92 Reports and Proceedings—Geological Society of London. 
The view that the acid rocks were, as a whole, older than the 
basic ones, was originally put forward by Prof. J. D. Forbes and 
Dr. F. Zirkel, and is supported by the memoir of 1874. Dr. Geikie 
admits that, around several of the centres indicated, basalts may 
frequently be seen resting on more acid rocks; but the latter he 
regards as being, in every case, of an intrusive character; he also 
allows that the tuffs intercalated with the basalts often contain 
fragments of felsite, but he does not accept this as a proof that the 
felsites must have been erupted before the basalts. Much of the 
divergence of opinion that has arisen appears, however, to be due to 
the circumstance that Dr. Geikie classes as basalt many of the dark- 
coloured lavas (augite-andesites, etc.), which were, in the original 
paper, grouped under the name of “felstones.”” In these “ felstones ” 
the granites and gabbros alike were shown to be intrusive; and it 
was also admitted that there were many intrusions of acid rocks of 
later date than both the ‘felstones” and the basaltic lavas. 
With respect to the existence of great volcanoes in the district, 
Dr. Geikie, while confirming most of the statements which were 
made in 1874 as to the several centres of eruption, prefers to refer 
the origin of the great plateaux of basaltic lava to “ fissure- 
eruptions.” He maintains that the numerous basic dykes of the 
district mark the actual cracks through which the lavas in question 
rose up and welled out at the surface. 
In opposition to this view, it was pointed out that the numbers 
and dimensions of the Tertiary dykes are not such as would warrant 
us in inferring that they formed the conduits through which the 
enormous masses of lava forming the plateaux were erupted; and 
the absence of all proofs of contact-metamorphism at their sides, and 
of evidence that the majority of them ever reached the surface at all, 
was commented upon. In 1874 it was pointed out that some of 
these dykes appeared to mark the radial fissures on which sporadic 
cones (“‘puys”’) were thrown up, after the great central volcanoes 
became extinct ; and this view is supported by the circumstance of 
the close analogies between the materials erupted at this later period, 
and the rocks which constitute some of the undoubtedly Post- 
Mesozoic dykes. 
Dr. Geikie supports his view that the plateau-basalts of the 
Western Isles of Scotland and of Antrim were formed by “ fissure- 
eruptions,” by facts which he noticed in the Snake-River country, 
in the year 1879, while he was making an excursion to the Yellow- 
stone Park, and also by observations made by Captain Dutton in 
the Grand Canon country, in Utah, and in New Mexico. 
With respect to Dr. Geikie’s own observations, it was pointed out 
that geologists who have had more time and opportunity for the 
detailed study of the district in question, like Captain Reynolds, 
Dr. Hayden, and Mr. Clarence King, all agree that there is abundant 
evidence of ordinary volcanic action having occurred in the Snake- 
River country ; and the last-mentioned author distinctly points out 
the great paucity of dykes, and the absence of any evidence of the 
existence of fissures.such as those from which ‘ fissure-eruptions ” 
are supposed to have taken place. 
