Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/449

 a hardly perceptible southerly dip of 2°, the sheets of dolerite, anamesite and basalt form a mural cliff about 700 feet high. Nowhere in the island can the bedded character of these rocks and their alternation of compact, columnar, amorphous, and amygdaloidal beds be more strikingly seen. They are traversed by veins and dykes of an exceedingly close-grained, sometimes almost flinty, basalt. But the conspicuous feature of the cliff is the hollow which has been worn out of these rocks, and which, after being partially filled with coarse conglomerate, has been buried under the huge pitchstone mass of the Scur. The conglomerate consists of water- worn fragments, chiefly of dolerite and basalt, but with some also of the white Oolitic sandstones, imbedded in a compacted sand derived from the waste of the older volcanic rocks. The grey porphyry, so conspicuous at the east end of the Scur, here disappears and leaves the conglomerate covered by one huge overlying mass of pitchstone.

An examination of the fragments of rock found in the conglomerate on which the great pitchstone ridge of Eigg stands, affords us some indication of the direction in which the river flowed. The occurrence of pieces of red sandstone, which no one who knows West-Highland geology can fail to recognize as of Cambrian derivation, at once makes it clear that the higher grounds from which they were borne could not have lain to the south or east, but to the north-west or north. From the fragments of white sandstone we may with some probability infer that the course of the stream came from the north, where the great white Oolitic sandstones rise to the surface. In short, there seems every probability that this old Tertiary river flowed southward through a forest-clad region, of which the red Cambrian mountains of Ross-shire and the white sandstone cliffs of Raasay and Skye are but fragments, that it passed over a wide and long tract of the volcanic plateau which has been so worn away that it now remains in mere islets left standing out of the deep Atlantic, that since then mountain and valley have alike disappeared, and that in Eigg a fragment of the river- valley has been preserved solely be- cause it has been sealed up under streams of vitreous lava which could better withstand the progress of waste. Thus the Scur of Eigg, like the fragments of the older basalt-plateaux of Auvergne, remains as a monument, not only of volcanic eruptions, but of a former land- surface, now effaced, and of the irresistible march of those slow and seemingly feeble agencies by which the denudation of a country is effected.

4. Summary of the Volcanic Geology of Eigg.

In conclusion let me briefly summarize the more important contributions made by the geology of Eigg to the history of the Tertiary volcanic rocks of Britain.

1. The volcanic rocks of this island rest unconformably upon strata of Oolitic age.

2. They consist almost wholly of a succession of nearly horizontal interbedded sheets of dolerite, anamesite, and basalt, forming an