cleavage. Under the blowpipe it fuses very readily into a dark grey, and
sometimes even black bead, which is slightly magnetic. From these
characters, I naturally thought that it was one of the pale species,
decomposed, of the genus augite;--a conclusion supported by the unaltered
rock being full of large separate crystals of black augite, and of balls
and irregular streaks of dark grey augitic rock. As the basalt ordinarily
consists of augite, and of olivine often tarnished and of a dull red
colour, I was led to examine the stages of decomposition of this latter
mineral, and I found, to my surprise, that I could trace a nearly perfect
gradation from unaltered olivine to the green wacke. Part of the same grain
under the blowpipe would in some instances behave like olivine, its colour
being only slightly changed, and part would give a black magnetic bead.
Hence I can have no doubt that the greenish wacke originally existed as
olivine; but great chemical changes must have been effected during the act
of decomposition thus to have altered a very hard, transparent, infusible
mineral, into a soft, unctuous, easily melted, argillaceous substance.
(D'Aubuisson "Traite de Geognosie" tome 2 page 569 mentions, on the
authority of M. Marcel de Serres, masses of green earth near Montpellier,
which are supposed to be due to the decomposition of olivine. I do not,
however, find, that the action of this mineral under the blowpipe being
entirely altered, as it becomes decomposed, has been noticed; and the
knowledge of this fact is important, as at first it appears highly
improbable that a hard, transparent, refractory mineral should be changed
into a soft, easily fused clay, like this of St. Jago. I shall hereafter
describe a green substance, forming threads within the cells of some
vesicular basaltic rocks in Van Diemen's Land, which behave under the
blowpipe like the green wacke of St. Jago; but its occurrence in
cylindrical threads, shows it cannot have resulted from the decomposition
of olivine, a mineral always existing in the form of grains or crystals.)