535.), also, has remarked on the large proportion of silica compared with
alumina, in six analyses of obsidian and pearlstone given in Brongniart's
"Mineralogy." Hence I conclude, that the foregoing concretions have been
formed by a process of aggregation, strictly analogous to that which takes
place in aqueous deposits, acting chiefly on the silica, but likewise on
some of the other elements of the surrounding mass, and thus producing the
different concretionary varieties. From the well-known effects of rapid
cooling (This is seen in the manufacture of common glass, and in Gregory
Watts's experiments on molten trap; also on the natural surfaces of lava-
streams, and on the side-walls of dikes.) in giving glassiness of texture,
it is probably necessary that the entire mass, in cases like that of
Ascension, should have cooled at a certain rate; but considering the
repeated and complicated alterations of nodules and thin layers of a glassy
texture with other layers quite stony or crystalline, all within the space
of a few feet or even inches, it is hardly possible that they could have
cooled at different rates, and thus have acquired their different textures.

The natural sphaerulites in these rocks very closely resemble those
produced in glass, when slowly cooled. (I do not know whether it is
generally known, that bodies having exactly the same appearance as
sphaerulites, sometimes occur in agates. Mr. Robert Brown showed me in an
agate, formed within a cavity in a piece of silicified wood, some little
specks, which were only just visible to the naked eye: these specks, when
placed by him under a lens of high power, presented a beautiful appearance:
they were perfectly circular, and consisted of the finest fibres of a brown
colour, radiating with great exactness from a common centre. These little
radiating stars are occasionally intersected, and portions are quite cut
off by the fine, ribbon-like zones of colour in the agate. In the obsidian
of Ascension, the halves of a sphaerulite often lie in different zones of
colour, but they are not cut off by them, as in the agate.) In some fine