518 
ELEMENTS OE GEOLOGY. 
great part scoriaceous and porous, such as were observed to 
have proceeded from Vesuvius and Etna, the resemblance 
seemed remote and equivocal. It was, in truth, like com¬ 
paring the roots of a tree with its leaves and branches, 
which, although they belong to the same plant, differ in 
form, texture, color, mode of growth, and position. The ex¬ 
ternal cone, with its loose ashes and porous lava, may be 
likened to the light foliage and branches, and the rocks con¬ 
cealed far below, to the roots. But it is not enough to say 
of the volcano, 
‘ ‘ Quantum vertice in auras 
^tlierias, tantum radice in Tartara tendit,” 
for its roots do literally reach downward to Tartarus, or to 
the regions of subterranean fire; and what is concealed far 
below is probably always more important in volume and ex¬ 
tent than what is visible above ground. 
We have already stated how frequently dense masses of 
strata have been removed by denudation from wide areas (see 
Chap. VI.); and this fact prepares us to expect a similar de¬ 
struction of whatever may once have formed the uppermost 
part of ancient submarine or sub¬ 
aerial volcanoes, more especially 
as those superficial parts are al¬ 
ways of the lightest and most 
perishable materials. The abrupt 
manner in which dikes of trap 
usually terminate at the surface 
(see Fig. 596), and the water-worn 
pebbles of trap in the alluvium 
which covers ihh dike, prove in¬ 
contestably that whatever was 
uppermost in these formations has been swept away. It is 
easy, therefore, to conceive that what is gone in regions of 
trap may have corresponded to what is now visible in active 
volcanoes. 
As to the absence of porosity in the trappean formations, 
the appearances are in a great degree deceptive, for all 
amygdaloids are, as already explained, porous rocks, into 
the cells of which mineral matter such as silex, carbonate of 
lime, and other ingredients, have been subsequently intro¬ 
duced (see p. 507); sometimes, perhaps, by secretion during 
the cooling and consolidation of lavas. In the Little Cum- 
bray, one of the Western Islands, near Arran, the amygda¬ 
loid sometimes contains elongated cavities filled with brown 
spar; and when the nodules have been washed out, the in- 
strata intercepted by a trap dike, 
and covered with alluvium. 
