Page:A Treatise on Geology, volume 1.djvu/168

 152 that about the same period the Lammermuir hills were raised; and the Cumbrian mountains received one of their great upward movements. It is important to remark in connection with this subject, that along the borders of the Grampian, Lammermuir and Cumbrian ranges, the red conglomerates contain enormous quantities of pebbles, which appear to have been gathered by inundations from the surface of the broken rocks of the neighbouring slates, gneiss, &c.: if in addition we remark the fact that, especially in Cumbria, these conglomerates fill valleys at the border of the tract of the slate mountains, we shall see the probability that the slate rocks were raised above the surface to be washed by atmospheric rains, or else so near the surface as to be exposed to the agitation of shallow water.— The former is the most probable view. The slate and mica schist tracts of the Isle of Man, Donegal, Galway, Wexford, Wicklow, Cavan, and Down, appear to have been similarly raised; and the same is supposed to be true for the Snowdon and Berwyn ranges in North Wales, and the Ocrynian chain of Devon and Cornwall. We must, however, remark on these last-mentioned cases that, on the south-east border of Wales certainly, and in Cornwall probably, there is no observable unconformity between the old red and the Silurian rocks.

Were the displacements thus shown to have happened in the bed of the sea over so large a portion of the British islands, sudden or gradual? To decide whether violent uplifting, or a gentle intumescence of the rocks, lifted the Grampians or the Cumbrian mountains, would be difficult in the present state of our knowledge; yet there are considerations which would render it probable that a considerable time elapsed in the process. Amongst others, this appears worthy of notice: the secondary strata, around these and other tracts, dip at high angles from the centre or axis of the older rocks, the most modern rocks occupying the lowest; ranges;