance in trying to force their way along the bedding- planes of the secondary strata, or between these strata and the overlying igneous series, or between the lower beds of that series, than in breaking through the thick and compact volcanic mass above. Hence it is that, in nature, the intrusive sheets are in reality found where we might expect to meet with them. These statements involve, no doubt, only the most elementary knowledge. Yet the want of a due appreciation of this knowledge, and of its application in the field, has led to grave misconceptions as to the age of the volcanic rocks of the Inner Hebrides — misconceptions in which I have myself fully shared. I am naturally anxious, therefore, to point out that, while their intrusive relations have been fully recognized, the pseudo-interbedded character of the intrusive sheets at the base of the great basaltic plateaux of our west coast has been confounded with the true interbedded character of the sheets forming the plateau above, and that hence the inference regarding the intercalation of contemporaneous volcanic rocks among the secondary rocks of the Hebrides is without foundation. There is no evidence of any truly contemporaneous volcanic rock, so far as I have yet ascertained, in any of the Liassic or Oolitic rocks of that region. The basalt-plateau, viewed as one great sheet, rests alternately on Cambrian, metamorphosed Lower Silurian, Liassic, Oolitic, and Cretaceous rocks, and unconformably upon them all, from Antrim to the north of Skye. Here and there, where it happened to be laid down upon more or less horizontal strata, it shows at its base intrusive sheets which seem to run parallel with it, as well as with the secondary strata, between which they have been thrust. And thus has arisen the apparent gradation of the Oolitic groups of Skye into an upper volcanic series — a gradation, however, which is quite deceptive, and which disappears when, after wider examination, we come to recognize the true intrusive character of the intercalated sheets, and the real unconformability of the basalt-plateau alike upon Palaeozoic and Secondary formations*.


 * The suggestion of Edward Forbes regarding the probable Oolitic date of the