decomposed, and to my surprise, I found in several sections that it was
impossible to distinguish, even approximately, the line of separation
between the decayed lava and the alternating beds of tuff. Since the
specimens have become dry, it is rather more easy to distinguish the
decomposed igneous rocks from the sedimentary tuffs. This gradation in
character between rocks having such widely different origins, may I think
be explained by the yielding under pressure of the softened sides of the
vesicular cavities, which in many volcanic rocks occupy a large proportion
of their bulk. As the vesicles generally increase in size and number in the
upper parts of a stream of lava, so would the effects of their compression
increase; the yielding, moreover, of each lower vesicle must tend to
disturb all the softened matter above it. Hence we might expect to trace a
perfect gradation from an unaltered crystalline rock to one in which all
the particles (although originally forming part of the same solid mass) had
undergone mechanical displacement; and such particles could hardly be
distinguished from others of similar composition, which had been deposited
as sediment. As lavas are sometimes laminated in their upper parts even
horizontal lines, appearing like those of aqueous deposition, could not in
all cases be relied on as a criterion of sedimentary origin. From these
considerations it is not surprising that formerly many geologists believed
in real transitions from aqueous deposits, through wacke, into igneous
traps.

In the valley of Tia-auru, the commonest rocks are basalts with much
olivine, and in some cases almost composed of large crystals of augite. I
picked up some specimens, with much glassy feldspar, approaching in
character to trachyte. There were also many large blocks of vesicular
basalt, with the cavities beautifully lined with chabasie (?), and
radiating bundles of mesotype. Some of these specimens presented a curious
appearance, owing to a number of the vesicles being half filled up with a