It became warmer and warmer, but he knew that the heat would soon decrease.

The character of the rocks changed, and he studied them as he went down, and continually made notes.
After a time the polished rocky sides of the shaft grew to be of a solemn sameness.

Clewe ceased to take notes; he lighted a cigar and smoked.

He tried to quietly imagine what he would come to when he got to the bottom; it would be some sort of a cave into which his shell had made an opening.

He wondered what sort of a cave it would be, and how high the roof of it was from the bottom.