rather more compact parts, irregular angular patches of the red jasper are
included, the edges of which insensibly blend into the surrounding mass;
other patches occur having an intermediate character between perfect jasper
and the ferruginous, decomposed, basaltic base. In these patches, and
likewise in the large vein-like masses of jasper, there occur little
rounded cavities, of exactly the same size and form with the air-cells,
which in the scoriaceous basalt are filled and lined with layers of
chalcedony. Small fragments of the jasper, examined under the microscope,
seem to resemble the chalcedony with its colouring matter not separated
into layers, but mingled in the siliceous paste, together with some
impurities. I can understand these facts,--namely, the blending of the
jasper into the semi-decomposed basalt,--its occurrence in angular patches,
which clearly do not occupy pre-existing hollows in the rock,--and its
containing little vesicles filled with chalcedony, like those in the
scoriaceous lava,--only on the supposition that a fluid, probably the same
fluid which deposited the chalcedony in the air-cells, removed in those
parts where there were no cavities, the ingredients of the basaltic rock,
and left in their place silica and iron, and thus produced the jasper. In
some specimens of silicified wood, I have observed, that in the same manner
as in the basalt, the solid parts were converted into a dark-coloured
homogeneous stone, whereas the cavities formed by the larger sap-vessels
(which may be compared with the air-vesicles in the basaltic lava) and
other irregular hollows, apparently produced by decay, were filled with
concentric layers of chalcedony; in this case, there can be little doubt
that the same fluid deposited the homogeneous base and the chalcedonic
layers. After these considerations, I cannot doubt but that the jasper of
Ascension may be viewed as a volcanic rock silicified, in precisely the
same sense as this term is applied to wood, when silicified; we are equally
ignorant of the means by which every atom of wood, whilst in a perfect
state, is removed and replaced by atoms of silica, as we are of the means