But through it, to my infinite surprise, streamed upward a steady brilliant light.

Could it be any volcanic fire?
In that case, surely I should have felt the heat.

Still, if on this there was doubt, it was of the utmost importance to our common safety to clear it up.

I examined the sides of the descent, and found that I could venture to trust myself to the irregular projection of ledges, at least for some way.

I left the cage and clambered down.