lateral boundaries, to mark the limits of the disruption;
or else, while the pith or marrow of the lava was still
in a fluid state, its upper surface became solid, and
formed a roof beneath which the molten stream flowed on
to lower levels, leaving a vast cavern into which the
upper crust subsequently plumped down. [Footnote: I feel
it is very presumptuous in me to hazard a conjecture on
a subject with which my want of geological knowledge
renders me quite incompetent to deal; but however incorrect
either of the above suppositions may be justly considered
by the philosophers, they will perhaps serve to convey
to the unlearned reader, for whose amusement (not
instruction) these letters are intended, the impression
conveyed to my mind by what I saw, and so help out the
picture I am trying to fill in for him.]

[Figure: fig-p050a.gif]

The enclosed section will perhaps help you a little to
comprehend what I am afraid my description will have
failed to bring before you.

[Figure: fig-p050.gif with following caption:
1 Gjas.
2 Lava deluge.
3 Original surface.
4 Thingvalla sunk to a lower level.
5 Astonished traveller.]

1. Are the two chasms called respectively Almanna Gja,