in the miide it is filled np with opaqoc
and brittle fquare cryilals o f a dead
udiite, or with green drops o f glafs,
that decay after they have been long
expofed to the air. The colour of
the lava is black, dark blue, purple,
reddifli brown, or yellowiili, but
ofteneil black or red. Where the
fire has operated very ilrongly, it is,
as it were, glazed, and looks like
refill. In the ilreams or great traéis
o f lava it is fometimes obferved, that
tlie criiil ill growing cold has fet itfelf
into folds ; but generally it forms a
refemblance of a rope or cable, fome-
times lengthways, and at other times
in the form of a circle, like unto
a great coiled cable, and generally
in fuch a manner that its thicknefs
continually augments from the centre
to the periphery. To this clafs I muft
-alfo place a black folid matter, that
ftrikes fire with ileel, and fometimes
takes the forms o f trees or branches;
and fome people have been inclined
to think they are petrified trees, but
I am rather o f opinion that it is a real
jafper.
jafper. Thirdly, pumice, black, reel,
and even white, which lail has moil
probably been difcoloured by the boiling
water. Fourthly, agate ; I pre-
ferve the received name, though it is
really nothing more than burned
glafs. In fome few places it is ioiind
white, tranfparent, and almoil in the
form of cryilal. The bluifh fort is
alio rare, but is found in large pieces:
the moil common is the black asate. cD *
that is found generally in ftrata, or
in fmall nodules, and fometimes almoft
in the iliape of cryftal, in oval, fquare,
or pentagonal forms. The aftronomer,
Mr. Ejnar Jonilen, has made ufe o f
this black glafs in his tubes, botli in
Copenhagen and Iceland, for his ob-
fervation o f the fun, and has found
them greatly preferable to the darkened
glafs. The green agate is found
rather coarfe and more reddifh, like
thick bottle glafs : it is called hraftin-
nuhroder,
Brimftone, which may be confidered
as the proper fuel of the fire, is found
in great abundance, pure and mineralized
;
If