1871.] GEIKIE TERTIARY VOLCANIC ROCKS, 309 



a hardly perceptible southerly dip of 2°, the sheets of dolerite, ana- 

 mesite and basalt form a mural cliff about 700 feet high. Nowhere 

 in the island can the bedded character of these rocks and their alter- 

 nation 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 hoUow 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 sand- 

 stones, 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- vaUey 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 con- 

 tributions made by the geology of Eigg to the history of the Ter- 

 tiary volcanic rocks of Britain. 



1. The volcanic rocks of this island rest unconformably upon 

 strata of Oolitic age. 



2. They consist almost whoUy of a succession of nearly horizontal 

 interbedded sheets of dolerite, anamesite, and basalt, forming an 



