Here we noticed a curious effect.

The melted litharge ran from the mouth of the furnace upon a floor of damp sand, and spread over it in a sheet.

Presently, as the heat of the mass vaporized the water in the sand below, the sheet of litharge, still slightly fluid, began to heave and swell, and a number of small cones rose from its surface.
Some of these cones reached the height of four inches, and then burst at the top, sending out a shower of red-hot fragments.

I removed one of these cones when the litharge was cool.

It had a regidar funnel-shaped crater, like that which Vesuvius had until three or four years ago.
The analogy is complete between these little cones and those on the lava-field at the foot of the volcano of Jorullo, the celebrated "hornitos;" the concentric structure of which, as described by Burkart, proves that they were formed in precisely the same manner.