One idea overpowered me almost, fear lest the rock should give way from which I was hanging.

This cord seemed a fragile thing for three persons to be suspended from.

I made as little use of it as possible, performing wonderful feats of equilibrium upon the lava projections which my foot seemed to catch hold of like a hand.
When one of these slippery steps shook under the heavier form of Hans, he said in his tranquil voice: "_Gif akt!_" "Attention!" repeated my uncle.
In half an hour we were standing upon the surface of a rock jammed in across the chimney from one side to the other.
Hans pulled the rope by one of its ends, the other rose in the air; after passing the higher rock it came down again, bringing with it a rather dangerous shower of bits of stone and lava.
Leaning over the edge of our narrow standing ground, I observed that the bottom of the hole was still invisible.
The same manoeuvre was repeated with the cord, and half an hour after we had descended another two hundred feet.
I don't suppose the maddest geologist under such circumstances would have studied the nature of the rocks that we were passing.

I am sure I did trouble my head about them.

Pliocene, miocene, eocene, cretaceous, jurassic, triassic, permian, carboniferous, devonian, silurian, or primitive was all one to me.