fo íTSH
F/ ^ :ÿ fo, fo '‘Tj|
rt
"grifo ; ; ' 4 -3 .,. - "'‘i
[ 388 ]
wards, which then gradually falls, and
during this time burfts in the manner
defcribed above.
My reafons for this opinion are
theie ; firft, we do not find the internal
grain of the bafalts melted or
vitrified, which however foon happens
by fufion, and for that purpofe
a very ftiiall degree of fire only.
is requifite. It confequently is very
hard to explain how this fubftance
could have been fo fluid, that no
traces of bubbles appear in it (at
leaft i have not been able to difcovef
any after the niceft examination into
the Scotch and Icelandic bafalts) and
yet when broken appear dull and uneven,
I know very well that lava is
feldom vitrified within ; but the great
number of bubbles and pores which
are found in the whole mafs, are more
than fufiicient proofs than it has not
been perfedly melted to its fmalleil
parts, but has only been brought to
be near fluid.
Secondly, the bafalts fo much refemble
the more fine trapp, both in
refped to their grain and original
com-
■i V ' i
m 'yt
f 389 i
compoiition, that they cah hardly be
tliftinguiilied in fmali fragments^ as
will be more plainly proved in thé
comparifon I hereafteC make* See
N" 24.
But the trapp in all probability has
never been melted^ at leaft not in thofe
parts where I have had opportunities
o f examining it.
Almoft ill ail the Weft Gothia ilra->
tified mountains, the uppermoft ftra-
turn is trapp ; and it muft be properly
-obferved that it always lies upon black
ailum flate. Is it therefore credible
that this fubftance^ which in many
places exceeds a hundred yards ill
depth, can have been perfedly melted
Without caufing the fiate lying beneath
it to lofe fome part of its blackneis,
even in thofe places where tlièy touclt
one another-, as this eíFed may be produced
ill a fmall culinary fire ?
There is befides a more fine kind
o f trapp, which is geiieraiiy found in
Veins or loads, and frequently in very
antient mountains, where not the leaft
traces o f fubterraneaii fire are to be
feen*
B b 3 The