238 DR JAMES GEIKIE ON 
action, and this view has been generally accepted by geologists. Indeed, the 
Feriée Islands are sometimes referred to as “a typical example of an up- 
heaved submarine volcano.” The greater lateral extension of the basalt-beds as 
compared to their thickness is supposed to indicate a flow under heavier pres- 
sure than that of the atmosphere alone. The phenomena presented by the old 
basaltic plateaux of Feerde, Iceland, and other countries, are contrasted with the 
appearances which are known to characterise the eruptive products of Hecla, 
Etna, Vesuvius, and other modern subaerial volcanic cones, and since these latter 
rarely or never form such vast successions of parallel and widely extended 
sheets of lava, the older basalts I refer to are believed to have been spread out 
upon the bed of the sea. But if we put aside the fact of their greater 
horizontal dimensions, we find that the basalt-rocks of the Ferde Islands 
present most of the features which are commonly supposed to be characteristic 
of subaerial lava-flows. They are generally vesicular, and often scoriform above 
and below; they exhibit layers and lines of pores and larger cavities, often 
flattened out in the plane of bedding ; now and again their upper surfaces have 
that slagey, wrinkled, and crumpled appearance which is so typical of certain 
modern lavas; while their middle parts are not more vesicular than are the 
central portions of undoubtedly subaerial flows. The absence. of fragmental 
accumulations, such as volcanic breccias and agglomerates, is really no proof of 
the submarine orign of the basalt-rocks of Ferée and other similar trappean 
plateaux. Such coarse accumulations are generally distributed round the 
immediate neighbourhood of the orifice from which they are ejected. The 
basalt beds of the Feerdes may quite well have been ejected from one or more 
central orifices, in which case the absence of fragmental materials would only 
serve to indicate that the focus or foci of eruption must have been at a con- 
siderable distance from the present islands. Or it may be that the whole 
series of basalt-rocks are the products of vast fissure-eruptions. But if this be 
their origin I certainly met with no direct evidence in its favour. None of the 
numerous dykes which we saw could possibly be the feeders that supplied the 
bedded basalts. Most of the dykes died out upwards—often wedging out in 
the middle of a basalt-bed or tuff—and the rock of the dyke could always be 
clearly distinguished from that in which it terminated. The dykes, in short, 
are only thin, irregular veins that ramify and split up into mere threads, and 
have no resemblance to those great wall-like basalt-dykes of supposed Miocene 
age, which are so common in Scotland. Veins exactly similar to those of the 
Feerdes, however, are very common in the Outer Hebrides. 
But whatever the particular origin of the bedded basalts of the Fzerde Islands 
may have been—whether they flowed from one or more foci like the lavas of 
modern volcanoes, or welled up from below along the lines of great fissures—all 
the evidence is against the view that they were erupted upon the bed of the 
