THE GEOLOGY OF THE F4ROE ISLANDS. 241 
but they would become less conspicuous as the outskirts of the igneous area 
were approached. The lavas would still continue to show scoriform crusts, but 
many of them would begin to exhibit a somewhat smoother or less rugged 
surface, showing, in place of great fields of cinder heaps, a wrinkled and 
crumpled appearance, such as is assumed by any viscid substance in motion. 
In short, the outlying parts of the igneous region would be invaded only by 
the more fluid and specifically heavier lavas, the lighter and more porous lavas 
and agglomerates would in great measure be restricted to the cone or cones 
and their vicinity. ‘Tufaceous deposits, however, would not be wanting in any 
part of the land to which the lavas might flow, and they might well extend 
even much further. The tuffs of the outlying regions, however, would generally 
be fine-grained, consisting of the volcanic dust which the winds had power to 
carry to considerable distances ; of volcanic mud or tuff ; and of the materials 
derived from the subaerial disintegration of the exposed lavas. Such fine- 
grained detritus, whether washed down the gentler slopes by rain or swept 
forward as mud, would tend to accumulate in the hollows of the ground. More- 
over, since the inclination of the surface in the outer zone of the volcanic area 
must have been as a rule very gentle, there would be no rapid flow of water, and 
therefore comparatively little aqueous erosion; and thus coarse gravel and 
shingle would be generally absent. But, just because the slope of the land was 
gentle and tolerably equable, fine tufaceous alluvia would tend to be widely dis- 
tributed—gathering thickly in the hollows, and thinning off where the ground 
rose in swells and undulations. In short, they would form rather sporadic 
patches and layers of variable depth, than widespread continuous sheets of 
equable thickness. These assumptions, as we have seen, are confirmed by 
direct observation ; the tuffs are much less continuous and of less uniform 
thickness than the basalt-rocks with which they are interstratified. 
Now let us suppose that, after many sheets of lava had been poured out 
and spread above the site of the future Suderée, a pause in the volcanic activity 
ensued, and the region referred to ceased for a time to be overflowed. We can 
readily believe that, under the climatic conditions prevailing in Miocene times, 
vegetation would sooner or later creep over the surface of the cooled basalts. 
Rain-water would gather in the depressions of the ground, and so give rise to 
swamps and pools and shallow straggling lakes, in which mineral and vegetable 
matter would gradually accumulate. It is not improbable that the basalt- 
plateaux might even be densely wooded. Such conditions might obtain for a 
prolonged period, so as to allow of the accumulation in swamps, bogs, and 
lakes of very considerable depths of vegetable matter, mixed with mud and 
clay and fine sand, and now and again with small stones or pebbly grit. At 
other times the suspension of volcanic activity might not be so prolonged, and 
a renewed incursion of lava might take place before the last current had 
VOL. XXX" PART I. 20 
